The crystal structure of Ca7Zr7Ta6O36 refined using synchrotron-radiation data

Single-crystal X-ray diffraction data [synchrotron radiation; lambda = 1.26 82 (4) Angstrom] are used to solve and refine the crystal structure of hept acalcium hexatantalum hepta-zirconium hexatriacontaoxide, Ca7Zr7Ta6O36. The structure adopts space group Fddd with cell dimensions a = 36.394 (1), b = 7.3674 (5), c = 31.006 (2) Angstrom. The structure was solved by direct me thods. Refinement using 1299 unique reflections leads to final values of R = 0.031 and wR = 0.034. The refined metal-atom ordering scheme is far from fully ordered and reminiscent of the A/B metal-atom ordering characteristic of the pyrochlore structure type. Bond-valence sums are calculated to conf irm the plausibility of the crystal chemistry of Ca7Zr7Ta6O36.