Structural and thermal effects of Hf4+ substitution in CaZr1-xHfxTi2O7 zirconolite

There is a requirement to further understand the structural and thermal properties of candidate Pu wasteform materials, and moreover gain a better understanding of composition-driven variation in these properties as they can impact disposability. Zirconolite (CaZrTi2O7) phases are a candidate wasteform system to immobilise Pu at scale and therefore it is necessary to understand (as far as possible) the isolated influence of cation substitution on specific lattice sites. CaZr1-xHfxTi2O7 is a model system for understanding the microstructural effects of Hf4+ substitution and underpin its viability as a neutron absorbing additive that could feasibly be co-immobilised with Pu. Hf4+ was capable of wholly substituting for Zr4+ at low-to-moderate concentration (i.e. x ≤ 0.60) after which some minor Hf-phase segregation was observed. Powder X-ray diffraction, Rietveld analysis and Raman spectroscopy were consistent with Hf4+ substituting in the Zr4+ site and confirmed no additional zirconolite polytypes were formed in addition to 2M. Hf L3-edge EXAFS analysis was consistent with Hf4+ occupying the 7-fold Zr4+ site in the zirconolite-2M structure consistent with the targeted substitution scheme. The thermal diffusivity and thermal conductivity of the zirconolite ceramics was generally observed to increase with elevated Hf4+ content although no clear compositional trends were identified.