Sadao Hoshino

Distribution and anharmonic thermal vibration of cations in α-AgI

Abstract The cation distribution in the superionic conductor α-AgI has been re-examined using neutron as well as X-ray diffraction data. A least squares analysis shows that the experimental data are very well explained by a structure model in which two silver ions are distributed over 12(d) sites of the space group Im3m with large asymmetric anharmonic thermal vibrations.

A Structural Phase Transition in Superionic Conductor Ag3SI

The phase transitions and the crystal structures of a superionic conductor Ag 3 SI were investigated by specific heat measurements and by X-ray as well as neutron diffraction methods. The transition entropies were measured to be 1.44 cal/mol·deg for the γ- β transition at 157 K and 2.71 cal/mol·deg for the β- α transition at 519 K. The least squares refinements of atomic positions and isotropic temperature factors were carried out. β-Ag 3 SI is cubic with space group Pm3m in which Ag is distributed into four positions apart 0.5 A from the face centered position toward [100], while in γ-Ag 3 SI below 157 K, Ag is located at one of these four positions obeying the symmetry of the rhombohedral space group R3. From these experimental results, it was found that the nature of the successive phase transitions in Ag 3 SI was not only of the order-disorder type but also of a structural phase transition. A comparison was made with the phase transition of CsPbCl 3 .

Neutron Scattering Study of Lattice Dynamics in CuBr. II. Anharmonic Effect on Debye-Waller Factor

Integrated intensities for the Bragg reflections of neutrons from CuBr single crystal with zincblende structure were measured at several temperatures. The experimental data both at room temperature and higher temperatures were interpreted consistently in terms of Matsubaras representations for anharmonic Debye-Waller factor. The harmonic and cubic anharmonic potential parameters for each atom obtained by the least squares refinement are α Cu =0.937, α Br =1.49 (×10 -12 erg A -2 ) and β Cu =1.00 (×10 -12 erg A -3 ). A discrepancy was found between the harmonic component of the mean square displacement of the Cu atom and that calculated from the phonon dispersion relation reported in Part I. In view of the lattice dynamical study by inelastic neutron scattering, the effects of the quartic anharmonicity which were ignored are considered to be included in the harmonic component determined in this analysis.

Structure and dynamics of solid state ionics

Abstract Crystal structures and dynamics of ionic diffusion of solid state ionics associated with their phase transitions are reviewed. The solid state ionics are classified according to the characteristic feature of their structures, and various types of structures are explained in detail. In particular, the anharmonic thermal vibration of mobile ions studied by X-ray and neutron diffraction is dealt with as an important characteristic of the solid state ionics. The mechanism of ionic diffusion and the dynamics of the motion of ions are discussed, in which the results obtained by various experimental methods including a computer simulation are introduced, in particular the neutron scattering studies are explained in detail.

Neutron Diffraction Study of the Structure in Cubic CsPbCl3

The crystal structure of cubic CsPbCl 3 was reinvestigated by neutron diffraction technique at 55°C. Two structure models were examined: (i) perovskite structure with anisotropic thermal vibration and (ii) disordered structure having several potential minima for the Cs and Cl atoms, originally proposed by Moller (1959). Both models explain the present neutron data equally well with a reliability index of about 9%. A set of parameters refined on the basis of the structure factor proposed by Moller is, however, revealed to be simply another representation of the perovskite model. As for the perovskite model, anomalously large thermal vibrations were obtained for the Cs and Cl atoms. It is concluded that such anomalous thermal vibrations are characteristics of the phase transitions of this substance.

The Existence of the Order Phase in Superionic Conductor Ag3SI

The ordered phase concerning the Ag distribution of superionic conductor Ag 3 SI was discovered below 157 K. The transition heat and the entropy change were measured to be 0.23 kcal/mol and 1.44 cal/mol.deg. respectively. The crystal structure of the newly found γ-phase was studied by X-ray diffraction method. The space group of the γ-phase may be C 4 3 –R3 and the transition to the β-phase is of first order but is very close to the second order one.

Incommensurate superlattice reflections in quasi one dimensional conductors, (MSe4)2I (M=Ta and Nb)

Abstract X-ray diffraction and conductivity measurements have been made on (MSE4)2I, (M=Ta and Nb). A phase transition of (NbSe4)2I was found and the transition temperature Tc was determined to be about 210K by the conductivity measurement. For (TaSe4)2I, Tc was about 260K as reported previously. Superlattice reflections due to the formation of charge density wave (CDW) in (TaSe4)2I were observed at points q =(±0.05, ±0.05, ±0.085) . The type of CDW of (NbSe4)2I is known to be identical to that of (TaSe4)2I. Temperature dependence of the threshold electric field of nonlinear conductivity is consistent with the result in previous reports.

Neutron Scattering Study on Distribution of Cations in α-AgI-Type Superionic Conductors

Neutron diffuse scattering from powder samples of α-AgI type superionic conductors AgI, Ag 3 SI and Ag 2 S was measured using a triple-axis spectrometer with a fixed position of the energy analyzer so as to measure only the elastic scattering caused by a disordered distribution of cations. The disorder scattering curves thus obtained were analyzed under the assumption that Ag atoms distribute over crystallographic 12(d) sites among the bcc lattice formed by anions, and probability functions α n for a simultaneous occupation in the n-th neighbor sites were obtained. The results are discussed comparing with the pair correlation function g Ag–Ag ( r ) being obtained theoretically as well as by the analysis of X-ray diffuse scattering based on a liquid-like distribution model. The liquid phase of AgI was also studied by the same neutron technique and the results are discussed together with those for the solid phase.

The Phase Transition and the Structures of Superionic Conductor Ag3SBr

The ordered phase concerning the Ag distribution of a superionic conductor Ag 3 SBr was discovered below 128 K. The transition heat and the entropy change were measured to be 0.23 kcal/mol and 1.77...

A Re-Entrant Glasslike Phase in Rb1-x(NH4)xH2PO4

The mixed crystal Rb 1- x (NH 4 ) x H 2 PO 4 shows ferroelectricity for small x and antiferroelectricity for large x . The frequency dispersion of the dielectric constant was studied in the concentration range 0.5≤ x . The low-frequency dispersion was found for 0.5≤ x ≤0.8 below 40 K. From the results, the phase diagram was obtained. Around x =0.75, a re-entrant glasslike phase with the dispersion of the dielectric constant was found.