Structural relations between weberite and zirconolite polytypes—refinements of doped 3T and 4M Ca 2Ta 2O 7 and 3T CaZrTi 2O 7

Abstract

New weberite-type Ca 2Ta 2O 7 and zirconolite-type CaZrTi 2O 7 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, Ca 0.8Ti 1.35Zr 1.3Th 0.15Al 0.4O 7, has a=7.228(1), c=16.805(1) Å. The 3T weberite-type polytype, Ca 1.92Ta 1.92Nd 0.08Zr 0.08O 7, has a=7.356(1), c=18.116(1) Å. Both 3T polytypes have space group P3 121, Z=6. The 4M Ca 2Ta 2O 7 polytype has the same composition, from electron microprobe analyses, as the 3T polytype, and has cell parameters: a=12.761(1), b=7.358(1), c=24.565(1) Å, β=100.17(1)°, 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 O M 4 tetrahedra is not maintained in the Ca 2Ta 2O 7 weberite-type polytypes, even though they have a fluorite-like fcc packing of metal atoms. One of the oxygen atoms moves from a tetrahedral Ta 3Ca interstice to an adjacent Ta 2Ca 4 octahedral interstice in the weberite polytypes.