SOLVING THE STRUCTURE OF DISORDERED MIXED SALTS

A detailed x-ray-absorption fine-structure (XAFS) investigation of two mixed alkali halides Rb0.76K0.24Br and RbBr0.62Cl0.38 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 dat a including multiple-scattering contributions revealed an rms buckling angular deviation of both mixtures from the average NaCl collinear st ructure of 7-9-degrees. The angles are defined by three atomic positio ns determined through double- and triple-scattering paths. These angle s are new parameters which should be added to characterize the buckled crystals. Adding to diffraction results the parameters determined fro m 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.