Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Hirofumi Akamatsu Last modified date:2021.06.23

Associate Professor / Applied inorganic chemistry laboratory / Department of Applied Chemistry / Faculty of Engineering


Papers
1. Tomoki Murata, Hirofumi Akamatsu, Daisuke Hirai, Fumiyasu Oba, Sakyo Hirose, Antiferroelectricity and robust dielectric response owing to competing polar and antipolar instabilities in tetragonal tungsten bronze K2RNb5O15 (R: rare-earth), PHYSICAL REVIEW MATERIALS, 10.1103/PhysRevMaterials.4.104419, 4, 10, 2020.10.
2. Sota Asaki, Hirofumi Akamatsu, George Hasegawa, Tomohiro Abe, Yuki Nakahira, Suguru Yoshida, Chikako Moriyoshi, Katsuro Hayashi, Ferroelectricity of Dion-Jacobsen layered perovskites CsNdNb(2)O(7)and RbNdNb2O7, JAPANESE JOURNAL OF APPLIED PHYSICS, 10.35848/1347-4065/abad46, 59, SP, 2020.11, Crystallography and dielectric properties in Dion-Jacobson layered perovskites, CsNdNb(2)O(7)and RbNdNb2O7, have been examined in dense polycrystalline samples, and polarization hysteresis loops that substantiate ferroelectricity have been observed at room temperature. The theoretical mechanism for the spontaneous polarization, "hybrid improper ferroelectric mechanism," induced by a combination of two types of non-polar octahedral rotations, is confirmed in these two phases. Our samples show remanent polarizations of 2-3 mu C cm(-2), which are much larger than those obtained in polycrystalline samples with the hybrid improper ferroelectricity reported so far. A dielectric constant in CsNdNb(2)O(7)exhibits an anomaly at 625 K, corresponding to the ferroelectric transition, as previously revealed by X-ray and neutron diffractometry. No dielectric anomaly is observed for RbNdNb(2)O(7)throughout the temperature range studied here (<= 773 K), which is consistent with the previous diffractometry showing the persistence of polar I2cmsymmetry up to 790 K..
3. George Hasegawa, Takaya Yano, Hirofumi Akamatsu, Katsuro Hayashi, Kazuki Nakanishi, Variation of meso- and macroporous morphologies in resorcinol-formaldehyde (RF) gels tailored via a sol-gel process combined with soft-templating and phase separation, JOURNAL OF SOL-GEL SCIENCE AND TECHNOLOGY, 10.1007/s10971-020-05236-9, 95, 3, 801-812, 2020.09, A wide-range control over multimodal pore systems in porous monoliths is a key technology for developing functional materials, as the favorable pore structures in different length scales are required to be tailored depending on their application fields. In the alkoxy-derived sol-gel systems of silica and organosilicates, the synthetic methodology of meso- and macroporous monoliths with tunable pore properties has been developed by combining the supramolecular self-assembly of a Pluronic surfactant and polymerization-induced phase-separation techniques. This strategy has been applied to the sol-gel process of phenolic resins, giving rise to the hierarchically porous polymer gels with ordered mesoporosity and the corresponding carbon monoliths after carbonization. However, the controllable size range has been limited so far. This study has explored the relationship between the starting composition and the bimodal pore properties in further detail aiming at a better control of pore properties in phenolic resins. The enlargement of mesopore size has been achieved, yet associated with broadening the mesopore size distribution and coarsening the macropore morphology, resulting in the particle aggregates. The systematical investigation also reveals that the addition of KCl can improve the micelle arrangement in macroframework and provide the narrower mesopore size distribution..
4. George Hasegawa, Yuto Akiyama, Moeko Tanaka, Ryo Ishikawa, Hirofumi Akamatsu, Yuichi Ikuhara, Katsuro Hayashi, Reversible Electrochemical Insertion/Extraction of Magnesium Ion into/from Robust NASICON-Type Crystal Lattice in a Mg(BF4)(2)-Based Electrolyte, ACS APPLIED ENERGY MATERIALS, 10.1021/acsaem.0c00943, 3, 7, 6824-6833, 2020.07, Reliable electrochemical investigations of electrode materials are indispensable for the development of next-generation energy storage devices. In the case of multivalent cation-based electrochemistry, intense attention should be paid to the cell configuration for obtaining reliable data. In particular, the electrolyte and reference electrode must be appropriately selected considering the potential window of electrolyte and the validity of reference. Here, we demonstrate the detailed electrochemical examination for the Mg2+-storage capability of the NASICON-type framework derived from Na3V2(PO4)(3) (NVP). A combination of the Mg(BF4)(2)-based electrolyte with high anodic stability and the reliable Ag pseudo-reference electrode offers decent electrochemical test results. Despite suffering from the polarization concerning magnesiation, the desodiated NVP electrode can deliver a well-defined discharge plateau at similar to 2.7 V (vs Mg2+/Mg) with the reversible capacity of >100 mAh g(-1) at room temperature. The impedance analysis results indicate that the increased charge transfer resistance on discharging due to the high energy barrier for desolvation of divalent cations is responsible for the large polarization but not extremely significant, allowing for the room-temperature operation. The findings obtained herein also highlight the importance of the structural robustness of host lattice, which is required to withstand the strong amorphization during Na+ extraction and Mg2+ insertion/extraction..
5. Hirofumi Akamatsu, Koji Fujita, Toshihiro Kuge, Arnab Sen Gupta, James M. Rondinelli, Isao Tanaka, Katsuhisa Tanaka, Venkatraman Gopalan, A-site cation size effect on oxygen octahedral rotations in acentric Ruddlesden-Popper alkali rare-earth titanates, PHYSICAL REVIEW MATERIALS, 10.1103/PhysRevMaterials.3.065001, 3, 6, 2019.06, We demonstrate inversion symmetry breaking induced by oxygen octahedral rotations in layered perovskite oxides KA(R)TiO(4) (A(R) = rare earth) using a combined experimental and theoretical approach including synchrotron x-ray diffraction, optical second harmonic generation, and first-principles lattice dynamics calculations. We experimentally find an interesting but counterintuitive phenomenon, i.e., the acentric-to-centric phase transition temperatures for K family are higher than those for previously reported Na family, in contrast to expectations based on the Goldschmidt tolerance factor, where the octahedral rotation instability toward the acentric phases would reduce with an increase in the radius of A-site alkali metal ions. Our detailed analysis of first-principles calculations for A(A)A(R)TiO(4) (A(A) = Na, K, Rb) reveals that the alkali metal and rare-earth ions play quite different roles in driving the octahedral rotations. Since rare-earth ions attract oxide ions more strongly than alkali metal ions due to the higher valence of the former in comparison with the latter (A(R)(3+) vs A(A)(+)), the optimization of coordination environment of rare-earth ions is the primary driving force of the octahedral rotations. Alkali metal ions serve to impose "bond strains" parallel to the layers, playing a secondary role in the octahedral rotations. Incorporation of large alkali metal ions generates a significant in-plane biaxial bond strain in A(R)O and TiO2 layers through the expanded A(A)O layers, and thereby facilitates the octahedral rotations because of the otherwise highly underbonding of rare-earth ions. Thus, the effect of A-site alkali metal size on the octahedral rotation instability can be explained in terms of the interlayer lattice mismatch. This understanding allows us to propose a geometric descriptor governing the structural instability in A(A)A(R)TiO(4) layered perovskites. We believe that control over the interlayer lattice mismatch could be a useful strategy to tune the octahedral rotations in layered compounds..
6. Ritesh Uppuluri, Hirofumi Akamatsu, Arnab Sen Gupta, Huaiyu Wang, Craig M. Brown, Kleyser E. Agueda Lopez, Nasim Alem, Venkatraman Gopalan, Thomas E. Mallouk, Competing Polar and Antipolar Structures in the Ruddlesden-Popper Layered Perovskite Li2SrNb2O7, Chemistry of Materials, 10.1021/acs.chemmater.9b00786, 31, 12, 4418-4425, 2019.06, Over the past few years, several studies have reported the existence of polar phases in n = 2 Ruddlesden-Popper layer perovskites by trilinear coupling of oxygen octahedral rotations (OOR) and polar distortions, a phenomenon termed as hybrid improper ferroelectricity. This phenomenon has opened an avenue to expand the available compositions of ferroelectric and piezoelectric layered oxides. In this study, we report a new polar n = 2 Ruddlesden-Popper layered niobate, Li2SrNb2O7, which undergoes a structural transformation to an antipolar phase when cooled to 90 K. This structural transition results from a change in the phase of rotation of the octahedral layers within the perovskite slabs across the interlayers. First-principles calculations predicted that the antipolar Pnam phase would compete with the polar A21am phase and that both would be energetically lower than the previously assigned centrosymmetric Amam phase. This phase transition was experimentally observed by a combination of synchrotron X-ray diffraction, powder neutron diffraction, and electrical and nonlinear optical characterization techniques. The competition between symmetry breaking to yield polar layer perovskites and hybrid improper antiferroelectrics provides new insight into the rational design of antiferroelectric materials that can have applications as electrostatic capacitors for energy storage..
7. Shiming Lei, Shalinee Chikara, Danilo Puggioni, Jin Peng, Mengze Zhu, Mingqiang Gu, Weiwei Zhao, Yu Wang, Yakun Yuan, Hirofumi Akamatsu, Moses H. W. Chan, Xianglin Ke, Zhiqiang Mao, James M. Rondinelli, Marcelo Jaime, John Singleton, Franziska Weickert, Vivien S. Zapf, Venkatraman Gopalan, Comprehensive magnetic phase diagrams of the polar metal Ca-3(Ru0.95Fe0.05)(2)O-7, PHYSICAL REVIEW B, 10.1103/PhysRevB.99.224411, 99, 22, 2019.06, Polar metals exist as a rather unique class of materials as they combine two seemingly mutually exclusive properties (polar order and metallicity) in one system. So far only a few polar metals have been unambiguously identified; the magnetic ones are exceptionally rare. Here we investigate a 5% Fe-doped polar metal Ca3Ru2O7, via electrical transport, magnetization, microstrain, and optical second-harmonic generation measurements. We report the full magnetic phase diagrams (in the field-temperature space) for magnetic field H parallel to a and H parallel to b, which exhibit distinct field-dependent magnetizations behavior. In particular, for H parallel to a we found a ferromagnetic incommensurate spin structure, which is absent in the pure Ca3Ru2O7. We propose a microscopic spin model to understand this behavior, highlighting the role of Fe doping in tipping the delicate balance of the underlying exchange-interaction energy in this system..
8. He Wang, George Hasegawa, Yuto Akiyama, Takahisa Yamamoto, Atsushi Inoishi, Hirofumi Akamatsu, Miki Inada, Tatsumi Ishihara, Katsuro Hayashi, A highly conductive Na3V2(PO4)(3) ceramic sheet prepared by tape-casting method, ELECTROCHIMICA ACTA, 10.1016/j.electacta.2019.03.057, 305, 197-203, 2019.05, Ceramic sheets of Na3V2(PO4)(3) (NVP) with NASICON-related structure have been prepared by tape-casting method. The addition of 60Na(2)O-10Nb(2)O(5)-30P(2)O(5) glass to NVP is demonstrated to enhance markedly the density of the sintered NVP sheet and its electrical conductivity. The best densified NVP sheet with a thickness of a few tens of mu m is obtained by sintering tape-casted NVP added with 5 wt% glass at 920 degrees C, which exhibits a total conductivity of 2.9 x 10(-5) S cm(-1) at room temperature. A Pt vertical bar NVP sheet vertical bar Pt cell works as a tiny solid-state battery with a cell potential of 1.7 V, in which NVP acts as both cathode and anode active materials via redox reactions of V4+/V3+ and V3+/V2+ as well as a Na+ ion conducting electrolyte between the two electrode layers. (C) 2019 Elsevier Ltd. All rights reserved..
