Solving the structure of disordered mixed salts

Abstract

A detailed x-ray-absorption fine-structure (XAFS) investigation of two mixed alkali halides ${\mathrm{Rb}}_{0.76}$${\mathrm{K}}_{0.24}$Br and ${\mathrm{RbBr}}_{0.62}$${\mathrm{Cl}}_{0.38}$ was performed. The concentrations of the mixtures had been chosen to produce a single homogeneous phase for each, and it was checked by XAFS that the salts were randomly mixed on the atomic level. Detailed analysis of the data including multiple-scattering contributions revealed an rms buckling angular deviation of both mixtures from the average NaCl collinear structure of 7--9\ifmmode^\circ\else\textdegree\fi{}. The angles are defined by three atomic positions determined through double- and triple-scattering paths. These angles are new parameters which should be added to characterize the buckled crystals. Adding to diffraction results the parameters determined from XAFS as input into a molecular-dynamics simulation the structures of the mixed salts with their fluctuations about the NaCL structure are solved and displayed.