Formation enthalpies of rare earth titanate pyrochlores |
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| Authors: | K.B. Helean S.V. Ushakov A. Navrotsky J. Lian J.M. Farmer |
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| Affiliation: | a Department of Chemical Engineering and Materials Science, Thermochemistry Facility, NEAT ORU, The University of California at Davis, One Sheilds Avenue, Davis, CA 95616, USAb Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109-2104, USAc Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2104, USAd Oak Ridge National Laboratory, Solid State Division, Oak Ridge, TN 37831-6056, USA |
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| Abstract: | High-temperature oxide melt solution calorimetry and Rietveld refinements of powder X-ray diffraction data were used to investigate the structure (Fd3m; Z=8) and energetics of a series of RE2Ti2O7 (RE=Sm-Lu) compounds with the pyrochlore structure as well as La2Ti2O7 with a layered perovskite-type structure. All of the RE-titanates were found to be stable in enthalpy with respect to their oxides. In the pyrochlore series, Lu2Ti2O7 was least stable in enthalpy (ΔHf-ox at 298 K=−56.0±4.0 kJ/mol); the most stable materials were Gd-, Eu-, and Sm2Ti2O7 with ΔHf-ox at 298 K=−113.4±2.7, −106.1±4.2, −115.4±4.2 kJ/mol, respectively. In general, as the radius ratio of the A- to B-site cations, RA/RB, decreases, the pyrochlore structure becomes less stable. The trend of ionic radius of the RE3+ cation vs. ΔHf-ox at 298 K is non-linear and approximately parallels the increasing “resistance” to ion-beam-induced amorphization as RA/RB decreases. |
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| Keywords: | Thermodynamics Pyrochlore Nuclear waste Enthalpy |
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