9. Yasuhiro Uchida, George Hasegawa, Kazunari Shima, Miki Inada, Naoya Enomoto, Hirofumi Akamatsu, Katsuro Hayashi, Insights into Sodium Ion Transfer at the Na/NASICON Interface Improved by Uniaxial Compression, ACS APPLIED ENERGY MATERIALS, 10.1021/acsaem.9b00250, 2, 4, 2913-2920, 2019.04, A robust ceramic solid electrolyte with high ionic conductivity is a key component for all-solid-state batteries (ASSBs). In terms of the demand for high-energy-density storage, researchers have been tackling various challenges to use metal anodes, where a fundamental understanding on the metal/solid electrolyte interface is of particular importance. The Na+ superionic conductor, so-called NASICON, has high potential for application to ASSBs with a Na anode due to its high Na+ ion conductivity at room temperature, which has, however, faced a daunting issue of the significantly large interfacial resistance between Na and NASICON. In this work, we have successfully reduced the interfacial resistance as low as 14 Omega cm(2) at room temperature by a simple mechanical compression of a Na/NASICON assembly. We also demonstrate a fundamental study of the Na/NASICON interface in comparison with the Na/beta ''-alumina counterpart by means of the electrochemical impedance technique, which elucidates a stark difference between the activation energies for interfacial charge transfer: similar to 0.6 eV for Na/NASICON and similar to 0.3 eV for Na/beta ''-alumina. This result suggests the formation of a Na+-conductive interphase layer in pressing Na metal on the NASICON surface at room temperature..
10. Matthew Brahlek, Vladimir A. Stoica, Jason Lapano, Lei Zhang, Hirofumi Akamatsu, I-Cheng Tung, Venkatraman Gopalan, Donald A. Walko, Haidan Wen, John W. Freeland, Roman Engel-Herbert, Structural dynamics of LaVO3 on the nanosecond time scale, STRUCTURAL DYNAMICS-US, 10.1063/1.5045704, 6, 1, 014502, 2019.01, Due to the strong dependence of electronic properties on the local bonding environment, a full characterization of the structural dynamics in ultrafast experiments is critical. Here, we report the dynamics and structural refinement at nanosecond time scales of a perovskite thin film by combining optical excitation with time-resolved X-ray diffraction. This is achieved by monitoring the temporal response of both integer and half-integer diffraction peaks of LaVO3 in response to an above-band-gap 800 nm pump pulse. We find that the lattice expands by 0.1% out of plane, and the relaxation is characterized by a biexponential decay with 2 and 12 ns time scales. We analyze the relative intensity change in half-integer peaks and show that the distortions to the substructure are small: the oxygen octahedral rotation angles decrease by similar to 0.3 degrees and La displacements decrease by similar to 0.2 pm, which directly corresponds to an similar to 0.8 degrees increase in the V-O-V bond-angles, an in-plane V-O bond length reduction of similar to 0.3 pm, and an unchanged out-of-plane bond length. This demonstration of tracking the atomic positions in a pump-probe experiment provides experimentally accessible values for structural and electronic tunability in this class of materials and will stimulate future experiments. (C) 2019 Author(s)..
11. George Hasegawa, Moeko Tanaka, Junie Jhon M. Vequizo, Akira Yamakata, Hajime Hojo, Makoto Kobayashi, Masato Kakihana, Miki Inada, Hirofumi Akamatsu, Katsuro Hayashi, Sodium titanium oxide bronze nanoparticles synthesized via concurrent reduction and Na+-doping into TiO2(B), NANOSCALE, 10.1039/c8nr08372j, 11, 3, 1442-1450, 2019.01, A mixed valence compound, sodium titanium oxide bronze (NaxTiO2-B), combines intriguing properties of high electric conductivity and good chemical stability together with a unique one-dimensional tunnel crystal structure available for cation storage. However, this compound has not been studied for a long period because of the strongly reductive condition at high temperature required for its preparation, which limits the morphological control such as the preparation of nanocrystals. For the first time in this paper, the topotactic synthesis of nano-sized NaxTiO2-B with high specific surface area (>130 m(2) g(-1)) from TiO2(B) nanoparticles has been demonstrated. The reaction of metastable TiO2(B) with NaBH4 allows carrier electrons to be doped simultaneously with incorporation of Na+ ions into the interstitial sites of the host Ti-O lattice at relatively low temperature. An electrochemical investigation of Li+- and Na+-ion storage behaviors suggests that the incorporated Na+ ions are mainly placed in the 6-fold coordination sites of bronze. In addition, optical measurements including time-resolved transient spectroscopy revealed that the doped electrons in the NaxTiO2-B nanoparticles are predominantly in the Ti3+ state and behave as a small polaron. The pelletized NaxTiO2-B nanoparticles shows a good electronic conductivity of 1.4 x 10(-2) S cm(-1) at 30 degrees C with an activation energy of 0.17 eV, which is attributable to the thermal barrier for the polaron hopping..
12. Mariola O. Ramirez, Tom T. A. Lummen, Irene Carrasco, Eftihia Barnes, Ulrich Aschauer, Dagmara Stefanska, Arnab Sen Gupta, Carmen de las Heras, Hirofumi Akamatsu, Martin Holt, Pablo Molina, Andrew Barnes, Ryan C. Haislmaier, Przemyslaw J. Deren, Carlos Prieto, Luisa E. Bausa, Nicola A. Spaldin, Venkatraman Gopalan, Emergent room temperature polar phase in CaTiO3 nanoparticles and single crystals, APL MATERIALS, 10.1063/1.5078706, 7, 1, 011103, 2019.01, Polar instabilities are well known to be suppressed on scaling materials down to the nanoscale, when the electrostatic energy increase at surfaces exceeds lowering of the bulk polarization energy. Surprisingly, here we report an emergent low symmetry polar phase arising in nanoscale powders of CaTiO3, the original mineral named perovskite discovered in 1839 and considered nominally nonpolar at any finite temperature in the bulk. Using nonlinear optics and spectroscopy, X-ray diffraction, and microscopy studies, we discover a well-defined polar to non-polar transition at a T-C = 350 K in these powders. The same polar phase is also seen as a surface layer in bulk CaTiO3 single crystals, forming striking domains with in-plane polarization orientations. Density functional theory reveals that oxygen octahedral distortions in the surface layer lead to the stabilization of the observed monoclinic polar phase. These results reveal new ways of overcoming the scaling limits to polarization in perovskites. (C) 2019 Author(s)..
13. Yasuhide Mochizuki, Yu Kumagai, Hirofumi Akamatsu, Fumiyasu Oba, Polar metallic behavior of strained antiperovskites ACNi(3) (A = Mg, Zn, and Cd) from first principles, PHYSICAL REVIEW MATERIALS, 10.1103/PhysRevMaterials.2.125004, 2, 12, 125004, 2018.12, Stable structures of metallic antiperovskites ACNi(3) (A = Mg, Zn, and Cd) under epitaxial strain are explored using first- principles lattice dynamics calculations. Although the ground states of ACNi(3) are cubic nonpolar phases without structural distortion, transitions to polar phases are predicted to arise in these compounds under compressive strain while maintaining metallic states. In particular, the polar phase of MgCNi3 would be attainable with moderate strain of a few percent. The polar distortions are considered to be generated by the enhancement of the hybridization between C 2p and Ni 3d states, which is a mechanism analogous to that of the previously discovered polar metal CeSiPt3..
14. Yasuhide Mochizuki, Hirofumi Akamatsu, Yu Kumagai, Fumiyasu Oba, Strain-engineered Peierls instability in layered perovskite La3Ni2O7 from first principles, PHYSICAL REVIEW MATERIALS, 10.1103/PhysRevMaterials.2.125001, 2, 12, 125001, 2018.12, Using first-principles calculations, we predict a strong coupling of lattice, charge, and spin degrees of freedom in Ruddlesden-Popper phase La3Ni2O7, which enables a phase control with epitaxial strain. While the bulk ground state is metallic, moderate compressive strain is found to trigger a Peierls transition to an insulating state in concurrence with a breathing distortion of NiO6 octahedra. The Peierls transition is microscopically interpreted as a band-gap opening arising from lifting and lowering of the Ni d-e(g) states due to the octahedral breathing..
15. Suguru Yoshida, Hirofumi Akamatsu, Ryosuke Tsuji, Olivier Hernandez, Haricharan Padmanabhan, Arnab Sen Gupta, Alexandra S. Gibbs, Ko Mibu, Shunsuke Murai, James M. Rondinelli, Venkatraman Gopalan, Katsuhisa Tanaka, Koji Fujita, Hybrid Improper Ferroelectricity in (Sr,Ca)(3)Sn2O7 and Beyond: Universal Relationship between Ferroelectric Transition Temperature and Tolerance Factor in n=2 Ruddlesden-Popper Phases, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 10.1021/jacs.8b07998, 140, 46, 15690-15700, 2018.11, Hybrid improper ferroelectricity, which utilizes non polar but ubiquitous rotational/tilting distortions to create polarization, offers an attractive route to the discovery of new ferroelectric and multiferroic materials because its activity derives from geometric rather than electronic origins. Design approaches blending group theory and first principles can be utilized to explore the crystal symmetries of ferroelectric ground states, but in general, they do not make accurate predictions for some important parameters of ferroelectrics, such as Curie temperature (T-C). Here, we establish a predictive and quantitative relationship between T-C and the Goldschmidt tolerance factor, t, by employing n = 2 Ruddlesden-Popper (RP) A(3)B(2)O(7) as a prototypical example of hybrid improper ferroelectrics. The focus is placed on an RP system, (Sr1-xCax)(3)Sn2O7 (x = 0, 0.1, and 0.2), which allows for the investigation of the purely geometric (ionic size) effect on ferroelectric transitions, due to the absence of the second-order Jahn-Teller active (d(0) and 6s(2)) cations that often lead to ferroelectric distortions through electronic mechanisms. We observe a ferroelectric-to-paraelectric transition with T-C = 410 K for Sr3Sn2O7. We also find that the T-C increases linearly up to 800 K upon increasing the Ca2+ content, i.e., upon decreasing the value of t. Remarkably, this linear relationship is applicable to the suite of all known A(3)B(2)O(7) hybrid improper ferroelectrics, indicating that the T-C correlates with the simple crystal chemistry descriptor, t, based on the ionic size mismatch. This study provides a predictive guideline for estimating the T-C of a given material, which would complement the convergent group-theoretical and first-principles design approach..
