HIGH-TEMPERATURE ENTHALPY OF La₂Hf₂O₇ IN THE TEMPERATURE RANGE 490–2120 K

Abstract

La₂Hf₂O₇ was prepared by the chemical method of inverse precipitation from ammonia solution of a mixture of La and Hf nitrates followed by hydroxide decomposition at 1250 °С in air and melting of the oxide mixture in a solar furnace. The La₂Hf₂O₇ formation was confirmed by XRD. Enthalpy increment measurements were carried out on this compound in the range 490–2120 K (for the first time in the temperature ranges 490–988 and 1740–2120 К) by drop method using a Setaram high-temperature differential calorimeter and high-temperature calorimetric device. From the fitted equation for the measured enthalpy increment data, the main thermodynamic functions, such as heat capacity, entropy, and Gibbs energy, were calculated in the temperature range 298–2120 K. The results are presented and compared with the published data and estimated using Neumann–Kopp rule.