Phase transformation in CuRh2O4 : A powder neutron diffraction study

The structure of the spinel-type oxide CuRh{sub 2}O{sub 4} has been determined between 25 and 650 C using powder neutron diffraction methods. A first-order phase transition from a tetragonal to a cubic structure occurred at ca. 575 C. The high-temperature cubic structure is a normal spinel (Fd{bar 3}m, a = 8.5487(2) {angstrom}, x = 0.2601(4)), whereas at room temperature the structure can be described as a distorted spinel (I{bar 4}{sub 1}/amd, = 6.1707(4) {angstrom}, c = 7.9183(7) {angstrom}). In both forms, Cu and Rh exclusively occupy the tetrahedral and octahedral sites, respectively. The room temperature tetragonal distortion is ascribed to the strong preference of Cu{sup 2+} for a square planar geometry. From valence bond sum analysis, it was concluded that the transition is not electronic in origin.