Osamu Kamishima

Pressure and temperature dependence of EXAFS Debye-Waller factors in diamond-type and white-tin-type germanium

Extended X-ray absorption fine-structure (EXAFS) spectra near the Ge K-edge in diamond- and white-tin-type Ge under high temperature and high pressure were measured using a cubic-anvil-type apparatus (MAX90) with synchrotron radiation from the Photon Factory, Tsukuba, Japan. Pressure values up to 10.6 GPa were estimated on the basis of the isothermal equation of state of the diamond-type Ge within an accuracy of 0.4 GPa. Pressures for the same cell assembly were also determined by X-ray diffraction experiment using the NaCl scale. The diamond-type Ge is of great advantage to the pressure calibrant of EXAFS measurements at elevated temperature because a harmonic approximation can be applied up to 900 K. By the phase transition from diamond- to white-tin-type phases, with an increase in coordination number, Ge—Ge distances increase. A sixfold-coordinated Ge atom in the white-tin-type structure has crystallographically non-equivalent two kinds of nearest-neighbour distances [2.530 (8) A and 2.697 (8) A at 12.8 GPa]. The harmonic effective interatomic potential, V(u) = 1/2αu2, was evaluated from the contribution to the thermal vibration, where u is the deviation of the bond distance from the location of the potential minimum. The potential coefficient, α, at 0.1 MPa is essentially temperature independent and is 9.06 eV A−2. At 9 GPa the potential coefficient is 9.71 eV A−2. The effective interatomic potential is influenced not only by pressure but also by changes in coordination number. The high-pressure white-tin-type phase has a broader potential and a relatively larger mean square amplitude of vibration than the diamond-type phase.

Raman scattering of single-crystal SrZrO3

Single-crystal SrZrO3 with perovskite-type structure was grown by a floating-zone method. The polarized Raman scattering spectra were measured at room temperature. The observed Raman spectra were interpreted on the basis of a factor group analysis for the group D2h. Although 24 Raman active modes were expected, 15 bands were observed and the mode assignments were made by using D2h symmetry. Vibrational mode frequencies calculated using a rigid-ion model were helpful for the assignment of B1g and B3g modes and the interatomic potentials were estimated.

Dielectric relaxation in Yb-doped SrZrO3

The dielectric constant of the proton conductor SrZr1−xYbxO3 (x = 0–0.1) was measured as a function of temperature and frequency. Two well-defined relaxation peaks were observed in SrZrO3 doped with more than 1 mol% of Yb. The assignment of the two dielectric relaxations is discussed in terms of IR spectra and by free energy calculation for a miscibility of dopant Yb ions. The Yb concentration dependence of the relaxation strength of the two dielectric relaxations is in agreement with the results calculated from the free energy. The two relaxations can be assigned to a reorientation of a single Yb–OH dipole and of Yb–OH dipoles associated with Yb-clusters. The attractive energy for Yb-clustering in SrZrO3 isevaluated at about −85 meV.

Local structure and mean-square relative displacement in SiO2 and GeO2 polymorphs

Extended X-ray absorption fine structure (EXAFS) spectra near the Si and Ge K-edge for SiO_2 and GeO_2 polymorphs were measured in transmission mode with synchrotron radiation at the Photon Factory, Tsukuba. The local structures and mean-square relative displacements were determined in \alpha-tridymite, \alpha-quartz and stishovite. In stishovite, Si is octahedrally coordinated and the four coplanar Si—O bonds [1.755 (8) A] are shorter than the other two axial bonds [1.813 (15) A]. The high-temperature phase tridymite [1.597 (3) A] has a smaller local bond distance than \alpha-quartz [1.618 (5) A]. The temperature variation of the local structural parameters for quartz-type GeO_2 (q-GeO_2) and rutile-type GeO_2 (r-GeO_2) have been determined in the temperature range 7–1000 K. The harmonic effective interatomic potential V(u)=\alpha{u}^2/2 was evaluated from the contribution to the thermal vibration, where u is the deviation of the bond distance from the location of the potential minimum. The potential coefficient \alpha for the Ge—O bond of the tetrahedron in q-GeO_2 is 24.6 eV A−2. The potential coefficients \alpha for the four coplanar Ge—O bonds and the two axial bonds of the octahedron in r-GeO_2 are 12.9 and 14.9 eV A−2, respectively. The potential coefficient \alpha for the second-nearest Ge—Ge distance in q-GeO_2 is 9.57 eV A−2. The potential coefficients \alpha for the second- and third-nearest Ge—Ge distances in r-GeO_2 are 11.6 and 7.18 eV A−2, respectively. The effective interatomic potential is largely influenced by the local structure, particularly by the coordination numbers. The phonon dispersion relations for q-GeO_2 and r-GeO_2 were estimated along [100] by calculating the dynamical matrix using the potential coefficients \alpha for the Ge—O and Ge—Ge motions. The quartz-type structure has a more complex structure with a wide gap between 103 and 141 meV and a highest energy of 149 meV, whereas the rutile-type structure has a continuous distribution and a highest energy of 126 meV.

