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Crystal chemistry and optimization of conductivity in 2A, 2M and 2H alkaline earth lanthanum germanate oxyapatite electrolyte polymorphs

Lookup NU author(s): Dr Stevin PramanaORCiD

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Abstract

Calcium (La10 - xCaxGe 6O27 - x/2, 0 ≤ x ≤ 2.57) and barium (La10 - xBaxGe 6O27 - x/2, 0 ≤ nominal x ≤ 3) doped lanthanum germanate apatites were synthesized by high temperature solid-state sintering. The lattice parameters obtained from Rietveld refinement of powder X-ray diffraction data showed that pseudomorphic transformations from 2A to 2H occurred as the alkaline earth was introduced. The likely appearance of an intervening 2M pseudomorph was not confirmed directly in this study; however, compositionally dependent inflections in cell constants and abrupt changes in oxygen mobility as a function of temperature support its existence, particularly for low alkaline earth contents. Oxygen excess apatites La 9AEGe6O26.5 (AE = Ca, Sr, Ba) prove most promising as solid oxide fuel cell (SOFC) electrolytes at intermediate temperatures, by delivering the optimal compromise between higher mobile oxygen concentration and lowering of activation energy. In contrast, stoichiometric La8AE2Ge6O26 showed inferior conductivity, because although the activation energy was reduced, the proportion of extrastoichiometric oxygen was negligible. Atomistic modeling suggests that interstitial oxygen is located near (0, 1/2, 1/2) and causes the O3 position to be split statistically into the (0.050, 0.278, 0.587) and (0.084, 0.335, 0.565) sites. These co-existing locations may facilitate inter-tunnel oxygen transport. © 2010 Elsevier B.V. All rights reserved.

Publication metadata

Author(s): Pramana SS, Baikie T, Kendrick E, Schreyer MK, Slater PR, White TJ

Publication type: Article

Publication status: Published

Journal: Solid State Ionics

Year: 2010

Volume: 181

Issue: 25-26

Pages: 1189-1196

Print publication date: 26/08/2010

Online publication date: 21/07/2010

ISSN (print): 0167-2738

ISSN (electronic): 1872-7689

Publisher: Elsevier BV

URL: https://doi.org/10.1016/j.ssi.2010.06.044

DOI: 10.1016/j.ssi.2010.06.044

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