Molar heat capacities and thermodynamic functions of CaHf Ti2O7( cr ) and CaZr0.26Hf0.74Ti2O7( cr)

Abstract

As part of an ongoing study of titanate-based ceramic materials for the disposal of surplus weapons plutonium, we report the molar heat capacities and thermodynamic functions for the zirconolite (CaZrTi2O7) analogue Hf-zirconolite (CaHfTi2O7) and a solid solution of the two: CaZr0.26Hf0.74Ti2O7. Measurements have been made on the solid solution to probe the extent (zirconolite + Hf-zirconolite) form ideal solutions. The molar heat capacity for both samples was measured from T= 13 K to T= 400 K in an adiabatic calorimeter and extrapolated to T= 1500 K with an equation fitted to the low-temperature results. The results at T= 298.15 K areΔ0298.15KSmo= (196.98 ± 0.39)J ·K−1· mol−1for CaHfTi2O7andΔ0298.15KSmo= (199.76 ± 0.39)J · K−1· mol−1for CaZr0.26Hf0.74Ti2O7.Recent crystallographic measurements on zirconolite established the existence of disorder at the Ti(2) site which results in a zero-point entropy contribution of (1/2) ·R·ln2 not previously included in the thermodynamic tabulation for zirconolite. The molar entropies of CaHfTi2O7and CaZr0.26Hf0.74Ti2O7reported here include this zero-point entropy, and the solid solution includes an additional zero-point entropy contribution of 4.765J · K−1· mol−1to account for the random mixing of Zr4+and Hf4+on the zirconium crystallographic site. The large zero-point entropy associated with the solid solution implies a substantial entropy stabilization contribution for other nuclear waste disposition mixtures.