Extended X-Ray Absorption Fine Structure Study on Anharmonic Pair Potential in CuBr

The Cu K-edge EXAFS (extended X-ray absorption fine structure) of CuBr has been measured at room temperature. An anharmonic pair potential for the Cu–Br bond introduced as Φ(u)=Φ0+(1/2!)αu2+(1/3!)βu3 in a central-field approximation is determined by numerical integration of the EXAFS function: obtained values of α and β are 2.28(4) eV/A2 and -8(1) eV/A3, respectively, and the Gruneisen parameter γ G=-(R0/6)(β/α) is obtained as 1.5(2) using these values. These values are concluded to be reasonable from comparison with previously obtained Gruneisen parameters and from studying phonon dispersion relations.

Anharmonicity on Raman active phonon modes of LaAlO3

The phonon-dispersion relations of LaAlO3 crystal are calculated using the first-principle calculations. The first-order Raman spectra in the LaGaO3 were measured in the temperature range of 20–300K for the rhombohedral phase. The temperature dependence of the linewidth of the Alg (132 cm-1) and that of Eg (489 cm-1) were analyzed using temperature-weighted two-phonon density of states (TDOS) due to the cubic term. The calculated results reproduce the observed ones in this temperature range. The scattering of a thermal phonon by observed phonons dominates phonon-phonon interaction on the rotational modes [the Alg (132 cm-1)]. Phonon-phonon interaction on the bending mode [the Eg (489 cm-1)] is dominated by both the scattering and the decay of the observed phonons into two phonons having lower energies. We found that this originates in the discrete spectra on the higher-frequency side in the phonon density of states that produces a difference in anharmonic effects on each peaks channels in the HT. This result is similar to that in the high-temperature phase in the case of LaGaO3 [1].

The First-Order Raman Spectra and Lattice Dynamics for YAlO3 Crystal

The phonon-dispersion curves of the YAlO 3 crystal are calculated on the basis of a rigid-ion model using the measured frequency values by Raman and infrared polarized spectroscopy at room temperat...

EXAFS study on temperature dependence of nearest neighbor distance in CuBr

Precise EXAFS (Extended X-ray Absorption Fine Structure) study of CuBr crystals, carried out in the temperature range from 300 to 631 K, leads to the fact that a potential minimum position for the nearest neighbor pairs of Cu and Br ions decreases monotonically. The mean interatomic distance, however, totally expands due to anharmonic vibrations with an increase in temperature.

Investigation of Cluster-Induced Distortion and Conductivity of Ionic Conductor CuBr

Theoretical investigation on the local distortion in the low-temperature γ-phase of superionic conductor CuBr is carried out in terms of the cluster-distortion coupling model. It is found that this cluster-induced local distortion, giving rise to the negative expansion, is quite consistent with experimental results. The specific heat and ionic conductivity involved in the 1st-order order-disorder phase transition are also studied, showing that the defects and distortions induced therefrom play quite important roles.

Cluster-induced local distortions and superionic transitions of ionic conductors AgI and PbF2

Theoretical investigation of the cluster-distortion coupling model is carried out in the low-temperature phases of two typical superionic conductors AgI and PbF 2 , which are characterized by two types of phase transitions, 1st-order order-disorder type and Faraday type, respectively. It is found that the static local distortion induced therefrom by clustering of mobile ions is quite consistent with the negative local distortion experimentally obtained from EXAFS measurement in β-AgI, and with the anomalous temperature dependence of the lattice constant of β-PbF 2 . The effective interaction energy between mobile ions decreases with temperature due to the static and dynamic distortions. The specific heat and ionic conductivity are also in agreement with experimental observations in both materials.