Predictions of strontium accommodation in A 2 B 2 O 7 pyrochlores
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Robin W. Grimes Department of Materials, Imperial College, London SW72BP, United Kingdom, and Los Angeles National Laboratory, Los Alamos, New Mexico 87545
John Maguire AFRL / MLMR, Air Force Research Laboratory, Wright Patterson Air Force Laboratory, Ohio 45433
Kurt Sickafus Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received 14 March 2002; accepted 16 May 2002)
A2B2O7 pyrochlore oxides are being considered as potential host materials for the immobilization of fission products. It is therefore important to establish the relative ability of these compounds to accommodate fission product ions. We address this issue by using computer simulations to predict the structures and relative equilibrium energies associated with solution of Sr2+ over an extensive compositional range. Results indicate that strontium is accommodated via substitution for A host cations with oxygen vacancy compensation. This results in a nonstoichiometric composition. Optimum compositions and defect clusters structures are identified by constructing contour energy maps.
I. INTRODUCTION A. Radiation tolerance
A2B2O7 pyrochlore oxides have attracted considerable attention as potential host materials for the immobilization of fission products and plutonium.1,2 Most of the interest has focused on the ability of pyrochlore and the related fluorite lattice to tolerate radiation damage. Indeed, pyrochlores were initially considered because natural pyrochlore (NaCaNb2O6F) contains U and Th and its long-term tolerance to self-irradiation could therefore be studied.3–5 Recently the radiation tolerance of an extensive series of A2B2O7 oxides was predicted by relating tolerance to the ability of the lattice to accommodate point defects, in particular cation antisite pairs, which form when trivalent cations swap sites with tetravalent cations.2 The previous study, which used energies calculated by atomic-scale computer simulations, concluded that the propensity of a material with composition A2B2O7 to accommodate radiation-induced defects depended strongly on the choice of A and B cations. Following experimental verification, 2 Sickafus et al. suggested that damage evolution through irradiation depends upon (i) the ability of the crystal structure to accommodate elevated defect concentrations and (ii) the nature of consequential interactions between the defects. Defect interactions can include annihilation through J. Mater. Res., Vol. 17, No. 8, Aug 2002
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interstitial-vacancy recombination or the formation of clusters precipitating as interstitial dislocation loops and voids. The latter is well known to cause void swelling.6,7 The activation energies for oxygen migration in pyrochlore oxides have also been studied through atomistic simulation.8 As with fluorites, oxygen migration in the pyrochlore lattice proceeds via a vacancy-mediated mechanism.9 Interestingly in certain compositions the oxygen vacancy can form a split configuration.8 Since we expect a suitable host for fission
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