16. Jason M. Munro, Hirofumi Akamatsu, Haricharan Padmanabhan, Vincent S. Liu, Yin Shi, Long-Qing Chen, Brian K. VanLeeuwen, Ismaila Dabo, Venkatraman Gopalan, Discovering minimum energy pathways via distortion symmetry groups, PHYSICAL REVIEW B, 10.1103/PhysRevB.98.085107, 98, 8, 085107, 2018.08, Physical systems evolve from one state to another along paths of least energy barrier. Without apriori knowledge of the energy landscape, multidimensional search methods aim to find such minimum energy pathways between the initial and final states of a kinetic process. However, in many cases, the user has to repeatedly provide initial guess paths, thus implying that the reliability of the final result is heavily user-dependent. Recently, the idea of "distortion symmetry groups" as a complete description of the symmetry of a path has been introduced. Through this, a new framework is enabled that provides a powerful means of classifying the infinite collection of possible pathways into a finite number of symmetry equivalent subsets, and then exploring each of these subsets systematically using rigorous group theoretical methods. The method, which we name the distortion symmetry method, is shown to lead to the discovery of previously hidden pathways for the case studies of bulk ferroelectric switching and domain wall motion in proper and improper ferroelectrics, as well as in multiferroic switching. These provide novel physical insights into the nucleation of switching pathways at experimentally observed domain walls in Ca(3)Ti2O(7), as well as how polarization switching can proceed without reversing magnetization in BiFeO3. Furthermore, we demonstrate how symmetry-breaking from a highly symmetric pathway can be used to probe the non-Ising (Bloch and Neel) polarization components integral to transient states involved in switching in PbTiO3. The distortion symmetry method is applicable to a wide variety of physical phenomena ranging from structural, electronic and magnetic distortions, diffusion, and phase transitions in materials..
17. Suguru Yoshida, Koji Fujita, Hirofumi Akamatsu, Olivier Hernandez, Arnab Sen Gupta, Forrest G. Brown, Haricharan Padmanabhan, Alexandra S. Gibbs, Toshihiro Kuge, Ryosuke Tsuji, Shunsuke Murai, James M. Rondinelli, Venkatraman Gopalan, Katsuhisa Tanaka, Ferroelectric Sr3Zr2O7: Competition between Hybrid Improper Ferroelectric and Antiferroelectric Mechanisms, ADVANCED FUNCTIONAL MATERIALS, 10.1002/adfm.201801856, 28, 30, 1801856, 2018.07, [URL], In contrast to polar cation displacements driving oxides into noncentrosymmetric and ferroelectric states, inversion-preserving anion displacements, such as rotations or tilts of oxygen octahedra about cation coordination centers, are exceedingly common. More than one nonpolar rotational mode in layered perovskites can lift inversion symmetry and combine to induce an electric polarization through a hybrid improper ferroelectric (HIF) mechanism. This form of ferroelectricity expands the compositional palette to new ferroelectric oxides because its activity derives from geometric rather than electronic origins. Here, the new Ruddlesden-Popper HIF Sr3Zr2O7, which is the first ternary lead-free zirconate ferroelectric, is reported and room-temperature polarization switching is demonstrated. This compound undergoes a first-order ferroelectric-to-paraelectric transition, involving an unusual change in the sense of octahedral rotation while the octahedral tilt remains unchanged. Our experimental and first-principles study shows that the paraelectric polymorph competes with the polar phase and emerges from a trilinear coupling of rotation and tilt modes interacting with an antipolar mode. This form of hybrid improper antiferroelectricity is recently predicted theoretically but has remained undetected. This work establishes the importance of understanding anharmonic interactions among lattice degrees of freedom, which is important for the discovery of new ferroelectrics and likely to influence the design of next-generation thermoelectrics..
18. Yanwei Cao, Zhen Wang, Se Young Park, Yakun Yuan, Xiaoran Liu, Sergey M. Nikitin, Hirofumi Akamatsu, M. Kareev, S. Middey, D. Meyers, P. Thompson, P. J. Ryan, Padraic Shafer, A. N'Diaye, E. Arenholz, Venkatraman Gopalan, Yimei Zhu, Karin M. Rabe, J. Chakhalian, Artificial two-dimensional polar metal at room temperature, NATURE COMMUNICATIONS, 10.1038/s41467-018-03964-9, 9, 2018.04, Polar metals, commonly defined by the coexistence of polar crystal structure and metallicity, are thought to be scarce because the long-range electrostatic fields favoring the polar structure are expected to be fully screened by the conduction electrons of a metal. Moreover, reducing from three to two dimensions, it remains an open question whether a polar metal can exist. Here we report on the realization of a room temperature two-dimensional polar metal of the B-site type in tri-color (tri-layer) superlattices BaTiO3/SrTiO3/LaTiO3. A combination of atomic resolution scanning transmission electron microscopy with electron energy-loss spectroscopy, optical second harmonic generation, electrical transport, and first-principles calculations have revealed the microscopic mechanisms of periodic electric polarization, charge distribution, and orbital symmetry. Our results provide a route to creating all-oxide artificial non-centrosymmetric quasi-two-dimensional metals with exotic quantum states including coexisting ferroelectric, ferromagnetic, and superconducting phases..
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20. Hirofumi Akamatsu, Yakun Yuan, Vladimir A. Stoica, Greg Stone, Tiannan Yang, Zijian Hong, Shiming Lei, Yi Zhu, Ryan C. Haislmaier, John W. Freeland, Long-Qing Chen, Haidan Wen, Venkatraman Gopalan, Light-Activated Gigahertz Ferroelectric Domain Dynamics, PHYSICAL REVIEW LETTERS, 10.1103/PhysRevLett.120.096101, 120, 9, 2018.02, Using time- and spatially resolved hard x-ray diffraction microscopy, the striking structural and electrical dynamics upon optical excitation of a single crystal of BaTiO3 are simultaneously captured on subnanoseconds and nanoscale within individual ferroelectric domains and across walls. A large emergent photoinduced electric field of up to 20 x 10(6) V/m is discovered in a surface layer of the crystal, which then drives polarization and lattice dynamics that are dramatically distinct in a surface layer versus bulk regions. A dynamical phase-field modeling method is developed that reveals the microscopic origin of these dynamics, leading to gigahertz polarization and elastic waves traveling in the crystal with sonic speeds and spatially varying frequencies. The advances in spatiotemporal imaging and dynamical modeling tools open up opportunities for disentangling ultrafast processes in complex mesoscale structures such as ferroelectric domains..
21. Minh An T. Nguyen, Arnab Sen Gupta, Jacob Shevrin, Hirofumi Akamatsu, Pengtao Xu, Zhong Lin, Ke Wang, Jun Zhu, Venkatraman Gopalan, Mauricio Terrones, Thomas E. Mallouk, Random anion distribution in MS
x
Se
2-x
(M = Mo, W) crystals and nanosheets, RSC Advances, 10.1039/c8ra01497c, 8, 18, 9871-9878, 2018.01, The group VIb dichalcogenides (MX
2
, M = Mo, W; X= S, Se) have a layered molybdenite structure in which M atoms are coordinated by a trigonal prism of X atoms. Ternary solid solutions of MS
x
Se
2-x
were synthesized, microcrystals were grown by chemical vapor transport, and their morphologies and structures were characterized by using synchrotron X-ray diffraction, Rietveld refinement, DIFFaX simulation of structural disorder, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Double aberration corrected scanning transmission electron microscopy was used to determine the anion distributions in single-layer nanosheets exfoliated from the microcrystals. These experiments indicate that the size difference between S and Se atoms does not result in phase separation, consistent with earlier studies of MX
2
monolayer sheets grown by chemical vapor deposition. However, stacking faults occur in microcrystals along the layering axis, particularly in sulfur-rich compositions of MS
x
Se
2-x
solid solutions..
22. Koji Fujita, Takahiro Kawamoto, Ikuya Yamada, Olivier Hernandez, Hirofumi Akamatsu, Yu Kumagai, Fumiyasu Oba, Pascal Manuel, Ryo Fujikawa, Suguru Yoshida, Masayuki Fukuda, Katsuhisa Tanaka, Perovskite-Type InCoO3 with Low-Spin Co3+: Effect of In-O Covalency on Structural Stabilization in Comparison with Rare-Earth Series, INORGANIC CHEMISTRY, 10.1021/acs.inorgchem.7b01426, 56, 18, 11113-11122, 2017.09, Perovskite rare-earth cobaltites ACoO(3) (A = Sc, Y, La Lii) have been of enduring interest for decades due to their unusual structural and physical properties associated with the spin-state transitions of low-spin Co3+ ions. Herein, we have synthesized a non rare-earth perovskite cobaltite, InCoO3, at 15 GPa and 1400 degrees C and investigated its crystal structure and magnetic ground state. Under the same high-pressure and high-temperature conditions, we also prepared a perovskite-type ScCoO3 with an improved cation stoichiometry in comparison to that in a previous study, where synthesis at 6 GPa and 1297 degrees C yielded a perovskite cobaltite with cation mixing on the A-site, (Sc0.95Co0.05)CoO3. The two perovskite phases have nearly stoichiometric cation compositions, crystallizing in the orthorhombic Pnma space group. In the present investigation, comprehensive studies on newly developed and well-known Pnma ACoO(3) perovskites (A = In, Sc, Y, Pr-Lu) show that InCoO3 does not fulfill the general evolution of crystal metrics with A-site cation size, indicating that InCoO3 and rare-earth counterparts have different chemistry for stabilizing the Pnma structures. Detailed structural analyses combined with first-principles calculations reveal that the origin of the anomaly for InCoO3 is ascribed to the A-site cation displacements that accompany octahedral tilts; despite the highly tilted CoO6 network, the In-O covalency makes In3+ ions reluctant to move from their ideal cubic-symmetry position, leading to less orthorhombic distortion than would be expected from electrostatic/ionic size mismatch effects. Magnetic studies demonstrate that InCoO3 and ScCoO3 are diamagnetic with a low-spin state of Co3+ below 300 K, in contrast to the case of (Sc0.95Co0.05)CoO3, where the high-spin Co3+ ions on the A-site generate a large paramagnetic moment. The present work extends the accessible composition range of the low-spin orthocobaltite series and thus should help to establish a more comprehensive understanding of the structure property relation..
23. Yu Kumagai, Kou Harada, Hirofumi Akamatsu, Kosuke Matsuzaki, Fumiyasu Oba, Carrier-Induced Band-Gap Variation and Point Defects in Zn3N2 from First Principles, PHYSICAL REVIEW APPLIED, 10.1103/PhysRevApplied.8.014015, 8, 1, 014015, 2017.07, The zinc nitride Zn3N2 is composed of inexpensive and earth-abundant Zn and N elements and shows high electron mobility exceeding 100 cm(2) V-1 s(-1). Although various technological applications of Zn3N2 have been suggested so far, the synthesis of high-quality Zn3N2 samples, especially single crystals, is still challenging, and therefore its basic properties are not yet well understood. Indeed, the reported band gaps of as-grown Zn3N2 widely scatter from 0.85 to 3.2 eV. In this study, we investigate the large gap variation of Zn3N2 in terms of the Burstein-Moss (BM) effect and point-defect energetics using first-principles calculations. First, we discuss the relation between electron carrier concentration and optical gaps based on the electronic structure obtained using the Heyd-Scuseria-Ernzerhof hybrid functional. The calculated fundamental band gap is 0.84 eV in a direct-type band structure. Second, thermodynamic stability of Zn3N2 is assessed using the ideal-gas model in conjunction with the rigid-rotor model for gas phases and first-principles phonon calculations for solid phases. Third, carrier generation and compensation by native point defects and unintentionally introduced oxygen and hydrogen impurities are discussed. The results suggest that a significant BM shift occurs mainly due to oxygen substitutions on nitrogen sites and hydrogen interstitials. However, gaps larger than 2.0 eV would not be due to the BM shift because of the Fermi-level pinning caused by acceptorlike zinc vacancies and hydrogen-on-zinc impurities. Furthermore, we discuss details of peculiar defects such as a nitrogen-on-zinc antisite with azidelike atomic and electronic structures..
