Thermal expansion and heat capacity of thulium orthotantalate

Abstract Monoclinic M′-orthotantalate thulium samples were obtain by co-precipitation and annealing at 1873 K and characterized by X-ray powder diffraction, scanning electron microscopy and chemical analysis. Using the high-temperature X-ray diffraction method, the temperature dependences of the monoclinic lattice parameters were determined at range 298–1273 K and instantaneous βV(i) and relative βV(r) volume expansion coefficients were calculated. The molar heat capacity of M′-TmTaO4 was measured by relaxation (4.1–25.1 K), adiabatic (5.6–348.8 K) and differential scanning (303–1333 K) calorimetry. Standard thermodynamic function (entropy, enthalpy increment and reduced Gibbs energy) were calculated on the smoothed values of molar capacity, without taking into account the contribution of phase transformations occurring below 4 K. The general shape of the anomalous Schottky contribution to the molar heat capacity was determined.