Enthalpies of Formation of Gd 2 (Ti 2-x Zr x )O 7 Pyrochlores
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Enthalpies of Formation of Gd2(Ti2-xZrx)O7 Pyrochlores K.B. Helean1, B.D. Begg2, A. Navrotsky1, B. Ebbinghaus3, W.J. Weber4 and R.C. Ewing5 1 Thermochemistry Facility, Department of Chemical Engineering and Materials Science, The University of California at Davis, Davis, CA 95616, U.S.A. 2 Materials Division, Australian Nuclear Science and Technology Organization, PMB 1, Menai, NSW 2234, Australia 3 Lawrence Livermore National Laboratory, Livermore, CA 94550, U.S.A. 4 Pacific Northwest National Laboratory, Richland, WA 99352, U.S.A. 5 Department of Nuclear Engineering and Radiological Sciences, The University of Michigan, Ann Arbor, MI 48109, U.S.A. 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) 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, ∆Hf = -113.4±2.7; x=0.5, ∆Hf = -94.0±3.0; x=1.0, ∆Hf = -74.2±4.9; x=1.5, ∆Hf = -64.5±2.0; x=2, ∆Hf = -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 ∆Hmix 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, 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, T∆Sconf. ≈ 10 kJ/mol. INTRODUCTION Recent estimates indicate that the United States will designate approximately 13 metric tons of weapons plutonium for immobilization [1]. A crystalline ceramic waste form is under development, largely based on the SYNROC concept [2,3]. This ceramic lies in the pseudoquaternary system consisting of CaHfTi2O7- CaPuTi2O7- CaUTi2O7- Gd2Ti2O7. One consequence of incorporating plutonium into a crystalline matrix is the loss of long-range periodicity due to the accumulation of radiation damage, i.e. metamictization. The response of the ceramic to alpha-decay damage is, therefore, of interest. Recent studies indicate that Gd2Ti2O7 pyrochlore (Fd3m) becomes metamict under heavy ion irradiation [4,5,6,7]. The material becomes amorphous concurrent with a structural transformation of pyrochlore (Fd3m) to an anion deficient fluorite (Fm3m) due to cation disorder. The higher symmetry leads to a reduction of the lattice parameter, a, by a factor of two and the elimination of diffraction maxima associated with the pyrochlore super-cell. The substitution of Zr for Ti in Gd2(Ti2-xZrx)O7 was shown to increase the dose required for amorphization until, for pure
Gd2Zr2O7, the material remained crystalline un
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