||Hiroshi Kageyama, Katsuro Hayashi, Kazuhiko Maeda, J. Paul Attfield, Zenji Hiroi, James M. Rondinelli, Kenneth R. Poeppelmeier, Expanding frontiers in materials chemistry and physics with multiple anions, Nature Communications, 9, Article No. 772, 2018.02.
||Katsuro Hayashi, Peter V. Sushko, Yasuhiro Hashimoto, Alexander L. Shluger, Hideo Hosono, Hydride Ions in Oxide Hosts Hidden by Hydroxide Ions, Nature Communications, 6, Article No. 4515, 2014.03, The true oxidation state of formally 'H(-)' ions incorporated in an oxide host is frequently discussed in connection with chemical shifts of (1)H nuclear magnetic resonance spectroscopy, as they can exhibit values typically attributed to H(+). Here we systematically investigate the link between geometrical structure and chemical shift of H(-) ions in an oxide host, mayenite, with a combination of experimental and ab initio approaches, in an attempt to resolve this issue. We demonstrate that the electron density near the hydrogen nucleus in an OH(-) ion (formally H(+) state) exceeds that in an H(-) ion. This behaviour is the opposite to that expected from formal valences. We deduce a relationship between the chemical shift of H(-) and the distance from the H(-) ion to the coordinating electropositive cation. This relationship is pivotal for resolving H(-) species that are masked by various states of H(+) ions.
||Katsuro Hayashi, Kazunari Shima, Fumiaki Sugiyama, A mixed aqueous/aprotic sodium/air cell using a NASICON ceramic separator, Journal of the Electrochemical Society, 10.1149/2.067309jes, 160, 9, 2013.07, An aqueous (mixed aqueous/aprotic) Na-air cell is fabricated and its discharge properties are examined experimentally. This design is attractive because Na is not only more abundant and cheaper than Li, but also potentially advantageous for enhancing cell output. The theoretical energy density of the aqueous Na-air cell exceeds that of a Li one under conditions where precipitation of hydroxides in the aqueous electrolyte is not allowed. Na 3Zr2Si2PO12 (NASICON) ceramics, which has a Na+ ion conductivity of 2 × 10-3 S · cm-1 at 50°C, is used as a solid electrolyte separator in the Na-air cell. Involvement of O2 gas in the cell reaction is verified by changing cell voltage in the presence or absence of O2 flow through the aqueous electrolyte. The Na-air cell exhibits an electrochemical discharge of ∼600 mAh· g-1 and energy density of ∼1500 Wh· kg-1 based on the weights of reacted Na and H2O. Its maximum areal output power is 11 mW· cm -2 and depends on the thickness of the NASICON ceramic separator, the concentration of NaOH in the aqueous electrolyte and the type of cathode, suggesting that cell performance can be improved by optimization of these components..
||Katsuro Hayashi, Peter V. Sushko, David Muñoz Ramo, Alexander L. Shluger, Satoshi Watauchi, Isao Tanaka, Satoru Matsuishi, Masahiro Hirano, Hideo Hosono, Nanoporous crystal 12CaO·Al2O3
A playground for studies of ultraviolet optical absorption of negative ions, Journal of Physical Chemistry B Materials, 10.1021/jp065793b, 111, 8, 1946-1956, 2007.03, A novel nanoporous material 12CaO·Al2O3 (C12A7) offers a possibility of incorporating large concentrations (>10 21 cm-3) of a wide range of extraframework anions inside its nanopores. We have investigated, both experimentally and theoretically, optical absorption associated with several types of such anions, including F-, OH-, O-, O2-, O2 2-, and O2 2-, and assigned their optical absorption bands. It is demonstrated that the chemical identity and concentration of extraframework anions can be controlled by an appropriate treatment of "as grown" C12A7. We also show that the position of the adsorption edge is, in turn, determined by the chemical identity of the extraframework species and can be varied in the range of ∼4-6 eV. We suggest that C12A7 is a unique host material, which can be used as a playground for studying negatively charged species that are unstable in other environments..
