Multiple - Scattering XAFS Analysis and Thermal Expansion of Alkali Halides

X-ray-absorption ne structure (XAFS) data of RbBr, RbCl and KBr at 30 K and 125 K were measured and analysed. An ionized atom multiple-scattering (MS) calculation and a correlated Debye model were used for tting FEFF5 theory to data. The modiications of FEFF5 necessary to obtain good ts 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 A around the center atom. Numerical calculations were performed to analyse the cause of the diierence found between the forward-scattering amplitudes of Rb + and Br ? focusing atoms. The second and third cumulants were determined of the rst and second neighbors and were used to calculate the temperature dependence of the thermal expansion coeecient including quantum eeects at low temperature. Agreement with macroscopic thermal expansion measurements was found.