Thermal expansion and thermodynamic properties of gadolinium hafnate ceramics

Gadolinium hafnate samples were obtained by co-precipitation and precursor annealing at a temperature of similar to 1773 K and characterised by X-ray powder diffraction, scanning electron microscopy and chemical analysis. The molar heat capacity of Gd2Hf2O7 ceramics with a pyrochlore structure was measured by adiabatic (8-340 K) and differential scanning calorimetry (315-1345 K). The heat capacity of gadolinium hafnate was extrapolated to 0 K without taking into account anomalies in the temperature range below 20 K. Standard thermodynamic functions were calculated based on the smoothed values of molar heat capacity: entropy, enthalpy change and reduced Gibbs energy. Using the high-temperature X-ray diffraction method, the temperature dependence of the Gd2Hf2O7 cubic crystal lattice parameter was determined in the range of 298-1273 K and coefficients of instantaneous (alpha(i)), relative (alpha(r)) and linear thermal expansion (TE) were calculated.