Ta-Nb order in the crystal structure of niobium-rich calciotantite

The crystal structure of niobium-rich calciotantite, Ca (Ta2.82Nb1.18) Oil, hexagonal, a 6.2261(6), c 12.280(1) Angstrom, V412.18(8) Angstrom(3), spac e group P6(3)22, Z = 1, D-calc. = 6.735 g/cm(3), has been refined to an R i ndex of 1.8% based on 414 observed (4 sigma) reflections measured with MoK alpha X-radiation. There is one unique Ca site occupied by Ca and coordinat ed by eight O-atoms with a <Ca-O> distance of 2.512 Angstrom. There are two unique Ta sites both occupied by Ta and Nb; the Ta(1) site is coordinated by seven O atoms in a distorted pentagonal dipyramidal arrangement, and the Ta(2) site is coordinated by six O atoms in a distorted octahedral arrange ment. The Ta(2)O-6 and CaO8 polyhedra occupy alternate vertices of a 6(3) n et to form an open network of edge-sharing polyhedra of the form [CaTaO8]. The Ta(1)O-7 polyhedra share four of their pentagonal edges to form a [Ta3O 11] sheet with triangular holes at the vertices of a 6(3) net. These sheets alternate along [001] to form a dense framework structure. The structure o f calciotantite is topologically similar to that of natrotantite, Na2Ta4O11 ; the [Ta3O11] sheets are topologically identical in both structures, and t he [CaTaO8] and [Na2TaO8] sheets are the same if they are written as [squar e CaTaO8] and [Na2TaO8]. The [Ta3O11] sheet is also topologically identical to the [(UO2)(3 phi 5)] (phi = O,OH) in billietite, protasite, becquerelit e and alpha-U3O8. Niobium is slightly preferentially ordered (relative to T a) at the Ta(1) site, indicating a slight difference in the crystal-chemica l behavior of Ta and Nb.