Maki Okube

Anharmonicity of gold under high-pressure and high-temperature

Abstract In order to examine the pressure effect on the anharmonicity of Au, EXAFS spectra near Au L 3 -edge were measured at the temperature range from 300 to 1000xa0K under pressures up to 14xa0GPa using large volume high-pressure devices and synchrotron radiation. The values of EXAFS Debye–Waller factors, σ 2 and σ 3, become smaller with the increase of pressure, which indicates that the potential width become narrower with pressure. The anharmonic effective pair potentials of Au, V ( u )= au 2 /2!+ bu 3 /3!, at 0.1xa0MPa, 6 and 14xa0GPa have been calculated. The pressure dependence of the coefficient of thermal expansion has also been evaluated.

XAFS study of GeO2 glass under pressure

Using a large-volume high-pressure apparatus, Li2O–4GeO2 glass and pure GeO2 gel have been compressed to 14 GPa at room temperature and their local structural changes have been investigated by an in situ XAFS (x-ray absorption fine-structure) method. On compression of Li2O–4GeO2 glass, the Ge–O distance gradually becomes short below 7 GPa, showing the conventional compression of the GeO4 tetrahedron. Abrupt increase in the Ge–O distance occurs between 8 and 10 GPa, which corresponds to the coordination number (CN) changing from 4 to 6. The CN change is completed at 10 GPa. On decompression, the reverse transition occurs gradually below 10 GPa. In contrast to the case for Li2O–4GeO2 glass, the Ge–O distance in GeO2 gel gradually increases over a pressure range from 2 to 12 GPa, indicating that continuous change in CN occurs. The Ge–O distance at 12 GPa is shorter than that of Li–4GeO2 indicating that the change in CN is not completed even at this pressure. On complete release of pressure, the Ge–O distance reverts to that of the starting gel.

Structural changes of quartz-type crystalline and vitreous GeO2 under pressure

Using a large volume high pressure apparatus, quartz-type crystalline GeO2 and Li2O-4GeO2 glass have been compressed up to 14 GPa at room temperature and their local structural changes have been investigated by an in-situ XAFS method. In quartz-type crystalline GeO2, the change of the coordination number from 4 to 6 begins above 8 GPa and finishes below 12 GPa. On decompression, reversal transition begins below 8 GPa and there is a large hysteresis. Almost no sixfold coordination of Ge is preserved after releasing pressure. Change of coordination number in vitreous Li2O-4GeO2 begins above 6 GPa and is completed below 10 GPa. Reversal transition begins below 10 GPa and the hysteresis is smaller than that of quartz-type GeO2. Change of coordination number in vitreous Li2O-4GeO2 is also reversal.

Anharmonic effective pair potentials of group VIII and Ib fcc metals

The temperature dependence of EXAFS Debye-Waller factors and anharmonic effective pair potentials of group VIII and Ib fcc metals were investigated by a cumulant expansion method. The EXAFS spectra near K- or L3-edges were measured at the temperatures from 30 to 800 K using synchrotron radiation from the Photon Factory, Tsukuba. The effective pair potentials, a x u2/2!+ b x u3/3! denote that the group Ib has obviously larger anharmonisity than the group VIII metals. The discrepancy between the Morse potential approximation and determined effective pair potential is discussed.

Anharmonic Effective Pair Potentials of γ- and α-CuBr at High Pressure

The anharmonic effective pair potentials V(u)=au2/2+bu3/3!+cu4/4! for Br–Cu bond in γ- and α-CuBr under pressure have been investigated by an extended X-ray absorption fine structure (EXAFS) technique. The EXAFS spectra near the Br K-edge were measured under high temperature and high pressure using a large-volume uniaxial press and synchrotron radiation from SPring-8. In the parameter fitting process, we have directly carried out the numerical integration of the EXAFS function and evaluated the precise anharmonic effective pair potentials. The anharmonic effective pair potential for each phase is influenced by pressure and becomes steeper with increasing pressure. The potential parameter a for γ-phase at 0.1 MPa increases by about 20% at the pressure of 4.8 GPa. The Gruneisen parameters γG for γ-CuBr are calculated from the obtained values of a and b as 1.4 and 1.6 at 0.1 MPa and 4.8 GPa, respectively. The fourth order term in the potential parameter is significant in α-CuBr at 7.0 GPa. The effective pair potentials do not change appreciably through the phase transition from γ to α-phase. The statistical distribution of Cu in α-CuBr and superionic conduction mechanism have been discussed based on the effective pair potential and distribution of Br–Cu distances. The probability of finding mobile Cu ions at the saddle-point position is several percent at 7.0 GPa and 1200 K in the superionic conducting α-CuBr.

Anharmonic effective pair potentials of gold under high pressure and high temperature

In order to examine the effect of pressure on the anharmonicity of Au, extended x-ray absorption fine-structure spectra near the Au L3 edge were measured in the temperature range from 300 to 1100 K under pressures up to 14 GPa using large-volume high-pressure devices and synchrotron radiation. The anharmonic effective pair potentials of Au, V (u) = au2 /2! + bu3 /3!, at 0.1 MPa, 6 and 14 GPa have been calculated. The pressure dependence of the thermal expansion coefficients has also been evaluated. The reliability of the anharmonic correction proposed on the basis of the Anderson scale has been discussed.

ANHARMONICITY OF PLATINUM UNDER HP AND HT

In order to examine the pressure effect on anharmonic thermal vibrations of Pt, Extended X-ray Absorption Fine Structure (EXAFS) spectra near Pt K-edge and L3-edge were measured in the temperature range from 300 to 800 K under pressures up to 6GPa, using large volume high-pressure devices and synchrotron radiation. We have investigated the anharmonic interatomic potentials by a cumulant expansion method. The values of EXAFS Debye-Waller factors, σ2 and σ3, become smaller with increase of pressure. The anharmonic effective pair potential V(u) =(1/2!)au 2 +(1/3!)bu 3 was evaluated and the potential coefficient a at 0.1 MPa and 6GPa are 4.8 and 5.0eV/Å2, respectively. The anharmonic potential parameter b decreases with increasing pressure.

Anharmonic effective pair potentials in CaTiO3, SrTiO3 and CaGeO3 perovskite

The temperature dependence of EXAFS Debye-Waller factors in CaTiO3, SrTiO3 and CaGeO3 perovskite was investigated with the cumulant expansion method. The CaGeO3 perovskite as an analogue of the Earths lower-mantle mineral was synthesized in a cubic anvil type apparatus under 10 GPa 1250 K The Ca, Ti and Ge K-edge EXAFS spectra were measured in transmission mode at temperature up to 1100 K The effective pair potentials, V(u)= alpha (u2/2+ beta u3/3!, were evaluated and the Grüneisen parameter were calculated. The potential coefficients alpha and beta for Ti-O bond in CaTiO3 are 6.9 eVA(-2) and -38 eVA(-3), respectively. Those for Ge-O bond in CaGeO3 are 9.8 eVA(-2) and -36 eVA(-3), respectively.