24. Arnab Sen Gupta, Hirofumi Akamatsu, Forrest G. Brown, Minh An T. Nguyen, Megan E. Strayer, Saul Lapidus, Suguru Yoshida, Koji Fujita, Katsuhisa Tanaka, Isao Tanaka, Thomas E. Mallouk, Venkatrarnan Gopalan, Competing Structural Instabilities in the Ruddlesden Popper Derivatives HRTiO4 (R = Rare Earths): Oxygen Octahedral Rotations Inducing Noncentrosymmetricity and Layer Sliding Retaining Centrosymmetricity, CHEMISTRY OF MATERIALS, 10.1021/acs.chemmater.6b04103, 29, 2, 656-665, 2017.01, We report the observation of noncentrosymmetricity in the family of HRTiO4 (R = Eu, Gd, Dy) layered oxides possessing a Ruddlesden Popper derivative structure, by second harmonic generation and synchrotron X-ray diffraction with the support of density functional theory calculations. These oxides were previously thought to possess inversion symmetry. Here, inversion symmetry is lifted by rotations of the oxygen-coordinated octahedra, a mechanism that is not active in simple perovskites. We observe a competition between rotations of the oxygen octahedra and sliding of a combined unit of perovskite rocksalt perovskite blocks at the proton layers. For the smaller rare earth ions, R = Eu, Gd, and Dy, which favor the octahedral rotations, noncentrosymmetricity is present but the sliding is absent. For the larger rare earth ions, R = Nd and Sm, the octahedral rotations are absent, but the sliding at the proton layers is present to optimize the length and direction of hydrogen.bonding in the crystal structure. The study reveals a new mechanism for inducing noncentrosymmetricity in layered oxides, and chemical structural effects related to rare earth ion size and hydrogen bonding that can turn this mechanism on and off. We construct a phase diagram of temperature versus rare earth ionic radius for the HRTiO4 family..
25. Koji Fujita, Takahiro Kawamoto, Ikuya Yamada, Olivier Hernandez, Naoaki Hayashi, Hirofumi Akamatsu, William Lafargue-Dit-Hauret, Xavier Rocquefelte, Masafumi Fukuzumi, Pascal Manuel, Andrew J. Studer, Christopher S. Knee, Katsuhisa Tanaka, LiNbO3-Type InFeO3: Room-Temperature Polar Magnet without Second-Order Jahn-Teller Active Ions, CHEMISTRY OF MATERIALS, 10.1021/acs.chemmater.6b02783, 28, 18, 6644-6655, 2016.09, Great effort has been devoted to developing single-phase magnetoelectric multiferroics, but room-temperature coexistence of large electric polarization and magnetic ordering still remains elusive. Our recent finding shows that such polar magnets can be synthesized in small-tolerance-factor perovskites AFeO(3) with unusually small cations at the A-sites, which are regarded as having a LiNbO3-type structure (space group R3c). Herein, we experimentally reinforce this finding by preparing a novel room-temperature polar magnet, LiNbO3-type InFeO3. This compound is obtained as a metastable quench product from an orthorhombic perovskite phase stabilized at 15 GPa and an elevated temperature. The structure analyses reveal that the polar structure is characterized by displacements of In3+ (d(10)) and Fe3+ (d(5)) ions along the hexagonal c-axis (pseudocubic [111] axis) from their centrosymmetric positions, in contrast to well-known perovskite ferroelectrics (e.g., BaTiO3, PbTiO3, and BiFeO3) where d(0) transition-metal ions and/or 6s(2) lone-pair cations undergo polar displacements through the so-called second-order Jahn-Teller (SOJT) distortions. Using density functional theory calculations, the electric polarization of LiNbO3-type InFeO3 is estimated to be 96 mu C/cm(2) along the c-axis, comparable to that of an isostructural and SOJT-active perovskite ferroelectric, BiFeO3 (90-100 mu C/cm(2)). Magnetic studies demonstrate weak ferromagnetic behavior at room temperature, as a result of the canted G-type antiferromagnetic ordering of Fe3+ moments below T-N similar to 545 K. The present work shows the functional versatility of small-tolerance-factor perovskites and provides a useful guide for the synthesis and design of room-temperature polar magnets..
26. Man‐Rong Li, Maria Retuerto, Peter W Stephens, Mark Croft, Denis Sheptyakov, Vladimir Pomjakushin, Zheng Deng, Hirofumi Akamatsu, Venkatraman Gopalan, Javier Sánchez‐Benítez, Felix O Saouma, Joon I Jang, David Walker, Martha Greenblatt, Low‐Temperature Cationic Rearrangement in a Bulk Metal Oxide, Angewandte Chemie, 128, 34, 10016-10021, 2016.08.
27. Man-Rong Li, Maria Retuerto, Peter W. Stephens, Mark Croft, Denis Sheptyakov, Vladimir Pomjakushin, Zheng Deng, Hirofumi Akamatsu, Venkatraman Gopalan, Javier Sanchez-Benitez, Felix O. Saouma, Joon I. Jang, David Walker, Martha Greenblatt, Low-Temperature Cationic Rearrangement in a Bulk Metal Oxide, ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, 10.1002/anie.201511360, 55, 34, 9862-9867, 2016.08, Cationic rearrangement is a compelling strategy for producing desirable physical properties by atomic-scale manipulation. However, activating ionic diffusion typically requires high temperature, and in some cases also high pressure in bulk oxide materials. Herein, we present the cationic rearrangement in bulk Mn2FeMoO6 at unparalleled low temperatures of 150-300 degrees C. The irreversible ionic motion at ambient pressure, as evidenced by real-time powder synchrotron X-ray and neutron diffraction, and second harmonic generation, leads to a transition from a Ni3TeO6-type to an ordered-ilmenite structure, and dramatic changes of the electrical and magnetic properties. This work demonstrates a remarkable cationic rearrangement, with corresponding large changes in the physical properties in a bulk oxide at unprecedented low temperatures..
28. Arnab Sen Gupta, Hirofumi Akamatsu, Forrest Brown, Megan Strayer, Minh An Nguyen, Thomas Mallouk, Venkatraman Gopalan, Noncentrosymmetry induced by oxygen octahedral rotations competing with octahedral sliding in Ruddlesden-Popper phases, HRTiO4 (R = rare earths), ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, 252, 2016.08.
29. Minh An Nguyen, Arnab Sen Gupta, Jacob Shervin, Hirofumi Akamatsu, Ana Laura Elias, Mauricio Terrones, Jun Zhu, Venkatraman Gopalan, Thomas Mallouk, Synthesis and characterizations of MoSxSe2-x and WSxSe2-x solid solutions, ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, 252, 2016.08.
30. Yoshiro Kususe, Suguru Yoshida, Koji Fujita, Hirofumi Akamatsu, Masafumi Fukuzumi, Shunsuke Murai, Katsuhisa Tanaka, Structural phase transitions in EuNbO3 perovskite, JOURNAL OF SOLID STATE CHEMISTRY, 10.1016/j.jssc.2016.04.032, 239, 192-199, 2016.07, The crystal structures of europium niobate, EuNbO3, have been examined over a wide temperature range between 20 and 500 K using synchrotron X-ray diffraction. We have observed two successive structural phase transitions at 360 and 460 K. Below 350 K, EuNbO3 adopts an orthorhombic perovskite structure (space group Imma), which is characterized by NbO6 octahedral tilting about the pseudocubic two-fold axis. The result differs from previous reports in which EuNbO3 was assigned to a cubic aristotype (space group Pm (3) over barm) of perovskite at room temperature. At around 360 K, EuNbO3 undergoes a first-order phase transition to a tetragonal symmetry (space group I4/mcm) with the NbO6 octahedral tilting about the pseudocubic four-fold axis. As the temperature is further increased, the I4/mcm tetragonal phase changes into the Pm (3) over barm cubic aristotype at 460 K. The tetragonal-to-cubic transformation is characterized as a continuous phase transition. (C) 2016 Elsevier Inc. All rights reserved..
31. Megan E. Strayer, Arnab Sen Gupta, Hirofumi Akamatsu, Shiming Lei, Nicole A. Benedek, Venkatraman Gopalan, Thomas E. Mallouk, Emergent Noncentrosymmetry and Piezoelectricity Driven by Oxygen Octahedral Rotations in n=2 Dion-Jacobson Phase Layer Perovskites, ADVANCED FUNCTIONAL MATERIALS, 10.1002/adfm.201504046, 26, 12, 1930-1937, 2016.03, The loss of centrosymmetry via oxygen octahedral rotations is demonstrated in the n = 2 Dion-Jacobson family of layered oxide perovskites, A'LaB2O7 (A' = Rb, Cs; B = Nb, Ta). Ab initio density functional theory calculations predict that all four materials should adopt polar space groups, in contrast to the results of previous experimental studies that have assigned these materials to the centrosymmetric P4/mmm space group. Optical second harmonic generation experiments confirm the presence of a noncentrosymmetric phase at ambient temperature. Piezoresponse force microscopy experiments also show that this phase is piezoelectric. To elucidate the symmetry-breaking and assign the appropriate space groups, the crystal structure of CsLaNb2O7 is refined as a function of temperature from synchrotron X-ray diffraction data. Above 550 K, CsLaNb2O7 adopts the previously determined centrosymmetric P4/mmm space group. Between 550 and 350 K, the symmetry is lowered to the noncentrosymmetric space group Amm2. Below 350 K, additional symmetry lowering is observed as peak splitting, but the space group cannot be unambiguously identified..
32. Forrest Brown, Arnab Sen Gupta, Hirofumi Akamatsu, Minh An Nguyen, Thomas Mallouk, Venkatraman Gopalan, Competing broken inversion symmetry and oxygen octahedral sliding phenomena in n=1 Ruddlesden popper derivative HRTiO4 (R=Nd, Sm, Eu, Gd, and Dy) family, ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, 251, 2016.03.
33. Sun Woo Kim, Zheng Deng, Man-Rong Li, Arnab Sen Gupta, Hirofumi Akamatsu, Venkatraman Gopalan, Martha Greenblatt, PbMn(IV)TeO6: A New Noncentrosymmetric Layered Honeycomb Magnetic Oxide, INORGANIC CHEMISTRY, 10.1021/acs.inorgchem.5b02677, 55, 3, 1333-1338, 2016.02, PbMnTeO6, a new noncentrosymmetric layered magnetic oxide was synthesized and characterized. The crystal structure is hexagonal, with space group P (6) over bar 2m (No. 189), and consists of edge-sharing (Mn4+/Te6+)O-6 trigonal prisms that form honeycomb-like two-dimensional layers with Pb2+ ions between the layers. The structural difference between PbMnTeO6, with disordered/trigonal prisms of Mn4+/Te6+, versus the similar chiral SrGeTeO6 (space group P312), with long-range order of Ge4+ and Te6+ in octahedral coordination, is attributed to a difference in the electronic effects of Ge4+ and Mn4+. Temperature-dependent second harmonic generation by PbMnTeO6 confirmed the noncentrosymmetric character between 12 and 873 K. Magnetic measurements indicated antiferromagnetic order at T-N approximate to 20 K and a frustration parameter (vertical bar theta vertical bar/TN) of similar to 2.16..
