Thermal expansion of chromites and zinc spinels

The aim of this work is the determination of the thermal expansion behaviour of chromium and zinc spinels by in situ XRPD. Two natural chromites (from Bushveld, South Africa and Xerolivado, Greece) and four synthetic phases (one Cr-spinel and three Zn-spinels) have been studied. A net change in the thermal expansion regime can be observed for all samples at temperatures T_a in the range 873-1073 K, depending on the chemical composition: the change is assumed to be related to the onset of the intracrystalline cation diffusion. The chromium spinels have a neat increase in thermal expansion coefficient above T_a, while the zinc samples show a decrease. It is difficult to rationalise the observed temperature behaviour of the cell parameters in terms of simple consideration based on cation radii and crystal geometry: proper interpretation has to include the crystal chemical role of site vacancies.