Thermophysical properties of M′-LuTaO4: Structural and calorimetric studies

Abstract

Lutetium orthotantalate with M′-fergusonite type structure was synthesized using a reverse coprecipitation method. Phase and chemical composition, as well as microstructure of the synthesized sample, were characterized by X-ray diffraction (XRD), μ-X-ray fluorescence and Fourier-transform infrared spectroscopies, and scanning electron microscopy. Heat capacity of M′-LuTaO4 was first studied by adiabatic and differential scanning calorimetry (DSC) in the temperature range from 10 to 1300 K. Using a temperature dependence of heat capacity, the standard thermodynamic functions (entropy Smo(T), enthalpy change Hmo(T)–Hmo(0) and derived Gibbs energy Фmo(T)) were calculated in the range of T→0–1300 K. The standard molar entropy of M′-LuTaO4 at 298.15 K is 123.12 ± 0.50 J K−1 mol−1. A comparison of the experimental heat capacity values, obtained by DSC, with those calculated using the empirical Neumann-Kopp rule showed a reasonable agreement between the two sets of data only up to ≈1000 K. The high-temperature evolution of the lattice parameters for M′-LuTaO4 was studied by high-temperature XRD (HTXRD). According to the high-temperature heat capacity study and the HTXRD measurements, there were no phase transformations up to 1300 K. Based on the HTXRD data, the linear and volume thermal expansion coefficients were obtained for the first time.