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Structural characterization of polycrystalline (Nd,Al)-substituted zirconolite

Published online by Cambridge University Press:  11 February 2011

Pascal Loiseau,
Daniel Caurant,
Noël Baffier and
Catherine Fillet
Pascal Loiseau
Affiliation:
Laboratoire de Chimie Appliquée de l'Etat Solide, ENSCP, 11 rue Pierre et Marie Curie 75231, Paris Cedex 05, FRANCE
Daniel Caurant
Affiliation:
Laboratoire de Chimie Appliquée de l'Etat Solide, ENSCP, 11 rue Pierre et Marie Curie 75231, Paris Cedex 05, FRANCE
Noël Baffier
Affiliation:
Laboratoire de Chimie Appliquée de l'Etat Solide, ENSCP, 11 rue Pierre et Marie Curie 75231, Paris Cedex 05, FRANCE
Catherine Fillet
Affiliation:
Commissariat à l'Energie Atomique, Centre d'Etudes de la Vallée du Rhône, DEN/DIEC/SCDV/LEBM, 30207 Bagnols-sur-Cèze, FRANCE
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Abstract

Zirconolite (formally CaZrTi2O7) is a crystalline phase particularly well adapted to actinide immobilization because of its excellent long-term behavior and its good containment capacity. Most of the French studies on zirconolite deal with minor actinides that are mainly responsible for the long-term radiotoxicity of high-level radioactive wastes. For these kind of studies, trivalent minor actinides (Am3+, Cm3+) can be simulated by a lanthanide ion with an ionic radius similar to that of Nd3+. Thus, several materials having the composition Ca1-xNdxZrTi2-xAlxO7 (0 ≤ x ≤ 0.8) were prepared by solid state reaction. These polycristalline materials were first characterized by X-ray diffraction and scanning electron microscopy associated with energy dispersive X-ray analysis in order to determine the nature of the crystalline phases formed. For low neodymium content (x ≤ 0.1), electron spin resonance of Nd3+ ions revealed that a significant proportion of these ions entered into trace amounts of perovskite. Nevertheless, all Ca1-xNdxZrTi2-xAlxO7 samples with x ≤ 0.6 can be considered as almost single phase zirconolite-2M. Structure refinement by the Rietveld method of Ca0.7Nd0.3ZrTi1.7Al0.3O7 showed that Nd3+ and Al3+ ions mainly entered respectively into the calcium site and into the split five-fold coordinated titanium site. Structural characterization of Ca0.3Nd0.7ZrTi1.3Al0.7O7 and Ca0.2Nd0.8ZrTi1.2Al0.8O7 samples confirmed that these compositions led to the crystallization of almost single phase zirconolite-3O, an orthorhombic polytype of zirconolite, whose structure was also refined by the Rietveld method. Results concerning neodymium location in Ca0.7Nd0.3ZrTi1.7Al0.3O7 and Ca0.3Nd0.7ZrTi1.3Al0.7O7 were qualitatively confirmed by optical absorption spectroscopy at low temperature.

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References

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