Short-range structure of solid and liquid AgBr determined by multiple-edge X-ray Absorption Spectroscopy

X-ray absorption measurements of solid and liquid AgBr in the T = 30-927 K range of temperature have been performed using synchrotron radiation. Accurate short-range structural parameters are determined using simultaneous Ag and Br K-edge EXAFS refinement in the framework of the GNXAS method for data analysis. Results are discussed in light of the peculiar structural and ionic conduction properties of AgBr and taking into account previous x-ray, neutron diffraction (ND), molecular-dynamics (MD), and EXAFS results. Detailed information about the asymmetry of the first-neighbor g(r) distribution is given, showing that anharmonic effects are important even at moderate temperatures. The foot and the most probable value of the first neighbor g(r) are found to shift up to about 0.25 Å toward shorter distances from 30 K to 670 K, while the long-range part of the distribution broadens at high temperatures. The average bondlength is found to be in agreement with known thermal expansion diffraction data correcting previous EXAFS results. Accurate EXAFS measurements of liquid AgBr as a function of temperature are presented and compared with existing ND and MD structural models. The reconstructed EXAFS signals related to previous structural models are shown to differ considerably with present experimental data, due to the extreme EXAFS sensitivity to short-range correlations. The first-neighbor peak of the Ag-Br partial distribution function is accurately determined in this work. The short-distance side of the distribution, probing the repulsive part of the interaction, shows a steeper onset located at about 2.4 Å, clearly shifted toward shorter distances with respect to previous broader MD simulations. The reconstructed g(r) broadens gradually at higher temperatures (up to 927 K). Existence of short-range Ag-Ag correlations resulting in a well-defined Ag-Ag short-distance peak in solid and liquid AgBr as suggested by recent ND reverse Monte Carlo data analysis is not confirmed.