PNNL

Abstract

A calorimetric investigation of the enthalpies of formation of Gd2(Ti2-xZrx)O7, where 0# x # 2 is underway. All samples exhibit pyrochlore (Fd3m) structural characteristics. However, where x=2 significant local disorder is observed in the Raman spectra. Preliminary data for the enthalpies of formation from the oxides in kJ/mol are: x=0, DHf = -113.4?2.7; x=0.5, DHf = -94.0?3.0; x=1.0, DHf = -74.2?4.9; x=1.5, DHf = -64.5?2.0; x=2, DHf = -52.2?4.8. Two additional samples, Gd1.80Zr2.15O7.00 (pyrochlore) and Gd2.15Zr1.87O7.00 (fluorite), were also studied. Their enthalpies of formation from the oxides in kJ/mole are -50.9?3.3 and -46.4?3.4 respectively. The substitution of Zr for Ti in Gd-pyrochlore destabilizes the material in enthalpy by approximately 60 kJ/mol. The DHmix for the Gd2(Ti2-xZrx)O7 solid-solution series is positive and can be described by a regular solution formalism with an estimated interaction parameter, W = +20 kJ/mol. The results of this study suggest that the pyrochlore to fluorite transition enthalpy in Gd2Zr2O7 is small, of the order of the configurational entropy contribution due to cation disorder at the transition temperature, TDSconf. " 10 kJ/mol.