||Katsuro Hayashi, Peter V. Sushko, Alexander L. Shluger, Masahiro Hirano, Hideo Hosono, Hydride ion as a two-electron donor in a nanoporous crystalline semiconductor 12CaO·7Al2O3, Journal of Physical Chemistry B Materials, 10.1021/jp053990p, 109, 50, 23836-23842, 2005.12, The 12CaO·7Al203 (C12A7) crystal with a nanoporous lattice framework exhibits high electrical conductivity with an activation energy of ∼ 1.5 eV when equilibrated in a hydrogen atmosphere above ∼800°C. The high conductivity is preserved in a quenched state below ∼600°C with a reduced activation energy of ∼0.8 eV, Such complex behavior in electrical conductivity is associated with incorporation of hydride ions (H-) in cages of the lattice framework. Electromotive force measurements reveal that the major carrier for the conductivity is electron with a small contribution by proton (H+), ruling out the possibility of direct intercage migration of the H- ion. A combination of these observations with the ab initio calculations leads to the conclusion that the electrons are thermally generated from the H- ion by the dissociation into two electrons and an proton, which is further converted to an OH- ion via reaction with an extraframework oxide ion (02-). The energy difference between the initial (H- + O2-) and the final (2e- + OH-) states as evaluated by the theoretical calculation is as small as ∼1 eV, which agrees well with an experimentally obtained enthalpy change, ∼1.4 eV. Thus, internal equilibration between the extraframework hydrogen and the oxygen species is responsible for the thermal generation of the carrier electron. It is also suggested that the same conductive (2e- + OH-) state is reached by the photoirradiation of H--containing C12A7. In this case the photoionization of H- forms an electron and an H o atom, which then forms an OH- ion and another electron with thermal assistance. The persistence of photoinduced conductivity is explained by the slow kinetics of the reverse process at room temperature..
||Katsuro Hayashi, Masahiro Hirano, Hideo Hosono, Thermodynamics and kinetics of hydroxide ion formation in 12CaO·7Al 4O 3, Journal of Physical Chemistry B Materials, 10.1021/jp050807j, 109, 24, 11900-11906, 2005.06, We have examined the thermodynamics and kinetics of hydroxide (OH) ions that formed in cages of 12CaO·7Al 2O 3 (C12A7) with nanoporous structures. It is confirmed using thermogravimetric-evolved gas analyses (TG-EGA) that hydration in C12A7 is mediated by a reaction between an oxide (O 2-) ion in the cage and an H 2O molecule in the atmosphere to form two OH - ions in the cages. To simply and exactly quantify the OH - content from infrared absorption measurements of OH-stretching band, we propose a method combined with a thermodynamic analysis, allowing the simultaneous determination of the molar extinction coefficient of the OH-band, enthalpy, and entropy for the hydration. Hydration enthalpy in C12A7 is extremely high compared with other oxides and was enhanced by the marked instability of O 2- ion in the cage. Consequently, high solubility of OH - ion is retained up to unusually high temperatures. Furthermore, we determined diffusion coefficients of species relevant to the hydration process and demonstrated that inward diffusion of OH - ions is the rate-determining process..
||Satoru Matsuishi, Yoshitake Toda, Masashi Miyakawa, Katsuro Hayashi, Toshio Kamiya, Masahiro Hirano, Isao Tanaka, Hideo Hosono, High-density electron anions in a nanoporous single crystal
[Ca24Al28O64]4+ (4e-), Science, 10.1126/science.1083842, 301, 5633, 626-629, 2003.08, We removed ∼100% of clathrated oxygen ions from the crystallographic cages in a single crystal of 12CaO·7Al2O3, leading to the formation of high-density (∼2 x 1021 cm-3) electrons highly localized in the cages. The resulting electron forms a structure that we interpret as an F+ center and migrates throughout the crystal by hopping to a neighboring cage with conductivity ∼100 siemens per centimeter, demonstrating that the encaged electron behaves as an anion. The electron anions couple antiferromagnetically with each other, forming a diamagnetic pair or singlet bipolaron. The resulting [Ca24Al28O64]4+(4e-) may be regarded as a thermally and chemically stable single crystalline "electride"..
