Anharmonicity effects on the extended x-ray-absorption fine structure: The case of β-AgI

Anharmonicity effects on the extended x-ray-absorption fine structure (EXAFS) of β-AgI have been studied within the temperature range 77–410 K through the cumulant analysis of the first-shell contribution. The distributions of I-Ag distances and the corresponding pair potentials have been reconstructed directly from cumulants. The shape of the EXAFS pair potential is temperature independent, while its equilibrium position shifts to lower values with increasing temperature. The I-Ag thermal expansion is influenced not only by the EXAFS pair-potential anharmonicity but also by its shift, so that Δ${\mathit{C}}_1$ is not equal to the ratio ${\mathit{C}}_3$/2${\mathit{C}}_2$. Thermal expansion measured by EXAFS is significantly larger than the one measured by diffraction; the difference is due to vibrational motion normal to the I-Ag bond direction, responsible also for peculiar correlation effects on the mean-square relative displacement (MSRD). The distribution of I-Ag distances sampled by EXAFS is consistent not only with the excluded-volume model but also with vibrational models leading to strong distortions of the first-shell tetrahedral cage. Harmonic and anharmonic contributions to the I-Ag MSRD have been singled out, the anharmonic fraction amounting to 20% at 300 K.