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Enthalpies of Formation of Gd2(Ti2-xZrx)O7 Pyrochlores

Published online by Cambridge University Press:  21 March 2011

K.B. Helean ,
B.D. Begg ,
A. Navrotsky ,
B. Ebbinghaus ,
W.J. Weber  and
R.C. Ewing
K.B. Helean
Affiliation:
Thermochemistry Facility, Department of Chemical Engineering and Materials Science, The University of California at Davis, Davis, CA 95616, U.S.A.
B.D. Begg
Affiliation:
Materials Division, Australian Nuclear Science and Technology Organization, PMB 1, Menai, NSW 2234, Australia
A. Navrotsky
Affiliation:
Thermochemistry Facility, Department of Chemical Engineering and Materials Science, The University of California at Davis, Davis, CA 95616, U.S.A.
B. Ebbinghaus
Affiliation:
Lawrence Livermore National Laboratory, Livermore, CA 94550, U.S.A.
W.J. Weber
Affiliation:
Pacific Northwest National Laboratory, Richland, WA 99352, U.S.A.
R.C. Ewing
Affiliation:
Department of Nuclear Engineering and Radiological Sciences, The University of Michigan, Ann Arbor, MI 48109, U.S.A.
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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.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 663: Symposium – Scientific Basis for Nuclear Waste management XXIV , 2000 , 691
Copyright
Copyright © Materials Research Society 2001

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