Molar heat capacity and thermodynamic functions of zirconolite CaZrTi2O7

Thermodynamics of formation for zirconolite (CaZrTi2O7) fromT=298.15 K toT=1500 K

“…Assuming that CaZrTi 2 O 7 is thermodynamically stable with respect to (CaTiO 3 + ZrTiO 4 ), an upper bound can be placed on the enthalpy of formation of ZrTiO 4 . If the T · r S o m term for reaction (4) can be neglected { r S o m for a (solid + solid) reaction is generally small}, then r H m ≈ r G m for this reaction which, in turn, is ≤ 0 as shown in equation (5): It is noted elsewhere (12) that the measured third-law entropy for zirconolite was bracketed by values obtained from the sum of the entropies of the constituent binary oxides (CaO, TiO 2 , and ZrO 2 ) and a perovskite-bearing assemblage (CaTiO 3 (4) is −(0.38 ± 0.24) kJ·mol −1 , and we conclude that our assumption is reasonable.…”

“…The value obtained (table 6, (12) and additional literature values (13,14,18) allows us to calculate the Gibbs free energy of formation, From an examination of the (CaO, TiO 2 , ZrO 2 ) phase diagram, (20,21) it is possible to consider the stability of zirconolite relative to various assemblages of binary and ternary oxides. Three additional reactions can be proposed:…”

“…Our value agrees remarkably well with the prediction of C p,m = 307. (12) we can now calculate the formation energetics of zirconolite at temperatures relevant to the optimization of nuclear waste form synthesis. The results are presented in table 7.…”

“…With this assumption, the sample composition is found to be {n(ZrO 2 )/n(CaZrTi 2 O 7 )} = 8.24·10 −2 . All calorimetric experiments (12) involving this sample were corrected for the presence of ZrO 2 (baddeleyite) by assuming a mechanical mixture of composition (8.24·10 −2 mol ZrO 2 + 1 mol CaZrTi 2 O 7 ). The results of enthalpy of drop-solution measurements using the bubbling technique for TiO 2 were obtained at T = 973 K and T = 1046 K. These results are listed in table 3.…”

“…(5) Early experiments with oxide-melt solution calorimetry using molten lead borate (2PbO·B 2 O 3 ) near T = 973 K as a solvent showed that the dissolution of TiO 2 , ZrO 2 , and compounds formed from these oxides was problematic. (10,11) However, recent advances using oxide-melt solution calorimetry, (10) some of which are described in detail below, make possible the determination of standard molar enthalpies of formation of oxides containing Ti and Zr at T = 298.15 K. We report in a following paper (12) the heat capacities and the third-law entropy of zirconolite from T = 20 K to T = 400 K obtained by using adiabatic calorimetry with appropriate extrapolations to T = 1500 K. Those results are used here to report the enthalpy and Gibbs free energy of formation of zirconolite from T = 298.15 K to T = 1500 K.…”