Correlation effects in the extended x-ray-absorption fine-structure Debye-Waller factors of AgI

Extended x-ray-absorption fine-structure (EXAFS) measurements at the edge I ${\mathit{L}}_{3}$ in AgI allowed us to determine the temperature dependence of the EXAFS Debye-Waller factors for the first-shell (I-Ag) and the second-shell (I-I) distances from 50 to 292 K. The comparison with previous diffraction measurements and calculated phonon density of states shows that the correlation of atomic motion affects the EXAFS Debye-Waller factor much more strongly for the I-Ag than for the I-I distance. Strengths and limitations of the Einstein and correlated Debye models in interpreting the experimental data are discussed. The experimental behavior is satisfactorily reproduced by a mixed model (Debye+Einstein) taking explicitly into account the phonon eigenvectors of the low-frequency optical modes calculated at the center of the first Brillouin zone. This model elucidates the connection between EXAFS Debye-Waller factor and phonon polarization and points to the potential of EXAFS as vibrational probe for heteroatomic crystals.