Anharmonic effective pair potentials of β- and α-AgI determined by I K-edge EXAFS

Abstract The anharmonic effective pair potentials V ( u )= au 2 /2+ bu 3 /3!+ cu 4 /4! for the I–Ag bond in β- and α-AgI have been investigated by the EXAFS technique. The EXAFS spectra near the I K-edge were measured from 7 to 800 K. The measurement near the I K-edge has the advantage for the preparation of uniform specimens to precise measurements at high temperatures. In the parameter fitting, we have directly carried out the numerical integration of EXAFS function and evaluated the precise anharmonic effective pair potentials. The phonon energies have been estimated using the potential parameter a by calculating the dynamical matrix. The Gruneisen parameters γ G are calculated from the obtained values of a and b as 2.05 and 1.75 for β- and α-AgI, respectively. The extent of anharmonicity is larger in β-AgI than in α-AgI though the effective pair potentials do not change appreciably through the β–α phase transition. The statistical distribution of Ag in α-AgI and superionic conduction mechanism have been discussed based on the effective pair potential and distribution of I–Ag distances. The time spent on a lattice site is greater than the time spent between the potential-energy minima. However, the probability of finding mobile ions at saddle-point position can be regarded as significant and is several percent at higher temperature in the superionic conducting AgI. The superionic conducting α-AgI has a broad interatomic potential which is similar to that of the octahedrally coordinated compounds and strongly correlated displacement in thermal vibration between cation and anion. The strongly correlated displacement can be attributed to covalent bonds which interact locally. Covalency is important to achieve large mean square displacements.