Buckled crystalline structure of mixed ionic salts.

X-ray absorption ne structure (XAFS) measurements of mixed salts Rb x K 1?x Br and RbBr y Cl 1?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 veriies the homogene-ity 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 diiraction displays the actual structure of the salts. 1 It is generally agreed that a fundamental understanding of the properties of a solid starts with the knowledge of its atomic structure. For pure compounds and elements, generally, especially at low temperature, the position of every atom is on a periodic array. The recently discovered aperiodic crystals are exceptions to this rule but it is believed that the ideal crystals of this type also have perfect long-range order. When the solid is composed of an alloy or mixture of atoms or molecules over a range of compositions and still remains a single phase, the perfect periodicity is broken by the randomness of the mixing and the diierence in the type of atoms. Yet, the average structure still remains periodic and produces sharp Bragg diiraction peaks. However, the actual distribution of atoms usually will not be centered on the periodic lattice. The problem could be further complicated by short range order such as a positive or negative tendency of like atoms to cluster. The issue of the actual location of atoms in such cases relative to the average lattice has not yet, to our knowledge, been addressed experimentally. In this Letter we investigate this issue for the mixed ionic salts Rb x K 1?x Br and RbBr y Cl 1?y where the cations sublattice is disordered in the former and the anion in the latter. The x-ray absorption ne structure technique is used in our investigation because it enables us to obtain both partial pair and 3-body correlation functions, which as we show, give suucient information to completely specify the actual structure! The two mixtures of interest are soluble in the solid for all compositions. The composition of the mixed salts that were investigated was chosen at the congruent melting point, namely, the minimum of the melting temperature where …