Structural relations between weberite and zirconolite polytypes-refinements of doped 3T and 4M Ca2Ta2O7 and 3T CaZrTi2O7

New weberite-type Ca2Ta2O7 and zirconolite-type CaZrTi2O7 polytypes have been prepared by doping with Nd/Zr and Th/Al, respectively, and their structures have been refined using single-crystal X-ray diffraction intensity data. The 3T zirconolite polytype, Ca0.8Ti1.35Zr1.3Th0.15Al0.4O7, has a = 7.228(l), c = 16.805(l) Angstrom. The 3T weberite-type polytype, Ca1.92Ta1.92Nd0.08Zr0.08O7, has a = 7.356(l), c = 18.116(l) Angstrom. Both 3T polytypes have space group P3(1)21, Z = 6. The 4M Ca2Ta2O7 polytype has the same composition, from electron microprobe analyses, as the 3T polytype, and has cell parameters: a = 12.761 (1), h = 7.358(l) c = 24.565(l) Angstrom, beta = 100.17(l)degrees, space group C2, Z = 16. The structural relationships between the different zirconolite and weberite polytypes are discussed. A consideration of the structures from the viewpoint of anion-centered tetrahedral arrays shows that zirconolite can be considered as an anion-deficient fluorite derivative phase. However, the fluorite-type topology of edge-shared OM4 tetrahedra is not maintained in the Ca2Ta2O7 weberite-type polytypes, even though they have a fluorite-like fcc packing of metal atoms. One of the oxygen atoms moves from a tetrahedral Ta3Ca interstice to an adjacent Ta2Ca4 octahedral interstice in the weberite polytypes. (C) 2003 Elsevier Inc. All rights reserved.