Calorimetric Study of the La₂Hf₂O₇ Heat Capacity in the Range 57–302 K

Abstract

The heat capacity of La₂Hf₂O₇ was studied for the first time in the range 57–302 K by adiabatic calorimetry. The heat capacity Ср of lanthanum hafnate changes monotonically and there are no anomalies. The values of heat capacity, entropy, enthalpy, and reduced Gibbs energy were determined in standard conditions: С^º_р (298,15 К) = 229,39 ± 0,92 J • mol^–1 • К^–1; Sº (298,15 К) = 246,9 ± 2 J • mol^–1 • К^–1; Фº(298,15 К) = 114,76 ± 1,72 J • mol^–1 • К^–1; Нº (298,15 К) – Нº (0 К) = 39403 ± 197 J • mol^–1 In the series of isostructural La₂Zr₂O₇ → La₂Hf₂O₇ compounds, atomic oscillation frequency in the lattice decreases and low-temperature heat capacity increases with greater mass of oscillator atoms from Zr to Hf.