Molar heat capacities and thermodynamic functions of CaHfTi2O7(cr) and CaZr0.26Hf0.74Ti2O7(cr)

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 are Delta(0)(298.15) S-K(m)o (196.98 +/- 0.39) J (.) K-1 (.) mol(-1) for CaHfTi2O7 and Delta(0)(291.15) S-K(m)o = (199.76 +/- 0.39) J (.) K-1 (.) mol(-1) for 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 CaHfTi2O7 and CaZr0.26Hf0.74Ti2O7 reported here include this zero-point entropy, and the solid solution includes an additional zero-point entropy contribution of 4.765 J (.) K-1 (.) mol(-1) to 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. (C) 2001 Academic Press.