34. Arnab Sen Gupta, Hirofumi Akamatsu, Megan E. Strayer, Shiming Lei, Toshihiro Kuge, Koji Fujita, Clarina dela Cruz, Atsushi Togo, Isao Tanaka, Katsuhisa Tanaka, Thomas E. Mallouk, Venkatraman Gopalan, Improper Inversion Symmetry Breaking and Piezoelectricity through Oxygen Octahedral Rotations in Layered Perovskite Family, LiRTiO4 (R = Rare Earths), ADVANCED ELECTRONIC MATERIALS, 10.1002/aelm.201500196, 2, 1, 2016.01, An improper mechanism for breaking inversion symmetry is revealed and thus inducing piezoelectricity in the family of layered perovskites, LiRTiO4 (R = rare earths), which are previously reported as centrosymmetric. Noncentrosymmetry in this family of compounds arises from TiO6 octahedral rotation represented by a(-)b(o)c(o)/b(o)a(-)c(o) in the perovskite blocks between RO rock salt and LiO antifl uorite layers. X-ray diffraction and optical second harmonic generation complemented by density functional theory predictions are crucial in determining the new structures. High transition temperature (T-ac) of up to 1200 K from noncentrosymmetric to centrosymmetric phase is observed. Piezoelectric coefficients (d(36)) of up to -15 pC/N are predicted, and piezoelectric force microscopy experiments confirm a piezoelectric response. The demonstrated improper mechanism in this and other layered oxide families, with a wide range of available topologies and chemistries, could aid in the search for high temperature piezoelectrics..
35. Takeshi Yajima, Fumitaka Takeiri, Kohei Aidzu, Hirofumi Akamatsu, Koji Fujita, Wataru Yoshimune, Masatoshi Ohkura, Shiming Lei, Venkatraman Gopalan, Katsuhisa Tanaka, Craig M. Brown, Mark A. Green, Takafumi Yamamoto, Yoji Kobayashi, Hiroshi Kageyama, A labile hydride strategy for the synthesis of heavily nitridized BaTiO3, NATURE CHEMISTRY, 10.1038/NCHEM.2370, 7, 12, 1017-1023, 2015.12, Oxynitrides have been explored extensively in the past decade because of their interesting properties, such as visible-light absorption, photocatalytic activity and high dielectric permittivity. Their synthesis typically requires high-temperature NH3 treatment (800-1,300 degrees C) of precursors, such as oxides, but the highly reducing conditions and the low mobility of N(3-)species in the lattice place significant constraints on the composition and structure-and hence the properties-of the resulting oxynitrides. Here we show a topochemical route that enables the preparation of an oxynitride at low temperatures (< 500 degrees C), using a perovskite oxyhydride as a host. The lability of H-in BaTiO3-xHx (x <= 0.6) allows H-/N3- exchange to occur, and yields a room-temperature ferroelectric BaTiO3-xN2x/3. This anion exchange is accompanied by a metal-to-insulator crossover via mixed O-H-N intermediates. These findings suggest that this 'labile hydride' strategy can be used to explore various oxynitrides, and perhaps other mixed anionic compounds..
36. Takeshi Yajima, Fumitaka Takeiri, Kohei Aidzu, Hirofumi Akamatsu, Koji Fujita, Wataru Yoshimune, Masatoshi Ohkura, Shiming Lei, Venkatraman Gopalan, Katsuhisa Tanaka, Craig M. Brown, Mark A. Green, Takafumi Yamamoto, Yoji Kobayashi, Hiroshi Kageyama, A labile hydride strategy for the synthesis of heavily nitridized BaTiO3, NATURE CHEMISTRY, 10.1038/NCHEM.2370, 7, 12, 1017-1023, 2015.12, Oxynitrides have been explored extensively in the past decade because of their interesting properties, such as visible-light absorption, photocatalytic activity and high dielectric permittivity. Their synthesis typically requires high-temperature NH3 treatment (800-1,300 degrees C) of precursors, such as oxides, but the highly reducing conditions and the low mobility of N(3-)species in the lattice place significant constraints on the composition and structure-and hence the properties-of the resulting oxynitrides. Here we show a topochemical route that enables the preparation of an oxynitride at low temperatures (< 500 degrees C), using a perovskite oxyhydride as a host. The lability of H-in BaTiO3-xHx (x <= 0.6) allows H-/N3- exchange to occur, and yields a room-temperature ferroelectric BaTiO3-xN2x/3. This anion exchange is accompanied by a metal-to-insulator crossover via mixed O-H-N intermediates. These findings suggest that this 'labile hydride' strategy can be used to explore various oxynitrides, and perhaps other mixed anionic compounds..
37. Martha Greenblatt, Manrong Li, Maria Retuerto, Zheng Deng, Mark Croft, David Vanderbilt, Meng Ye, Peter Stephens, Joke Hadermann, David Walker, Joachim Hemberger, Christoph Grams, Chang-Qing Jin, Wen-Min Li, Joon Jang, Felix Saouma, Venkatraman Gopalan, Hirofumi Akamatsu, New polar and magnetic corundum - type oxides, A2BB ' O6:High pressure synthesis, ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, 250, 2015.08.
38. Man-Rong Li, Mark Croft, Peter W. Stephens, Meng Ye, David Vanderbilt, Maria Retuerto, Zheng Deng, Christoph P. Grams, Joachim Hemberger, Joke Hadermann, Wen-Min Li, Chang-Qing Jin, Felix O. Saouma, Joon I. Jang, Hirofumi Akamatsu, Venkatraman Gopalan, David Walker, Martha Greenblatt, Mn2FeWO6: A New Ni3TeO6-Type Polar and Magnetic Oxide, ADVANCED MATERIALS, 10.1002/adma.201405244, 27, 13, 2177-+, 2015.04, Mn22+Fe2+W6+O6, a new polar magnetic phase, adopts the corundum-derived Ni3TeO6-type structure with large spontaneous polarization (P-S) of 67.8 mu C cm-2, complex antiferromagnetic order below approximate to 75 K, and field-induced first-order transition to a ferrimagnetic phase below approximate to 30 K. First-principles calculations predict a ferrimagnetic (udu) ground state, optimal switching path along the c-axis, and transition to a lower energy udu-udd magnetic double cell..
39. Takahiro Kawamoto, Koji Fujita, Ikuya Yamada, Tomohiko Matoba, Sung Joo Kim, Peng Gao, Xiaoqing Pan, Scott D. Findlay, Cedric Tassel, Hiroshi Kageyama, Andrew J. Studer, James Hester, Tetsuo Irifune, Hirofumi Akamatsu, Katsuhisa Tanaka, Room-Temperature Polar Ferromagnet ScFeO3 Transformed from a High-Pressure Orthorhombic Perovskite Phase, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 10.1021/ja507958z, 136, 43, 15291-15299, 2014.10, Multiferroic materials have been the subject of intense study, but it remains a great challenge to synthesize those presenting both magnetic and ferroelectric polarizations at room temperature. In this work, we have successfully obtained LiNbO3-type ScFeO3, a metastable phase converted from the orthorhombic perovskite formed under 15 GPa at elevated temperatures. A combined structure analysis by synchrotron X-ray and neutron powder diffraction and high-angle annular dark-field scanning transmission electron microscopy imaging reveals that this compound adopts the polar R(3)c symmetry with a fully ordered arrangement of trivalent Sc and Fe ions, forming highly distorted ScO6 and FeO6 octahedra. The calculated spontaneous polarization along the hexagonal c-axis is as large as 100 mu C/cm(2). The magnetic studies show that LiNbO3-type ScFeO3 is a weak ferromagnet with TN = 545 K due to a canted G-type antiferromagnetic ordering of Fe3+ spins, representing the first example of LiNbO3-type oxides with magnetic ordering far above room temperature. A comparison of the present compound and rare-earth orthorhombic perovskites RFeO3 (R = La-Lu and Y), all of which possess the corner-shared FeO6 octahedral network, allows us to find a correlation between TN and the Fe-O-Fe bond angle, indicating that the A-site cation-size-dependent octahedral tilting dominates the magnetic transition through the Fe-O-Fe superexchange interaction. This work provides a general and versatile strategy to create materials in which ferroelectricity and ferromagnetism coexist at high temperatures.
40. Hirofumi Akamatsu, Koji Fujita, Toshihiro Kuge, Arnab Sen Gupta, Atsushi Togo, Shiming Lei, Fei Xue, Greg Stone, James M. Rondinelli, Long-Qing Chen, Isao Tanaka, Venkatraman Gopalan, Katsuhisa Tanaka, Inversion Symmetry Breaking by Oxygen Octahedral Rotations in the Ruddlesden-Popper NaRTiO4 Family, PHYSICAL REVIEW LETTERS, 10.1103/PhysRevLett.112.187602, 112, 18, 2014.05, Rotations of oxygen octahedra are ubiquitous, but they cannot break inversion symmetry in simple perovskites. However, in a layered oxide structure, this is possible, as we demonstrate here in A-site ordered Ruddlesden-Popper NaRTiO4 (R denotes rare-earth metal), previously believed to be centric. By revisiting this series via synchrotron x-ray diffraction, optical second-harmonic generation, piezoresponse force microscopy, and first-principles phonon calculations, we find that the low-temperature phase belongs to the acentric space group P(4) over bar 2(1)m, which is piezoelectric and nonpolar. The mechanism underlying this large new family of acentric layered oxides is prevalent, and could lead to many more families of acentric oxides..
41. Yuko Nakatsuka, Hirofumi Akamatsu, Shunsuke Murai, Koji Fujita, Katsuhisa Tanaka, Superspin glass behavior of amorphous FeO-SiO2 thin films, JAPANESE JOURNAL OF APPLIED PHYSICS, 10.7567/JJAP.53.05FB11, 53, 5, 2014.05, We have prepared amorphous xFeO center dot(100 - x)SiO2 (x = 38.2, 54.8, and 67.5 in mol %) thin films by a pulsed laser deposition method and examined their magnetic properties. The temperature dependence of zero-field-cooled magnetization manifests a cusp-like peak, i.e., spin-freezing in a low temperature region. The spin-freezing temperature depends on the frequency of ac magnetic field, and the dependence is explainable in terms of the scaling law. The aging-memory effect characteristic of spin glass is also observed. Furthermore, superparamagnetic behavior is found in the variation of magnetization with magnetic field at high temperatures. These observations indicate that there exist magnetic clusters of Fe2+ ions in the amorphous FeO-SiO2 thin films and that the intercluster interactions bring about the superspin glass transition at low temperatures. (C) 2014 The Japan Society of Applied Physics.
