High-temperature crystallographic and thermodynamic investigations on synthetic zirconolite (CaZrTi2O7)

Abstract

In this paper, we report the crystal structure and thermodynamic parameters of zirconolite (CaZrTi2O7) which is a potential ceramic matrix for incorporation of nuclear high-level waste. The room temperature structural studies revealed a two layer monoclinic (C2/c) structure for the compound. Thermal expansion behavior of zirconolite was investigated by the high-temperature XRD. The average thermal expansion coefficient of zirconolite, measured in the temperature range 298–1273 K, is found to be: a-axis (A) = 12.410(2) + 1.06(8) × 10−4 [T] + 3.1(5) × 10−8 [T]2/K, b-axis (A) = 7.252(1) + 6.2(4) × 10−5 [T] + 4.9(2.5) × 10−9 [T]2/K and c-axis (A) = 11.351(2) + 7.6(7) × 10−5 [T] + 3.2(4) × 10−8 [T]2/K. The standard molar enthalpy of formation of zirconolite (CaZrTi2O7, s) at 298.15 K determined employing high-temperature solution calorimetry is found to be − 3716.5 ± 3.7 kJ mol−1. The isobaric heat capacity of the CaZrTi2O7(s) has been derived from the enthalpy increment (H T–H 298.15) data. Based on the experimental data for the compound and the available thermodynamic data for the component oxides, thermodynamic functions like: $${\text{Cp}}_{\text{m}}^{\circ } ,S_{\text{m}}^{\circ } , H_{{{\text{m}},{\text{T}}}}^{0} , { }G_{\text{m,T}}^{\circ } , \left( {H_{\text{T}}^{\circ } - H_{298.15}^{\circ } } \right), - \left( {\frac{{G_{\text{T}}^{\circ } - H_{298.15}^{\circ } }}{T}} \right), \Delta_{\text{f}} H_{{{\text{m}},298.15}}^{\circ } \,\, {\text{and}}\,\,\Delta_{\text{f}} G_{{{\text{m}},{\text{T}}}}^{\circ }$$ for zirconolite (CaZrTi2O7, s) have been generated.