Crystal Chemistry and Cation Ordering in Zirconolite 2M

Abstract

ABSTRACTStructural studies of single phase or nearly single phase zirconolite ceramic samples have been conducted using electron microscopy and microanalysis, X-ray diffraction, neutron diffraction, and spectroscopic methods. We show that it is possible to produce a complete series of zirconolite 2M samples with substitution of 2Ti by Nb+Fe in the HTB layer. The samples are single phase up to about 80% Nb +Fe substitution, with the appearance of a minor perovskite phase at higher Nb+Fe levels. Electron probe microanalysis reveals that the samples are homogeneous and close to their nominal compositions, except for those containing perovskite, which have a slight excess of Zr and a deficiency in the Fe content. The lattice parameters and the positions of certain Raman bands are non-linear as a function of composition, suggesting the possibility of cation ordering over the three available Ti sites within the HTB layer. Rietveld refinement of Synchrotron X-ray powder data for the Nb+Fe end-member have been conducted for the disordered case and for six trial models each with a different ordering scheme. Results of this exercise indicate that Fe preferentially occupies the Ti2 (split) site with partial ordering of Nb and the remaining Fe over the Ti1 and Ti2 octahedra. The preference of Fe for the five coordinated Ti2 site has been confirmed by 57Fe Mossbauer spectroscopy.