Analysis and structural determination of Nd-substituted zirconolite-4M

The structure of a new polytype of zirconolite, zirconolite-4M, has been determined using X-ray and neutron powder diffraction, high resolution transmission electron microscopy, and selected area electron diffraction. Zirconolite-4M occurs when zirconolite is doped with 0.5-0.8 Nd per formula unit. Its structure consists of four hexagonal tungsten bronze (HTB) type layers interleaved alternately with layers of Ca, Zr polyhedra (as in zirconolite-2M) and Ca, Ti polyhedra (as in pyrochlore). Nd substitutes on the Ca and Zr sites. The compositions of the zirconolites were determined using an extrapolation technique based on an analysis of the impurity lines in the diffraction pattern. Cation site occupancies were determined with composition constraints applied and these were consistent with the expected zirconolite-4M cation site occupancies. Observed zirconolite-4M lattice parameters correlated with expected values for a zirconolite and pyrochlore type stacking sequence. (C) 1997 Academic Press.