Anharmonic effective pair potentials in α-,β-, and γ-CuI determined by extended X-ray absorption fine structure

The anharmonic effective pair potentials V(u) = au2/2 + bu3/3! + cu4/4! for I–Cu bond in γ-, β- and α-CuI have been investigated by the EXAFS technique. The EXAFS spectra near the I L3-edge were measured from 73 to 873 K. In the parameter fitting, we have directly carried out the numerical integration of EXAFS function and evaluated the precise anharmonic effective pair potentials. The effective pair potentials do not change appreciably through the phase transitions, though the extent of anharmonicity is larger in α-CuI than in γ- and β-CuI. The superionic conducting α-CuI has a broad interatomic potential which is similar to those in CuBr and CuCl and the displacements of cation and anion correlate strongly with each other in thermal vibration.