||Katsuro Hayashi, Satoru Matsuishi, Naoto Ueda, Masahiro Hirano, Hideo Hosono, Maximum incorporation of oxygen radicals, O- and O2
-, into 12CaO·7Al2O3 with a nanoporous structure, Chemistry of Materials, 10.1021/cm020959g, 15, 9, 1851-1854, 2003.05, The effects of oxygen partial pressure (pO2) up to 400 atm on the generation of active oxygen radicals in 12CaO·7Al2O3 (C12A7) was examined using a hot isostatic pressing technique. Concentrations of superoxide (O2
-) and oxygen anion radicals (O-), which were analyzed from a combination of electron paramagnetic spin resonance (EPR) and Raman spectroscopy, increased with pO2. The total concentration of the radicals reached 1.7 × 1021 cm3, which is comparable to the theoretical maximum, confirmed that dominant radical incorporation process is O2- (cage) + O2 (atmosphere) → O- (cage) + O2
- (cage). The radical incorporated C12A7 has a large magnetic susceptibility that obeys Curie's law down to 2.5 K without magnetic ordering, although a weak exchange interaction was suggested by the line narrowing of the EPR signal..
||Katsuro Hayashi, Satoru Matsuishi, Toshio Kamiya, Masahiro Hirano, Hideo Hosono, Light-induced conversion of an insulating refractory oxide into a persistent electronic conductor, Nature, 10.1038/nature01053, 419, 6906, 462-465, 2002.10, Materials that are good electrical conductors are not in general optically transparent, yet a combination of high conductivity and transparency is desirable for many emerging opto-electronic applications. To this end, various transparent oxides composed of transition or post-transition metals (such as indium tin oxide) are rendered electrically conducting by ion doping. But such an approach does not work for the abundant transparent oxides of the main-group metals. Here we demonstrate a process by which the transparent insulating oxide 12CaO·7Al2O3 (refs 7-13) can be converted into an electrical conductor. H- ions are incorporated into the subnanometre-sized cages of the oxide by a thermal treatment in a hydrogen atmosphere; subsequent irradiation of the material with ultraviolet light results in a conductive state that persists after irradiation ceases. The photo-activated material exhibits moderate electrical conductivity (∼0.3 S cm-1) at room temperature, with visible light absorption losses of only one per cent for 200-nm-thick films. We suggest that this concept can be applied to other main-group metal oxides, for the direct optical writing of conducting wires in insulating transparent media and the formation of a high-density optical memory..
||Katsuro Hayashi, Masahiro Hirano, Satoru Matsuishi, Hideo Hosono, Microporous crystal 12CaO·7Al2O3 encaging abundant O- radicals, Journal of the American Chemical Society, 10.1021/ja016112n, 124, 5, 738-739, 2002.02, Extremely high concentrations (>1020 cm-3) of active oxygenic radicals, O- and O2
-, have been created in the zeolitic crystal, 12CaO·7Al2O3 (C12A7), which can accommodate anions in its cavities. An increase in oxygen pressure and a decrease in water vapor pressure at high temperature enhance the formation of the radicals. The oxidation of Pt is observed on the surface of the material as a result of reaction with the active oxygens..
||Katsuro Hayashi, Takahisa Yamamoto, Taketo Sakuma, Grain orientation dependence of the PTCR effect in niobium-doped barium titanate, Journal of the American Ceramic Society, 10.1111/j.1151-2916.1996.tb08780.x, 79, 6, 1669-1672, 1996.04, The positive temperature coefficient of resistivity (PTCR) effect is directly measured in single grain boundaries in 0.1-mol%-Nb-doped BaTiO3 with 1 mm coarse grains. The PTCR effect largely depends on grain boundary structure. Random grain boundaries exhibit the PTCR effect as in polycrystalline samples, but the PTCR effect does not appear in highly coherent boundaries such as small-angle boundaries, twin boundaries, and coincidence site lattice (CSL) boundaries with low ∑ values. For ∑= 3 boundaries, the resistance increase above the Curie temperature is a function of deviation angle. A small PTCR effect is observed in ∑ = 3 boundaries with a deviation angle of about 9° in contrast with ideal ∑ = 3 boundaries and boundaries with a deviation of about 4°..