Extended x-ray-absorption fine-structure study of alkali-metal halides under high pressure

The extended x-ray-absorption fine structure (EXAFS) of three alkali-metal halides (NaBr, KBr, and RbCl) is investigated at pressures ranging from 0 to 8 GPa. Parameter fitting is used to extract the change of the nearest-neighbor distance and thus the pressure, as well as the absolute values of the second cumulant as a function of pressure, ${\mathrm{\sigma }}^2$(p), and the third cumulant at zero pressure, ${\mathrm{\sigma }}^{\mathrm{}\left(3\right)\mathrm{}}$(0). For all three materials, a sharp reduction with pressure of ${\mathrm{\sigma }}^2$(p) is found. Classical statistical mechanics is then invoked to calculate the first, second, and third moments of the nearest-neighbor distance and thus their second and third cumulants. The integration is done with the Monte Carlo technique. For the potential energy U in the Boltzmann factor, the generalized Huggins-Mayer pair potential is used along with a three-body potential term due to charge transfer. Generally, model calculations and EXAFS data match well. High-pressure EXAFS data can therefore be regarded as a help to assess, and even exclude, potential parameters from the literature.