Thermal expansion behaviour of some oxide spinels

Abstract

Compounds which crystallize with the cubic spinel structure are believed to be unusually anharmonic. This work is concerned with investigating the anharmonicity in MgA 204 and ZnFe204 spinels. The experimental method depends on exploiting Grlineisen's law which states that in the case of a harmonic solid, the coefficient of thermal expansion is proportional to the heat capacity at that temperature. The heat capacity data of MgA 2o4 and ZnFe2o4 spinels are known over the low temperature range between 77.5 K and room temperature. Therefore if the thermal expansion is measured over the same temperature range, the validity of GrUneisen's law can be tested, and some measure of anharmonicity can be derived from any discrepancies. A combination of X-ray equipment with low temperature attachments has been used to measure the lattice constants of the two spinels over a range of temperature 77.5 - 300.1 K0 • This experimental data, corrected by means of an internal standard, was smoothed by curve fitting, to provide results for the coefficient of thermal expansion for the two compounds over a corresponding range of temperature. Finally, these new results are compared with the thermal expansion and Gruneisen parameter behaviour of silicon and related materials and with the behaviour of other spinels where data are already available.