42. Yasuhide Akizuki, Ikuya Yamada, Koji Fujita, Hirofumi Akamatsu, Tetsuo Irifune, Katsuhisa Tanaka, AgCu3V4O12: a Novel Perovskite Containing Mixed-Valence Silver ions, INORGANIC CHEMISTRY, 10.1021/ic402579v, 52, 24, 13824-13826, 2013.12, A novel silver-containing perovskite, Ag-Cu3V4O12, was synthesized under high-pressure and high-temperature conditions. It crystallizes in an A-site-ordered perovskite structure (space group Im (3) over bar), in which silver ions occupy the 12-coordinated A sites forming regular icosahedra, and exhibits metallic behavior. Bond-valencesum calculations and X-ray photoemission spectroscopy reveal that Ag ions are present in the mixed-valence state, most likely attributable to the coexistence of Ag+ and Ag3+, unlike the case of well-known perovskite-type AgNbO3 and AgTaO3 containing only Ag+ ions. We discuss metallic conduction in relation to electronic structure calculations..
43. Hirofumi Akamatsu, Koji Fujita, Yuko Nakatsuka, Shunsuke Mural, Katsuhisa Tanaka, Magneto-optical properties of Eu2+-containing aluminoborosilicate glasses with ferromagnetic interactions, OPTICAL MATERIALS, 10.1016/j.optmat.2012.10.029, 35, 11, 1997-2000, 2013.09, Magnetic, optical, and magneto-optical properties have been examined for aluminoborosilicate glasses with high concentration of divalent europium ions. An analysis on temperature dependence of magnetic susceptibility yields positive Weiss temperatures, indicating that magnetic moments of 4f electrons of Eu2+ interact ferromagnetically with each other in the glasses. The glasses show large Faraday rotation angles in the visible range; glass with a composition of 58.0EuO center dot 12.0Al(2)O(3)center dot 20.0B(2)O(3)center dot 10.0SiO(2) (mol%) has a Verdet constant of -1.03 min/Oe cm (-300 rad/T m) at 633 nm. Wavelength dependence of the Verdet constant is interpreted in terms of Eu2+ 4f(7) to 4f(6)5d(1) electronic transitions based on the Van Vleck-Hebb theory. (C) 2012 Elsevier B.V. All rights reserved:.
44. Takahiro Kawamoto, Koji Fujita, Hirofumi Akamatsu, Tetsuya Nakamura, Toyohiko Kinoshita, Masaichiro Mizumaki, Naomi Kawamura, Motohiro Suzuki, Yoshiro Kususe, Shunsuke Murai, Katsuhisa Tanaka, Ferromagnetic amorphous oxides in the EuO-TiO2 system studied by the Faraday effect in the visible region and the x-ray magnetic circular dichroism at the Eu M-4,M-5 and L-2,L-3 edges, PHYSICAL REVIEW B, 10.1103/PhysRevB.88.024405, 88, 2, 2013.07, Amorphous Eu2TiO4 and EuTiO3 have been studied by a combination of the Faraday effect in the visible region and polarization-dependent x-ray absorption spectroscopy at the Eu M-4,M-5 and L-2,L-3 edges to examine the role of Eu 4f-5d exchange interactions on the ferromagnetic behavior. The bulk-sensitive x-ray absorption spectra (XAS) for Eu L-2,L-3 edges show that most of the europium ions are present as the divalent state in the amorphous Eu2TiO4 and EuTiO3. The Eu M-4,M-5 edge x-ray magnetic circular dichroism (XMCD) signals, measured for the amorphous Eu2TiO4, dramatically increase upon cooling through the Curie temperature (16 K) determined by a superconducting quantum interference device (SQUID) magnetometer. Sum-rule analysis of the XMCD at Eu M-4,M-5 edges measured at 10 K yields a 4f spin magnetic moment of 6.6 mu(B) per Eu2+ ion. These results confirm that the ferromagnetic properties exclusively arise from 4f spins of Eu2+. In addition, for both the amorphous Eu2TiO4 and EuTiO3, the temperature and magnetic-field dependence of Eu L-2,L-3 edge XMCD signals can be scaled with the corresponding magnetization measured by SQUID, indicating that the 5d magnetic polarization of Eu2+ is involved in the process to cause the ferromagnetic interaction between Eu2+ ions. We further discuss the origin of ferromagnetism in the amorphous system on the basis of the energy diagram of Eu 4f and 5d levels deduced from the Faraday effect in the visible region. From the wavelength dependence of Faraday rotation angles of the amorphous EuO-TiO2 system in comparison with those of the divalent Eu chalcogenides as reported previously, it is found that the magnitude of crystal-field splitting of Eu 5d levels in the former is on the same order as that in the latter, which explains an enhanced ferromagnetic exchange interaction between Eu 4f and 5d states..
45. Katsuhisa Tanaka, Koji Fujita, Yuya Maruyama, Yoshiro Kususe, Hideo Murakami, Hirofumi Akamatsu, Yanhua Zong, Shunsuke Murai, Ferromagnetism induced by lattice volume expansion and amorphization in EuTiO3 thin films, JOURNAL OF MATERIALS RESEARCH, 10.1557/jmr.2013.60, 28, 8, 1031-1041, 2013.04, Lattice volume expansion or amorphization renders EuTiO3 ferromagnetic, although the stable phase of crystalline EuTiO3 is an antiferromagnet. The lattice volume expansion is induced into the crystalline EuTiO3 thin film by utilizing the lattice mismatch between the thin film and a substrate. The magnetization at low temperatures monotonically increases with an increase in lattice volume for the crystalline EuTiO3 thin film, coincident with the results of calculations based on the hybrid Hartree-Fock density functional approach. The ferromagnetic interaction between Eu2+ ions is enhanced by the amorphization as well; the amorphous EuTiO3 thin film becomes a ferromagnet, and the Curie temperature is higher for amorphous Eu2TiO4 than for its crystalline counterpart. The phenomenon, that is, the volume expansion-and amophization-induced ferromagnetism, is explained in terms of the competition between ferromagnetic and antiferromagnetic interactions among Eu2+ ions..
46. Hirofumi Akamatsu, Yu Kumagai, Fumiyasu Oba, Koji Fujita, Katsuhisa Tanaka, Isao Tanaka, Strong Spin-Lattice Coupling Through Oxygen Octahedral Rotation in Divalent Europium Perovskites, ADVANCED FUNCTIONAL MATERIALS, 10.1002/adfm.201202477, 23, 15, 1864-1872, 2013.04, First-principles calculations reveal that in divalent europium perovskites EuMO3 (M = Ti, Zr, and Hf), antiferromagnetic superexchange interactions via nd states of the B-site M cations (n = 3, 4, and 5, respectively) are enhanced by rotations of the MO6 octahedra. The octahedral rotations involved in a structural change from cubic $ Pm{ar 3}m $ to orthorhombic Pbnm structures not only reduce energy gaps between the Eu 4f and M nd bands but also point the M nd orbitals at the Eu sites, leading to a significant overlap between the M nd and Eu 4f orbitals. These results reveal that the octahedral rotations are indispensable for antiferromagnetic ordering observed for EuZrO3 and EuHfO3, and put these perovskites into a class of materials exhibiting a novel type of strong coupling between their magnetism and octahedral rotations..
47. A. Winterstein, Hirofumi Akamatsu, D. Möncke, K. Tanaka, M. A. Schmidt, L. Wondraczek, Magnetic and magneto-optical quenching in (Mn2+, Sr2+) metaphosphate glasses, Optical Materials Express, 10.1364/OME.3.000184, 3, 2, 184-193, 2013.02, Transition metal ions such as Mn2+, Fe2+, or Co2+ provide an interesting alternative to rare earth dopants in optically active glasses. In terms of their magneto-optical properties, they are not yet very well exploited. Here, we report on the effect of Mn2+ on Faraday rotation in a metaphosphate glass matrix along the join MnxSr1-x(PO3)2 with x = 0...1. Mn2+ shows small optical extinction in the visible spectral range and, compared to other transition metal ions, a high effective magnetic moment. At high Mn- levels, however, the magneto-optical activity of Mn2+ is strongly quenched due to ionic clustering. The magnetic properties of the heavily Mn2+-loaded phosphate matrix are dominated by a superexchange interaction in the Mn2+-O-Mn2+ bridge with antiparallel spin alignment between Mn2+ and O2- species. The apparent paramagnetic potential of Mn2+ species can therefore not be exploited at room temperature..
48. Katsuhisa Tanaka, Koji Fujita, Yuya Maruyama, Yoshiro Kususe, Hideo Murakami, Hirofumi Akamatsu, Shunsuke Murai, Effect of substrate strain and interface on magnetic properties of EuTiO3 thin film, 2012 MRS Spring Meeting Nanocomposites, Nanostructures and Heterostructures of Correlated Oxide Systems, 10.1557/opl.2012.962, 1454, 149-159, 2012.12, Bulk EuTiO3 is known as a compound in which spin and soft phonon mode is strongly coupled. Recent theoretical study suggests that application of stress or formation of strain leads to a drastic change in magnetic and dielectric properties of EuTiO3 and that so-called multiferroic properties emerge under such a situation. In the present study, effect of strain induced by a substrate, on which EuTiO3 thin film is deposited, on the magnetic properties of the film has been experimentally examined. By using a pulsed laser deposition method, EuTiO3 thin film has been deposited on different kinds of substrate, i.e., LaAlO3, SrTiO3, and DyScO3; the lattice parameter of these compounds is smaller than, just the same as, and larger than that of EuTiO3, respectively. X-ray diffraction analysis confirms that the strain induced in the plane of as-deposited EuTiO3 thin films on different substrates is coincident with the lattice parameter of the substrate compounds. Also, all the as-deposited EuTiO3 thin films manifest elongation of lattice in a direction perpendicular to the film surface. Temperature dependence of magnetization indicates that all the thin films exhibit ferromagnetic behavior at low temperatures. The magnetization at 2 K under a magnetic field of 100 Oe is the highest for EuTiO3 on DyScO3 and the lowest for EuTiO3 on LaAlO3. The experimental result is coincident with the first-principles calculations which predict that ferromagnetic spin configuration becomes more stable as the lattice volume of EuTiO3 is increased..
49. Hirofumi Akamatsu, Koji Fujita, Hiroyuki Hayashi, Takahiro Kawamoto, Yu Kumagai, Yanhua Zong, Koji Iwata, Fumiyasu Oba, Isao Tanaka, Katsuhisa Tanaka, Crystal and Electronic Structure and Magnetic Properties of Divalent Europium Perovskite Oxides EuMO3 (M = Ti, Zr, and Hf): Experimental and First-Principles Approaches, INORGANIC CHEMISTRY, 10.1021/ic2024567, 51, 8, 4560-4567, 2012.04, A comparative study of the crystal and electronic structure and magnetism of divalent europium perovskite oxides EuMO3 (M = Ti, Zr, and Hf) has been performed on the basis of both experimental and theoretical approaches playing complementary roles. The compounds were synthesized via solid-state. reactions. EuZrO3 and EuHfO3 have an orthorhombic structure with a space group Pbnm at room temperature contrary to EuTiO3, which is cubic at room temperature. The optical band gaps of EuZrO3 and EuHfO3 are found to be about 2.4 and 2.7 eV, respectively, much larger than that of EuTiO3 (0.8 eV). On the other hand, the present compounds exhibit similar magnetic properties characterized by paramagnetic-antiferromagnetic transitions at around 5 K, spin flop at moderate magnetic fields lower than 1 T, and the antiferromagnetic nearest-neighbor and ferromagnetic next-nearest-neighbor exchange interactions. First-principles calculations based on a hybrid Hartree-Fock density functional approach yield lattice constants, band gaps, and magnetic interactions in good agreement with those obtained experimentally. The band gap excitations are assigned to electronic transitions from the Eu 4f to M nd states for EuMO3 (M = Ti, Zr, and Hf and n = 3, 4, and 5, respectively)..
