Zirconolite (formally CaZrTi_2O_7) 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 (Am~(3+),Cm~(3+)) can be simulated by a lanthanide ion with an ionic radius similar to that of Nd~(3+).Thus,several materials having the composition Ca_(1-x)Nd_xZrTi_(2-x)Al_xO_7 (0<=x<=0.08) 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 Nd~(3+) ions revealed that a significant proportion of these ions entered into trace amountsof perovskite.Nevertheless,all Ca_(1-x)Nd_xZrTi_(2-x)Al_xO_7 samples with x<=0.6 can be considered as almost single phase zirconolite-2M.Structure refinement by the Rietveld method of Ca_(0.7)Nd_(0.3)ZrTi_(1.7)Al_(0.3)O_7 showed that Nd~(3+) and Al~(3+) ions mainly entered respectively into the calcium site ad into the split five-fold coordinated titanium site.Structural characterization of Ca+_(0.3)Nd_(0.7)ZrTi(1.3)Al_(0.7)O_7 and Ca_(0.2)Nd_(0.8)ZrTi_(1.2)Al_(0.8)O_7 sampls 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.Reslts concerning neodymium location in Ca_(0.7)Nd_(0.3)ZrTi_(1.7)Al_(0.3)O_7 and Ca_(0.3)Nd_(07)ZrTi_(1.3)Al_(0.7)O_7 were qualitatively confirmed by optical absorption spectroscopy at low temperature.
展开▼
