Structural Effect of Pu Substitutions on the Zr-Site in Zirconolite
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Structural Effect of Pu Substitutions on the Zr-Site in Zirconolite B.D. Begg, R.A. Day and A. Brownscombe Materials Division, ANSTO, PMB 1, Menai, NSW, 2234, Australia. Email: [email protected] ABSTRACT As the level of Pu4+ substituted on the Zr-site in CaZr1-xPuxTi2O7 zirconolite increased, from x=0.1 to 0.6, a series of structural transitions occurred from zirconolite-2M to zirconolite-4M and subsequently from zirconolite-4M to pyrochlore. The solid-solution limit for Pu4+ substituted on the Zr-site in zirconolite-2M was ~ 0.15 formula units. Zirconolite-4M was only stable over a narrow compositional range, centered about CaZr0.59Pu0.41Ti2O7, whilst the pyrochlore structure was stabilized with CaZr0.4Pu0.6Ti2O7 stoichiometry. The stability of the zirconolite polytypes is therefore sensitive to the average effective ionic size of the ions occupying the seven-coordinated Zr-site. The reduction in Pu from Pu4+ to Pu3+ destabilized the zirconolite-4M, producing a mixture of perovskite and possibly zirconolite-3T. The CaZr0.4Pu0.6Ti2O7 pyrochlore was also predominantly transformed to perovskite as a result of this reduction of Pu. INTRODUCTION In 1998 a synroc-based titanate ceramic was selected for the geological immobilization of impure surplus Pu in the US [1]. An important factor in this decision was the twenty years of synroc research for the immobilization of high-level waste from reprocessed nuclear fuel. Within the context of Pu immobilization, the principal actinide-bearing phases that have been investigated have been zirconolite and the closely related titanate-based pyrochlore. Both structures are extremely durable and exhibit a high degree of compositional flexibility [2,3]. Zirconolite exhibits a range of stoichiometries and polytypes, described by the general formula CaZrxTi3-xO7. The zirconolite polytypes are characterized by the number of layers that constitute a repeat and the manner in which they are stacked. Like pyrochlore, all the zirconolite polytypes are derived from an anion-deficient fluorite superstructure. The polytype most commonly referred to as zirconolite is zirconolite-2M. This is monoclinic, has a two layer repeat and displays CaZrxTi3-xO7 stoichiometry for 0.80
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