Thermodynamic properties of, and fergusonite-to-scheelite phase transition in, gadolinium orthoniobate GdNbO4 ceramics

Abstract

Gadolinium orthoniobate ceramics, GdNbO4, were prepared by a conventional solid state reaction method. The synthesized sample was characterized by means of X-ray diffraction and μ-X-ray fluorescence spectrometry methods. The temperature dependence of the heat capacity of GdNbO4 was first measured by relaxation, adiabatic and differential scanning calorimetry in the temperature range of 4–1308 K. The high-temperature enthalpy increments of GdNbO4 were measured by drop calorimetry between 434 and 1745 K. A small step-like change in the Cp,m(T) associated with a fergusonite-to-scheelite phase transition was detected. The temperature of a second order phase transition was determined to be 1078 K. Based on the fitted values of the Cp,m(T), the thermodynamic functions (entropy, enthalpy change and Gibbs energy) were calculated in the range from T→0–1750 K.