X-ray absorption fine structure (XAFS) measurements of mixed salts ${\mathrm{Rb}}_{\mathit{x}}$${\mathrm{K}}_{1\mathrm{-}\mathit{x}}$Br and ${\mathrm{RbBr}}_{\mathit{y}}$${\mathrm{Cl}}_{1\mathrm{-}\mathit{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°. 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 Å in interatom bond distances. A computer simulation based on the information from XAFS and diffraction displays the actual structure of the salts.

• Received 21 July 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.3485