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Thermochemistry of Substituted Perovskites in the NaTixNb1-xO3-0.5x System

Published online by Cambridge University Press:  01 February 2011

Hongwu Xu
Affiliation:
Thermochemistry Facility, Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, CA 95616, U.S.A.
Alexandra Navrotsky
Affiliation:
Thermochemistry Facility, Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, CA 95616, U.S.A.
M. Lou Balmer
Affiliation:
Pacific Northwest National Laboratory, P.O. Box 999, MSIN K8-93, Battelle Blvd., Richland, WA 99352, U.S.A.
Yali Su
Affiliation:
Pacific Northwest National Laboratory, P.O. Box 999, MSIN K8-93, Battelle Blvd., Richland, WA 99352, U.S.A.
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Abstract

A suite of perovskite phases with the compositions NaTixNb1-xO3-0.5x, 0 ≤ x ≤ 0.2, has been synthesized for the first time using the sol-gel method. Rietveld analysis of powder XRD data reveals that with increasing Ti content, the orthorhombic perovskite structure becomes more cubic-like, as evidenced by the smaller differences among its three cell parameters. Enthalpies of formation of the synthesized phases from the constituent oxides and from the elements have been determined by drop solution calorimetry into molten 3Na2O·4MoO3 solvent at 974 K. As Ti4+ substitutes for Nb5+, the formation enthalpies become less exothermic in a nearly linear manner. This behavior suggests that the Ti→Nb substitution destabilizes the perovskite structure, presumably because of the concomitant occurrence of O2- vacancies, which compensate the charge imbalance between Ti4+ and Nb5+ in the structure.

Type
Research Article
Copyright
Copyright © Materials Research Society 2002

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Thermochemistry of Substituted Perovskites in the NaTixNb1-xO3-0.5x System
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