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 Grüneisen 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.