Buckled crystalline structure of mixed ionic salts.

X-ray absorption fine structure (XAFS) measurements of mixed salts Rb[sub [ital x]]K[sub 1[minus][ital x]]Br and RbBr[sub [ital y]]Cl[sub 1[minus][ital y]] at their congruent melting points compositions reveal that their actual structure buckles about the NaCl average structure with an rms angular deviation from collinearity of 7--9[degree]. XAFS measures the buckling directly through three-body correlations, and verifies the homogeneity and randomness of the mixtures. The characteristic ionic sizes are found to be dependent on concentration, causing changes to 0.1 A in interatom bond distances. A computer simulation based on the information from XAFS and diffraction displays the actual structure of the salts.