High temperature crystallographic and thermodynamic investigations on synthetic calzirtite (Ca₂Zr₅Ti₂O₁₆)

Abstract

In the context of formation and stability of synthetic rock (SYNROC), studies on crystal structure and thermodynamic parameters of a mineral analogous material, namely calzirtite has been undertaken. The ambient temperature structural studies revealed a tetragonal (I4₁/acd) fluorite related super-structure with about 5% intermixing of Ti and Zr sites for calzirtite. High temperature structural studies indicated the retention of tetragonal structure of calzirtite up to 1673 K. The average axial thermal expansion coefficient of calzirtite between 297 and 1473 K is found to be 12.7 × 10⁻⁶/K for a-axis and 9.2 × 10⁻⁶/K along the c-axis. The thermodynamic parameters of calzirtite have been determined employing high temperature solution calorimetry. The standard molar enthalpy of formation of Ca₂Zr₅Ti₂O₁₆(s) at 298 K is found to be −8964.16 ± 12.59 kJ mol⁻¹. The isobaric heat capacity of the compound has been derived from the enthalpy increment (H_T-H₂₉₈) data measured by drop calorimetric technique. Based on the experimental results thermodynamic functions like C⁰_p,m, S⁰_m, H⁰, G⁰, (G⁰_T-H⁰₂₉₈)/T, Δ_fH⁰₂₉₈ and Δ_fG⁰_T for calzirtite (Ca₂Zr₅Ti₂O₁₆,s) has been generated.