Ionic Size Limits for A Ions in Brannerite (ATi 2 O 6 ) and Pyrochlore (CaATi 2 O 7 ) Titanate Structures ( A = tetraval
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Ionic Size Limits for A Ions in Brannerite (ATi2O6) and Pyrochlore (CaATi2O7) Titanate Structures ( A = tetravalent rare earths and actinides) E. R. Vance, M. L. Carter, M. W. A. Stewart, R. A. Day, B. D. Begg and C. J. Ball Materials Division, Australian Nuclear Science and Technology Organisation Menai, NSW 2234, Australia ABSTRACT The lower limit of the size of the octahedral A4+ ion in the ATi2O6 brannerite structure is just smaller than that of Ce/Pu. Attempts to expand the A ion size beyond that of Th by (a) substituting a Ba ion plus two U5+ ions for three A ions or (b) substituting one Ba plus one hexavalent ion for two A ions did not succeed. Ge, Sn and Zr substitutions in the Ti site of ThTi2O6 do not exceed 0.2 formula unit in ceramic preparations. These and other coupled substitutions in the B site of ThTi2O6 showed that the average B site size could tolerate deviations of < 1%. Ce4+ is unusually stabilised in air atmospheres at temperatures close to the melting point of 1400oC in the A site of brannerite. Lattice parameter data on different endmember ATi2O6 brannerites are given. The lower and upper size limits for the eightfold A ions in the pyrochlore structure are around 0.100 and 0.117 nm respectively. A BaUTi2O7 stoichiometry did not produce a pyrochlore structure, and when fired in either argon or air yielded a mixture of BaUTiO6, whose structure is still uncertain, plus brannerite and rutile. INTRODUCTION Brannerite and pyrochlore-structured titanates are phases of central interest in the recently-developed ceramics [1,2] designed for the immobilisation of surplus plutonium. Brannerite, UTi2O6, is normally a minor phase in the ceramic, but its abundance depends on the chemical impurities accompanying the Pu. It has an orthorhombic crystal structure and both the U and Ti ions are octahedrally co-ordinated to oxygen ions [3]. Many relevant properties have been studied only recently, namely aqueous dissolution rates [4,5], radiation damage response [5,6] and incorporation of impurities [5,7]. The pyrochlore structured titanate is a major phase in the ceramic and the composition of relevance to the present discussion is CaATi2O7 (A = tetravalent actinide). Here Ca and actinides are in 8-fold coordination, while the Ti ions are octahedrally bound to oxygens. Information on impurity incorporation in especially brannerite is largely empirical to date, so the object of this paper is to try to rationalise the impurity solid solubility limits in brannerite and pyrochlore in terms of ionic radius. Data [8] are already available to show that the lower size limit of the 8-fold A site in pyrochlore-structured CaATi2O7 is close to that of Ce4+ and Pu4+ as the substitution of only ~ 0.3-0.4 formula units (f.u.) of Zr for these ions in the A site in both cases destabilises pyrochlore and forms zirconolite (see Table 1 for
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the relevant octahedral and 8-fold crystal radii [10] for the ions used in the present investigation). Table I. Octahedral and 8-fold crystal radii (nm) after Shannon [9]. Ion U4+ U6+
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