LOW-TEMPERATURE THERMODYNAMIC FUNCTIONS OF GADOLINIUM HAFNATE Gd₂Hf₂O₇)

Abstract

The isobar heat capacity of Gd₂Hf₂O₇ is studied for the first time by adiabatic calorimetry in the range 80–298.15 K. It is shown that С_р of gadolinium hafnate changes monotonically and no anomalies are observed. The temperature dependences of the principal thermodynamic functions of Gd₂Hf₂O₇ in the range 10–300 K are established. The heat capacity, entropy, reduced Gibbs energy, and enthalpy are determined for standard conditions: C°_p(298.15 K) = 224.2 ± 0.9 J • mol^–1• K^–1; S°(298.15 K) = 237.1 ± 2 J • mol^–1 • K^–1; Ф' (298.15 K) = 109.4 ±  1.6 J • mol^–1• K^–1; H°(298.15 K) – H°(0 K) = 38084 ±  190 J • mol^–1. The temperature dependence of the Gd₂Hf₂O₇ heat capacity is evaluated using the Neumann–Kopp rule of additivity. The difference between the experimental and evaluated heat capacities of Gd₂Hf₂O₇ in the range 80–298.15 K is no more than 3.4%, while the difference in  C°_p(298.15 K) is 0.4%.