Enthalpies of formation of Gd₂(Ti_2-xZr_x)O₇ pyrochlores

A calorimetric investigation of the enthalpies of formation of Gd₂(Ti_2-xZr_x)O₇, where 0≤ x ≤ 2 is underway. All samples exhibit pyrochlore (Fd3 m) peaks in their XRD patterns. 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, ΔH_f = -113.4±2.7; x=0.5, ΔH_f = -94.0±3.0; x=1.0, ΔH_f = -74.2±4.9; x=1.5, ΔH_f = -64.5±2.0; x=2, ΔH_f = -52.2±4.8. Two additional samples, Gd_1.80Zr_2.15O_7.00 (pyrochlore) and Gd_2.15Zr_1.87O_7.00 (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. Replacing Ti with Zr, i.e. when x=2, destabilizes the pyrochlore in enthalpy by approximately 60 kJ/mol. The ΔH_mix for the Gd₂(Ti_2-xZr_x)O₇ solid-solution series is positive and can be described by a regular solution formalism with an estimated interaction parameter, Ω = +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, TΔS_conf. ≈ 10 kJ/mol.