50. Yanhua Zong, Koji Fujita, Hirofumi Akamatsu, Seisuke Nakashima, Shunsuke Murai, Katsuhisa Tanaka, Local Structure of Amorphous EuO-TiO2 Thin Films Probed by X-Ray Absorption Fine Structure, JOURNAL OF THE AMERICAN CERAMIC SOCIETY, 10.1111/j.1551-2916.2011.04849.x, 95, 2, 716-720, 2012.02, We have investigated the local structure of amorphous oxides in EuO-TiO2 system to understand the mechanism that gives rise to curious ferromagnetic properties recently observed in the system. X-ray absorption spectroscopy has been performed on amorphous EuTiO3 and Eu2TiO4 thin films prepared by the pulsed-laser-deposition method. The Ti K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data analyses reveal that the coordination number of Ti4+ in the amorphous thin films is around 4, which is in sharp contrast to their crystalline counterparts, where Ti4+ ions are octahedrally coordinated by oxide ions. It is therefore inferred that Ti4+ acts as a network-forming cation in the random network structure of the amorphous oxides. From the Eu L-3-edge EXAFS data analyses, it is found that the coordination number of Eu2+ is much lower and the nearest Eu-O bond length is shorter in the amorphous thin films than in the crystalline counterparts. The amorphization-induced enhancement of ferromagnetic interactions observed in the EuO-TiO2 system is discussed in terms of the local environment around Eu2+ ions..
51. Yanhua Zong, Koji Fujita, Hirofumi Akamatsu, Shunsuke Murai, Katsuhisa Tanaka, Ferromagnetic properties with reentrant spin-glass behavior in amorphous EuZrO3 thin film, Physica Status Solidi (C) Current Topics in Solid State Physics, 10.1002/pssc.201000703, 8, 11-12, 3051-3054, 2011.11, We report on the preparation, structure, and magnetic properties of amorphous EuZrO3, an amorphous ferromagnetic oxide. The samples were prepared in the form of thin film through a pulsed-laser-deposition method and their structure was confirmed to be amorphous by X-ray diffraction and high-resolution transmission electron microscope observation. Amorphous EuZrO3 thin films exhibit a ferromagnetic transition and reenter into a spin glass phase at a lower temperature, while crystalline EuZrO3 is an antiferromagnet as reported previously. The difference in magnetic properties between amorphous and crystalline EuZrO3 is suggested to arise from the difference in their structures..
52. Hirofumi Akamatsu, Jun Kawabata, Koji Fujita, Shunsuke Murai, Katsuhisa Tanaka, Magnetic properties of oxide glasses containing iron and rare-earth ions, PHYSICAL REVIEW B, 10.1103/PhysRevB.84.144408, 84, 14, 2011.10, Measurements of fundamental magnetic properties including not only dc and ac susceptibilities but also magnetic aging effects have been performed for aluminoborate glasses with high concentrations of iron and rare-earth R3+ ions (R = Sm, Gd, and Tb) in order to give an insight into the magnetic structures and interactions in amorphous oxides containing both 3d transition metal and 4f rare-earth ions, which manifest magnetic interactions that differ from each other. We demonstrate that the antiferromagnetic interactions between iron and rare-earth ions as well as those between iron ions play a significant role for their magnetic properties, while those between rare-earth ions are of little importance. Most of the rare-earth ions remain paramagnetic even below the spin-freezing temperatures under the strong molecular field caused by the spin-glass freezing of the iron ions, as in the case of rare-earth garnet ferrites..
53. Hirofumi Akamatsu, Yu Kumagai, Fumiyasu Oba, Koji Fujita, Hideo Murakami, Katsuhisa Tanaka, Isao Tanaka, Antiferromagnetic superexchange via 3d states of titanium in EuTiO3 as seen from hybrid Hartree-Fock density functional calculations, PHYSICAL REVIEW B, 10.1103/PhysRevB.83.214421, 83, 21, 2011.06, A superexchange mechanism between Eu2+ 4f spins via the 3d states of nonmagnetic Ti4+ ions is proposed through first-principles calculations based on a hybrid Hartree-Fock density functional approach to explain G-type antiferromagnetism in EuTiO3. This mechanism is supported by systematic calculations for related Eu2+-based perovskite oxides. In EuTiO3, the competition between the antiferromagnetic superexchange and an indirect ferromagnetic exchange via the Eu 5d states leads to a delicate balance between antiferromagnetic and ferromagnetic phases. The superexchange mechanism involving the Ti 3d states hints at the microscopic origin of the strong spin-lattice coupling in EuTiO3..
54. Hirofumi Akamatsu, Koji Fujita, Yanhua Zong, Naohiro Takemoto, Shunsuke Murai, Katsuhisa Tanaka, Impact of amorphization on the magnetic properties of EuO-TiO2 system, Physical Review B - Condensed Matter and Materials Physics, 10.1103/PhysRevB.82.224403, 82, 22, 2010.12, Recently, we have reported amorphization-induced ferromagnetism in a magnetic oxide
whereas crystalline EuTiO3 is an antiferromagnet, its amorphous counterpart shows a ferromagnetic transition. In this paper, we have investigated the magnetic properties of amorphous EuO-TiO2 system in detail. The present compounds, i.e., amorphous EuTiO3 and Eu 2 TiO4, exhibit a paramagnetic-ferromagnetic transition followed by a spin-glass transition upon cooling down, thereby being characterized as reentrant ferromagnets. Interestingly, the magnetic ordering temperatures of amorphous EuTiO3 and Eu2 TiO4 are comparable to and higher than those of their crystalline counterparts, respectively, indicating the enhancement of ferromagnetic interactions by amorphization. The origin of these features is discussed in terms of both the essential structural difference between amorphous and crystalline oxides and the magnetism unique to the crystalline EuO-TiO2 compounds. We also find a correlation between the magnitude of ferromagnetic interactions and the covalency of Eu-O bonds. © 2010 The American Physical Society..
55. Hirofumi Akamatsu, Koji Fujita, Yanhua Zong, Naohiro Takemoto, Shunsuke Murai, Katsuhisa Tanaka, Impact of amorphization on the magnetic properties of EuO-TiO2 system, PHYSICAL REVIEW B, 10.1103/PhysRevB.82.224403, 82, 22, 2010.12, Recently, we have reported amorphization-induced ferromagnetism in a magnetic oxide; whereas crystalline EuTiO3 is an antiferromagnet, its amorphous counterpart shows a ferromagnetic transition. In this paper, we have investigated the magnetic properties of amorphous EuO-TiO2 system in detail. The present compounds, i.e., amorphous EuTiO3 and Eu2TiO4, exhibit a paramagnetic-ferromagnetic transition followed by a spin-glass transition upon cooling down, thereby being characterized as reentrant ferromagnets. Interestingly, the magnetic ordering temperatures of amorphous EuTiO3 and Eu2TiO4 are comparable to and higher than those of their crystalline counterparts, respectively, indicating the enhancement of ferromagnetic interactions by amorphization. The origin of these features is discussed in terms of both the essential structural difference between amorphous and crystalline oxides and the magnetism unique to the crystalline EuO-TiO2 compounds. We also find a correlation between the magnitude of ferromagnetic interactions and the covalency of Eu-O bonds..
56. Yanhua Zong, Kazuma Kugimiya, Koji Fujita, Hirofumi Akamatsu, Kazuyuki Hirao, Katsuhisa Tanaka, Preparation and magnetic properties of amorphous EuTiO3 thin films, JOURNAL OF NON-CRYSTALLINE SOLIDS, 10.1016/j.jnoncrysol.2010.05.014, 356, 44-49, 2389-2392, 2010.10, Amorphous EuTiO3 thin films have been prepared by a pulsed laser deposition method, and their structural and magnetic properties have been investigated. High-resolution transmission electron microscope image and selected-area electron diffraction pattern as well as X-ray diffraction pattern confirm the amorphous nature of the thin films. Eu-151 conversion-electron Mossbauer effect measurements show that almost all of the europium ions are present as Eu2+ in the thin films. The amorphous EuTiO3 thin film exhibits a positive Weiss temperature of 8.7 K, indicating predominant ferromagnetic interactions among Eu2+ ions. The signature of a ferromagnetic-like transition is observed at around 5.5 K. (C) 2010 Elsevier B.V. All rights reserved..
57. Yanhua Zong, Koji Fujita, Hirofumi Akamatsu, Shunsuke Murai, Katsuhisa Tanaka, Antiferromagnetism of perovskite EuZrO3, JOURNAL OF SOLID STATE CHEMISTRY, 10.1016/j.jssc.2009.10.014, 183, 1, 168-172, 2010.01, Polycrystalline EuZrO3 has been synthesized by the solid-state reaction between EuO and ZrO2, and its structural and magnetic properties have been investigated. Rietveld analysis of the X-ray diffraction pattern indicates that EuZrO3 crystallizes in an orthorhombic perovskite structure. Eu-151 Mossbauer effect measurement reveals that almost all the europium ions are present as the divalent state and Occupy distorted sites with non-axial electric field gradients, in agreement with the orthorhombic structure. In contrast to previous reports, an antiferromagnetic transition was observed around 4.1 K. The magnetic structure below the Neel temperature has been discussed. (C) 2009 Elsevier Inc. All rights reserved..
58. Hirofumi Akamatsu, Koji Fujita, Shunsuke Murai, Katsuhisa Tanaka, Ferromagnetic Eu2+-based oxide glasses with reentrant spin glass behavior, PHYSICAL REVIEW B, 10.1103/PhysRevB.81.014423, 81, 1, 2010.01, In most amorphous insulating magnets, the magnetic structure is dominated by the random distribution of magnetic moments as well as the predominant antiferromagnetic interaction among them, inevitably leading to a transition from high-temperature paramagnetic to low-temperature spin glass phase. In this paper, we report our discovery of ferromagnetic amorphous oxides with reentrant spin glass behavior. Unlike most oxide glasses, there is a strong tendency for the magnetic interaction of Eu2+ ions to be ferromagnetic in oxide glasses, as obviously indicated by the positive values of Weiss temperature. Comprehensive investigations of low-temperature magnetic properties for the present Eu2+-containing glasses have revealed a typical behavior of reentrant ferromagnets. We discuss the possible mechanisms behind the ferromagnetic interactions, as well as the origin of reentrant spin glass nature, based on the specific electronic structure of Eu2+ compounds..
