Enthalpies of Formation of Gd₂(Ti_2-xZr_x)O₇ Pyrochlores

Abstract

A calorimetric investigation of the enthalpies of formation of Gd2(Ti2-xZrx)O7, where 0≤ × ≤ 2 is underway. All samples exhibit pyrochlore (Fd3m) 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, ΔHf = -113.4±2.7; x=0.5, ΔHf = -94.0±3.0; x=1.0, ΔHf = -74.2±4.9; x=1.5, ΔHf = -64.5±2.0; x=2, ΔHf = -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. Replacing Ti with Zr, i.e. when x=2, destabilizes the pyrochlore in enthalpy by approximately 60 kJ/mol. The ΔHmix 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, ŝ = +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, TΔSconf. ≍ 10 kJ/mol.

Phase equilibria in the ZrO2–GdO1.5 system at 1400–1700°C

“…This assumption would explain why virtually the same homogeneity range (45 ± 1 to 55 ± 1 mol% GdO 1.5 ) for the pyrochlore phase is reported by several groups in the literature [3,10,19], and also found in this work by XRD and TEM. It is also consistent with the results of recent calorimetric study [35], where the enthalpy difference between pyrochlore and fluorite structures of Gd 2 Zr 2 O 7 was estimated as 10 kJ mol −1 . However, further experimental and theoretical studies are necessary for a quantitative determination of the long-range order parameter as a function of temperature and composition.…”

Up-Date of a Thermodynamic Database of the ZrO2-Gd2O3-Y2O3-Al2O3 System for TBC Applications

“…The tie-lines in the ZrO 2 -Gd 2 O 3 -Al 2 O 3 system were used to maintain consistency with the isothermal section at 1523 K [15] by adjusting the enthalpy of formation of GdAM phase. The calculated thermodynamic data for the Gd 2 O 3 -Al 2 O 3 and ZrO 2 -Gd 2 O 3 systems in comparison with experimental data [16,24,[43][44][45][46][47] are given in Table 4. Thermodynamic descriptions of the ZrO 2 -Al 2 O 3 [15,25] and …”

Phase diagram of the ZrO2–Gd2O3–Al2O3 system

“…The parameters of the ZrO 2 -Gd 2 O 3 system have been assessed using calorimetric data for the phases with pyrochlore and fluorite structures [16][17][18]33,34 and phase equilibrium data. 5,[35][36][37][38] The system ZrO 2 -Al 2 O 3 has been assessed by Fabrichnaya et al 21 However, the parameters of this system have been re-assessed in this study because of the necessity to use other models for the fluorite and tetragonal phases than were used in work.…”