The thermodynamics of formation, molar heat capacity, and thermodynamic functions of ZrTiO4(cr)

As part of an ongoing study of titanate-based ceramic materials for the dis posal of surplus weapons-grade plutonium, we report thermodynamic propertie s of a sample of zirconium titanate (ZrTiO4) quenched from a high-temperatu re synthesis, The standard enthalpy of Formation Delta H-f(m)o was obtained by using high-temperature oxide-melt solution calorimetry. The molar heat capacity C-p,C-m, was measured from T = 13 K to T = 400 K in an adiabatic c alorimeter and extrapolated to T=1800 K by using an equation fitted to the low-temperature results. The results at T = 298.15 K are Delta H-f(m)o = - (2024.1 +/- 4.5) kJ . mol(-1) Delta S-T(o)m(o) = (116.71 +/- 0.31) J . K-1 . mol(-1), and Delta (f)G(m)(o) = -(1915.8 +/- 4.5) kJ . mol(-1); the molar entropy includes a contribution of 2Rln 2 to account for the random mixing of Zr4+ and Ti4+ on a four-fold crystallographic site. Values for the stan dard molar Gibbs energies and enthalpies of formation of ZrTiO4, Delta (f)G (m)(o) and Delta H-f(m)o, and for the free energies and enthalpies for the reaction to form ZrTiO4(cr) from ZrO2(cr) and TiO2(cr), are tabulated over the temperature interval, 0 less than or equal to (T/K) less than or equal to 1800. From these results, we conclude that ZiTiO(4) is not stable with r espect to (ZrO2 + TiO2) at T = 298.15 K, but becomes so at T = (1250 +/- 15 0) K. (C) 2001 Academic Press.