Motohiko Ishii

Crystal structures of V3S4 and V5S8

Abstract Crystal structures of the ordered phases of V3S4 and V5S8 were refined with single crystal data. Both are monoclinic. Chemical compositions, space groups and lattice constants are as follows: VS1.47, I2 m (No. 12), a = 5.831(1), b = 3.267(1), c = 11.317(2)A, β = 91.78(1)° and VS1.64, F 2 m (No. 12), a = 11.396(11), b = 6.645(7), c = 11.293(4), A, β = 91.45(6)°. In both structures, short metal-metal bonds were found between the layers as well as within them. In comparison with the structure of Fe7S8, the stability of NiAs-type structure was discussed based on the detailed metal-sulfur distances.

STARK SPLITTING SCHEME AND AN INDUCED EMISSION CROSS SECTION OF TBALO3

Detailed absorption and emission spectra on TbAlO3 are measured at room temperature and liquid-nitrogen temperature. By decomposition of the spectra, the Stark splitting scheme is established for 7F6, 7F5, and 5D4 multiplets of Tb3+ ions in TbAlO3 together with the peak energies in the Fourier transform infrared spectrum at liquid-nitrogen temperature. The multiplets are split into their maximum number of levels, which shows that the point symmetry is low for the Tb3+ ion site in TbAlO3. An induced emission cross section is estimated using the results of decomposition at liquid-nitrogen temperature to be 3.55×10−19 cm2. This value is comparable to the cross sections reported for Nd:Y3Al5O12, which is a well known solid-state laser material. The population inversion threshold is also estimated for TbAlO3 in comparison with Nd:Y3Al5O12. It is found that the threshold value for TbAlO3 is only about three times larger than that for Nd:Y3Al5O12. This TbAlO3 may have the potential to be a laser material with a ...

Structure and vibrational spectra of argyrodite family compounds Cu8SiX6 (X=S, Se) and Cu8GeS6

Abstract Infrared transmission and Raman spectra of the argyrodite family compounds Cu8SiX6 (X=S,Se) and Cu8GeS6 have been measured. Structure data of Cu8SiSe6 and Cu8GeS6 at 300 K have been obtained through Rietveld analysis. On the basis of these results, the frequencies and intensities of infrared or Raman active modes of these compounds have been calculated, and the relations between the vibrational spectra and the crystal structures have been studied.

Preparation, crystal structure and silver ionic conductivity of the new compound Ag8TiS6

Abstract The new compound Ag 8 TiS 6 has been prepared by solid state reaction. The crystal structure, phase transition and transport properties have been studied using X-ray diffraction, Raman spectra, ionic conductivity measurement and the e.m.f method. The dc conductivities of silver ion of Ag 8 TiS 8 reach values around 10 −3 Ω −1 cm −1 at ambient temperatures. The transference numbers of silver ion are found to be close to unity.

Behavior of vanadium dioxide single crystals synthesized under the various oxygen partial pressures at 1500 K

Abstract The effects of nonstoichiometry upon the behavior of vanadium dioxide single crystals in the vicinity of the semiconductor/metal transition temperature (Tc) were experimentally investigated. According to the electrical and thermal measurements, more stoichiometric vanadium dioxide exhibited the less electrical conductivity gap, the larger thermal and electrical hysteresis, and the lower transition temperature than the increased nonstoichiometric one near transition. Infrared absorptions and X-ray observations indicated the local and overall lattice distortion in the nonstoichiometric crystal due to the existence of V5+ ions. Furthermore, an intermediate phase between the low-temperature monoclinic and the high-temperature tetragonal phases was found in the nonstoichiometric VO2. On the other hand, no evidence for this intermediate phase was found in the stoichiometric one. Finally, some comparisons and discussions of our present data with the previously published ones were made.

Powder X-ray diffraction of low-temperature phases of argyrodite-family compound Ag7TaS6

Abstract Low-temperature X-ray diffraction studies revealed the existence of two low-temperature phases of an argyrodite-family compound Ag7TaS6 that contains mobile Ag ions at room temperature, the phase II (between about 280 and 170 K) and the phase III (between about 170 and 10 K). The powder pattern of the phase II could be indexed on the basis of an orthorhombic unit cell, A = 7.442, B = 7.403 and C = 10.531 A . A probable assignment on the basis of tetragonal superstructure model was proposed for the powder pattern of the phase III.

Raman spectroscopic studies of silver-tantalum sulfide, silver-niobium sulfide and silver-tantalum selenide with argyrodite structures

Abstract Raman spectra of the superionic conductors Ag 7 TaS 6 , Ag 7 NbS 6 and Ag 7 TaSe 6 , which crystallize in the argyrodite structures, have been studied in the temperature range from 300 to 80K for the first time. In these compounds, there exist the nearly isolated tetrahedral MX 4 (M=Ta, Nb; X=S, Se) units, and the peaks due to the M-X stretching modes of these units have been identified. As for Ag 7 NbS 6 , the observed Nb-S stretching peaks have shown the shifts or the splittings, which suggest the phase transitions, with decreasing temperature. In the low frequency region below 80 cm −1 , the peaks due to the vibration modes of the mobile silver ions have been observed at 80K, and the results have been discussed as compared with those of Ag 2 S and Ag 2 Se.

Raman studies of (Fe, V)3S4 solid solution

Abstract Raman scattering of FexV3-xS4 with 0⩽x⩽2 has been studied. The observed spectra of V3S4, one of the end members of this solid solution, have been assigned to the Raman active lattice modes based on the C2h3 space group symmetry. On the basis of the results obtained for V3S4, the nature of the metal-metal interactions and the site distributions of Fe atoms in (Fe, V)3S4 have been discussed.

Phase transition of copper selenide studied by powder x-ray diffractometry

Abstract The temperature dependence of the lattice parameters of CuSe up to 420 K showed that a phase transition from hexagonal to orthorhombic observed at about 398 K with decreasing temperature is second order and another transition from orthorhombic to hexagonal at 323 K is first order. The latter hexagonal phase has weak superlattice reflections in addition to main sublattice reflections. The intensities of weak superlattice reflections were increased by grinding the powder of CuSe at room temperature and they did not disappear over the temperature range of the orthorhombic phase only upon heating. These superlattice reflections of this new but metastable phase can be indexed based on a monoclinic lattice.

Phase transition and crystal structure of silver-ion conductor Ag12−nM+nS6 (M=Ti, Nb, Ta)

Abstract Phase transition of Ag-ion conductor of argyrodite sulfides, Ag 12− n M + n S 6 (M=Ti, Nb or Ta), has been investigated through thermal analyses and the low-temperature X-ray diffraction methods. It was found that all the room-temperature phase with the cubic F 43 m structure transforms into the several less symmetric forms at lower temperatures than 293 K. In Ag 8 TiS 6 , the low temperature modification of type β crystallizes in the orthorhombic symmetry with a =15.095 A, b =7.462 A, c =10.641 A, space group Pna 2 1 (No. 33), Z =4. The structure was determined from the Rietveld refinement of powder X-ray diffraction data and the final R factors were R wp =0.13 and R F =0.048. The structure of Ag 8 TiS 6 contains four tetrahedral TiS 4 units and Ag and S atoms. The coordination of the Ag atoms by sulfur is distorted tetrahedral (Ag–S, 2.28–2.79 A), trigonal planar (Ag–S, 2.36–2.89 A), or linear (Ag–S, 2.46–2.64 A).