The thermodynamics of formation, molar heat capacity, and thermodynamic functions of ZrTiO₄(cr)

As part of an ongoing study of titanate-based ceramic materials for the disposal of surplus weapons-grade plutonium, we report thermodynamic properties of a sample of zirconium titanate (ZrTiO₄) quenched from a high-temperature synthesis. The standard enthalpy of formation Δ_fH_m^o was obtained by using high-temperature oxide-melt solution calorimetry. The molar heat capacity C_{p, m} was measured from T = 13 K to T = 400 K in an adiabatic calorimeter and extrapolated to T = 1800 K by using an equation fitted to the low-D temperature results. The results at T = 298.15 K are Δ_fH_m^o = -(2024.1 ± 4.5) kJ·mol⁻¹, Δ_O^TS_m^o = (116.71 ± 0.31) J·K⁻¹·mol⁻¹, and Δ_fG_m^o = -(1915.8 ± 4.5) kJ·mol⁻¹; the molar entropy includes a contribution of 2Rln 2 to account for the random mixing of Zr⁴⁺ and Ti⁴⁺ on a four-fold crystallographic site. Values for the standard molar Gibbs energies and enthalpies of formation of ZrTiO₄; Δ_fG_m^o and Δ_fH_m^o, and for the free energies and enthalpies for the reaction to form ZrTiO₄(cr) from ZrO₂(cr) and TiO₂(cr), are tabulated over the temperature interval, 0 ≤ (T /K) ≤ 1800. From these results, we conclude that ZrTiO₄ is not stable with respect to (ZrO₂ + TiO₂) at T = 298.15 K, but becomes so at T = (1250 ± 150) K.