Multiple-scattering x-ray-absorption fine-structure analysis and thermal expansion of alkali halides

Abstract

X-ray-absorption fine-structure (XAFS) data of RbBr, RbCl, and KBr at 30 K and 125 K were measured and analyzed. An ionized-atom multiple-scattering calculation and a correlated Debye model were used for fitting the theory used in the computer code feff5 to data. The modifications of feff5 necessary to obtain good fits to the data are discussed. The results demonstrate the domination of single-scattering and focusing paths in XAFS and the determination of vibrational information through at least 10 \AA{} around the center atom. Numerical calculations were performed to analyze the cause of the difference found between the forward-scattering amplitudes of ${\mathrm{Rb}}^{+}$ and ${\mathrm{Br}}^{\mathrm{\ensuremath{-}}}$ focusing atoms. The second and third cumulants were determined of the first and second neighbors and were used to calculate the temperature dependence of the thermal-expansion coefficient including quantum effects at low temperature. Agreement with macroscopic thermal-expansion measurements was found.