Zirconolite—a fluorite-related superstructure

Abstract

Synthetic zirconolite, CaZrTi₂O₇, has attracted interest recently, as it has been proposed as a constituent phase of an artificial rock (SYNROC) which may immobilise, in solid solution, the elements occurring in high-level nuclear reactor wastes1. Although it is known that zirconolite is a superstructure phase, derived by ordering of an anion-deficient fluorite (CaF₂−_x or Mo₂−_x) type of substructure², the crystal structure proper, and thus the nature of the ordering, is not known. This has hampered discussion of the crystal chemistry of the phase. A crystal structure determined from powder X-ray diffraction intensities and adequate for such discussion purposes is reported here.