Solid solution of metal oxides in the zirconolite phase CaZrTi2O7. I. Heterotype solid solutions

Abstract Structure refinement based on powder X-ray diffraction intensities has been carried out on zirconolite phases containing ∼10 mole% of various oxides in solid solution. Unambiguous crystallographic location of solute cations was not always possible, but it is shown that the large cations Th iv , Nd iii , Gd iii , Yb iii , and Sc iii substitute for Ca ii and Zr iv in the structure, and that the smaller Fe iii , Co ii , Mg ii , Ta v , and Sn iv substitute for Ti iv , while some exchange of Ti iv and Zr iv also occurs. There is evidence that both cation and anion arrays are intact in these solid solutions, so that the overall stoichiometry is always M 4 O 7 , i.e., the formal charges of substituting cations are mutually dependent. It is shown that the structure will tolerate substitution of Nd iii or Th iv for Ca ii only if Zr iv is simultaneously substituted by cations of lower charge, e.g., Sc iii or Mg ii : if these are not available, an appropriate amount of Ti iii becomes stabilized in the structure, and this may be supplied from a suitable excess of TiO 2 in the reaction mixture. The extent of the heterotype solid solutions has not been explored: however, in the case of Sn iv , substitution for Ti iv can be complete. Zirconolite-related phases with rhombohedral substructure that form at high solute concentrations are discussed.