Local structure of ionic solid solutions: Extended x-ray-absorption fine-structure study

The first-neighbor distances in the ionically bonded rocksalt-structured solid solutions (${\mathrm{K}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Rb}}_{\mathrm{x}}$)Br and Rb(${\mathrm{Br}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{I}}_{\mathrm{x}}$) differ substantially from the average, or virtual-crystal, distance. There is, nonetheless, a significant variation in the first-neighbor distances with composition, indicating that the Pauling concept of the conservation of ionic radii is not valid. This change, relative to the change in the lattice constant, is about twice as large as that observed in covalently bonded, zincblende-structured solid solutions, resulting from the different crystal structures for these two alloy systems.