MULTIPLE-SCATTERING X-RAY-ABSORPTION FINE-STRUCTURE ANALYSIS AND THERMAL-EXPANSION OF ALKALI-HALIDES

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-sc attering calculation and a correlated Debye model were used for fittin g the theory used in the FEFF5 computer code to data. The modification s of FEFF5 necessary to obtain good fits to the data are discussed. Th e results demonstrate the domination of single-scattering and focusing paths in XAFS and the determination of vibrational information throug h at least 10 angstrom around the center atom. Numerical calculations were performed to analyze the cause of the difference found between th e forward-scattering amplitudes of Rb+ and Br- focusing atoms. The sec ond and third cumulants were determined of the first and second neighb ors and were used to calculate the temperature dependence of the therm al-expansion coefficient including quantum effects at low temperature. Agreement with macroscopic thermal-expansion measurements was found.