59. Hirofumi Akamatsu, Satoshi Oku, Koji Fujita, Shunsuke Murai, Katsuhisa Tanaka, Magnetic properties of mixed-valence iron phosphate glasses, PHYSICAL REVIEW B, 10.1103/PhysRevB.80.134408, 80, 13, 2009.10, Magnetic properties of mixed-valence iron phosphate glasses, where there coexist Fe2+ and Fe3+ ions, have been investigated. The molar fraction of Fe3+ with respect to the total iron ion, [Fe3+]/[Fe-total], can be controlled by melting the glass at varied temperatures. Experiments of magnetic aging and memory effects as well as dynamic and static scaling analyses of relaxation time and nonlinear magnetic susceptibility have been performed to get insight into the nature of low-temperature magnetic phase of the glass system. The experimental results reveal that the iron phosphate glasses undergo paramagnet-spin-glass transitions at low temperatures. Temperature dependence of magnetic specific heat suggests that as the temperature is lowered, the magnetic moments start to be frozen at a temperature significantly higher than the spin-glass transition temperature accompanied by a deviation in magnetic susceptibility from Curie-Weiss law. The ratio of the absolute value of Weiss temperature to spin-glass transition temperature increases as the ratio [Fe3+]/[Fe-total] becomes larger. This behavior is explainable in terms of the difference in single-ion anisotropy between Fe3+ and Fe2+ ions..
60. Yasuaki Tokudome, Kazuki Nakanishi, Kazuyoshi Kanamori, Koji Fujita, Hirofumi Akamatsu, Teiichi Hanada, Structural characterization of hierarchically porous alumina aerogel and xerogel monoliths, JOURNAL OF COLLOID AND INTERFACE SCIENCE, 10.1016/j.jcis.2009.06.042, 338, 2, 506-513, 2009.10, Detailed nanostructures have been investigated for hierarchically porous alumina aerogels and xerogels prepared from ionic precursors via sol-gel reaction. Starting from AlCl3 center dot 6H(2)O and poly(ethylene oxide) (PEO) dissolved in a H2O/EtOH mixed solvent, monolithic wet gels were synthesized using propylene oxide (PO) as a gelation initiator. Hierarchically porous alumina xerogels and aerogels were obtained after evaporative drying and supercritical drying, respectively. Macroporous structures are formed as a result of phase separation, while interstices between the secondary particles in the micrometer-sized gel skeletons work as mesoporous structures. Alumina xerogels exhibit considerable shrinkage during the evaporative drying process, resulting in relatively small mesopores (from 5.4 to 6.2 nm) regardless of the starting composition. For shrinkage-free alumina aerogels, on the other hand, the median mesopore size changes from 13.9 to 33.1 nm depending on the starting composition; the increases in PEO content and H2O/EtOH volume ratio both contribute to producing smaller mesopores. Small-angle X-ray scattering (SAXS) analysis reveals that variation of median mesopore size can be ascribed to the change in agglomeration state of primary particles. As PEO content and H2O/EtOH ratio increase, secondary particles become small, which results in relatively small mesopores. The results indicate that the agglomeration state of alumina primary particles is influenced by the presence of weakly interacting phase separation inducers such as PEO. (C) 2009 Elsevier Inc. All rights reserved..
61. Hirofumi Akamatsu, Yanhua Zong, Yosefu Fujiki, Kazuaki Kamiya, Koji Fujita, Shunsuke Murai, Katsuhisa Tanaka, Structural and Magnetic Properties of CdFe2O4 Thin Films Fabricated via Sputtering Method, IEEE TRANSACTIONS ON MAGNETICS, 10.1109/TMAG.2008.2001508, 44, 11, 2796-2799, 2008.11, CdFe2O4 thin films have been prepared by a sputtering method, and their structural and magnetic properties have been examined. As-deposited film shows magnetic transition like that of cluster-spin glass or superspin glass, and possesses large magnetization even at room temperature. The magnetic behaviors are partly attributed to crystal structure with a random distribution of cations in CdFe2O4 nanocrystals. Annealing of the as-deposited film at 200 degrees C leads to an increase in magnetization while the magnetization is decreased when the film is annealed at 400 degrees C. The effect of annealing on the magnetic properties has been discussed in terms of the change of microstructure as well as chemical structure of the thin films..
62. Hirofumi Akamatsu, Koji Fujita, Shunsuke Murai, Katsuhisa Tanaka, Magneto-optical properties of transparent divalent iron phosphate glasses, APPLIED PHYSICS LETTERS, 10.1063/1.2952460, 92, 25, 2008.06, We have prepared glasses having xFeO center dot(100-x)P2O5 (mol %) (x=50.0,54.0,57.1) compositions by melting under mild reducing condition and found that these glasses exhibit fairly high transmittance in the visible range and large Faraday effect at the wavelength of about 400 nm. Fe-57 Mossbauer spectra confirm that almost all the iron ions are present as Fe2+ in the glasses. A spin glass transition is observed at low temperatures in the temperature dependence of magnetic susceptibility. Intense optical absorption in the ultraviolet and infrared wavelength ranges occurs by the charge transfer transition from O2- to Fe2+ and the intra-atomic d-d transition, respectively. The analysis on the wavelength dependence of the Faraday rotation angle using the Van Vleck-Hebb theory has revealed that the charge transfer transition contributes more significantly to the Faraday effect owing to the large effective transition probability, which is comparable to those reported for glasses containing 4f rare-earth ions. The magneto-optical figure of merit shows a maximum at around 380 nm. (c) 2008 American Institute of Physics..
63. Hirofumi Akamatsu, Katsuhisa Tanaka, Koji Fujita, Shunsuke Murai, Magnetic phase transitions in Fe2O3-Bi2O3-B2O3 glasses, JOURNAL OF PHYSICS-CONDENSED MATTER, 10.1088/0953-8984/20/23/235216, 20, 23, 2008.06, Magnetic and structural properties of iron- containing bismuth borate glasses, whose composition is denoted as xFe(2)O(3) center dot( 80.0 - x)Bi2O3 center dot 20.0B(2)O(3), in mol% ( 18.2 <= x <= 40.0), have been explored. The glasses manifest intriguing magnetic behaviors explainable in terms of the coexistence of a spin glass phase and magnetic clusters. The x = 18.2 glass shows a spin glass transition at 3.5 K, while the contribution of magnetic clusters to the magnetic properties becomes more significant as the content of Fe2O3, x, is increased. We have performed detailed experiments on the x = 32.0 glass for which two different magnetic transitions are observed, as demonstrated by the temperature and frequency of the ac magnetic field dependences of the dc and ac susceptibilities. It is revealed from the measurements of magnetic ageing and memory effects that the magnetic clusters are frozen to form a superspin glass- like state with strong inter- cluster interactions at low temperatures. Transmission electron microscopy clarifies that the magnetic clusters are ascribable not to nanocrystals but to some phases possessing amorphous structures. Also, we have found from the observation of exchange bias effects that an interplay of the clusters with the spin glass phase brings about an exchange anisotropy field after cooling in the presence of magnetic field..
64. Katsuhisa Tanaka, Hirofumi Akamatsu, Seisuke Nakashima, Koji Fujita, Magnetic properties of disordered oxides with iron and manganese ions, JOURNAL OF NON-CRYSTALLINE SOLIDS, 10.1016/j.jnoncrysol.2007.01.107, 354, 12-13, 1347-1352, 2008.02, Magnetic properties of disordered oxides involving oxide glasses have been investigated. Spin glass-like transition is observed for Fe2O3-TeO2 and MnO-TeO2 glasses in temperature dependence of dc susceptibility, although magnetic transition does not take place for MnO-TeO2 glasses with low concentration of manganese ion above 2 K at least. The mechanism of magnetic transition observed for Fe2O3-TeO2 glasses is discussed on the basis of dc susceptibility obtained under conditions of field cooling and zero field cooling, magnetic field dependence of magnetization at low temperatures, and frequency dependence of spin-freezing temperature derived from temperature dependence of ac susceptibility. Magnetic properties of disordered ZnFe2O4 thin film prepared by a radio frequency sputtering method have been also studied. The disordered ZnFe2O4 thin film exhibits ferrimagnetic behavior with high magnetization even at room temperature. At the same time, the thin film shows spin-freezing at around 320 K. Temperature dependence of nonlinear susceptibility leads to a conclusion that this transition is explainable in terms of superparamagnetism with magnetic interaction among clusters. (c) 2007 Elsevier B.V. All rights reserved..
65. Hirofumi Akamatsu, Shunsuke Murai, Koji Fujita, Katsuhisa Tanaka, Magnetic properties of amorphous Fe2O3-R 2O3 (R = La, Gd and Tb) thin films fabricated by sputtering method, Glass - The Challenge for the 21st Century - 9th ESG Conference with the Annual Meeting of the ICG Glass - The Challenge for the 21st Century - 9th ESG Conference with the Annual Meeting of the ICG, 39-40, 207-212, 2008, Amorphous oxide thin films of Fe2O3-R 2O3 (R = La, Gd and Tb) systems have been deposited on silica glass substrates by using a radio frequency sputtering method, and magnetic properties of the thin films have been examined. The Fe 2O3-La2O3 thin films exhibit cusp-like maxima of dc magnetic susceptibility in their temperature dependences as well as magnetic aging and memory effects characteristic of typical spin glasses. For Fe2O3-Gd2O3 and Fe 2O3-Tb2O3 systems, magnetic moments of iron ions take part in formation of a spin glass state, as indicated by the magnetic aging effects, while those of rare-earth ions remain to be in a paramagnetic state even at very low temperatures..
66. Hirofumi Akamatsu, Katsuhisa Tanaka, Koji Fujita, Shunsuke Murai, Spin dynamics in oxide glass of Fe2O3-Bi2O3-B2O3 system, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 10.1016/j.jmmm.2006.10.649, 310, 2, 1506-1507, 2007.03, We have investigated magnetic aging and memory effects in the magnetically ordered phase of 32Fe2O(3)(center dot)48Bi(2)O(3)center dot 20B(2)O(3) (mol%) glass, which exhibits a spin glass-like transition at about 15 K (T,,) and a superparamagnetic blocking at about 28 K (T-sp). The present glass manifests magnetic aging effects similar to those observed in spin glasses in the temperature regimes of not only T < T-seg but also T-sg < T < T-sp, indicating that superparamagnetic clusters existing in the glass strongly interact with each other. The interactions among the magnetic clusters lead to cooperative spin dynamics as observed in superspin glasses. (c) 2006 Elsevier B.V. All rights reserved..
67. Hirofumi Akamatsu, Katsuhisa Tanaka, Koji Fujita, Shunsuke Murai, Spin dynamics in Fe2O3-TeO2 glass: Experimental evidence for an amorphous oxide spin glass, PHYSICAL REVIEW B, 10.1103/PhysRevB.74.012411, 74, 1, 2006.07, We have examined spin dynamics including magnetic aging and memory effects as well as critical slowing down for 20Fe(2)O(3)center dot 80TeO(2) (mol %) glass. Scaling analysis on critical slowing down reveals that the present glass exhibits a critical behavior as observed in a prototype of spin glasses. Aging and memory effects peculiar to spin glasses have been observed in the magnetically ordered phase of the present glass. These experimental results strongly confirm that the 20Fe(2)O(3)center dot 80TeO(2) glass is converted into a spin glass phase at a very low temperature. It is thought that a disordered structure of the oxide glass gives rise to randomness and frustration in the magnetic structure, leading to the spin glass phase transition..