The crystal structure of synthetic natrotantite

Ideal natrotantite has the composition Na2Ta4O11, space group R-3c, a = 6.2092(1), c = 36.619(1) Å, Z = 6. The structure of synthetic natrotantite has been refined by the Rietveld method to Rp = 8.67, Rwp = 9.24, RB = 2.33 % using X-ray powder diffractometer data. The structure consists of layers of (l) edge-sharing NaO7 polyhedra and TaO7 octahedra and (2) edge-sharing TaO7 pentagonal bipyramids, alternating along Z. Natrotantite is topologically identical to, but not isomorphous with Na2Nb4O11 ; the Na2Nb4O11 structure is a monoclinic distortion of the natrotantite structure. The structure is also similar to that of calciotantite from which it differs in its layer (l) topology and in its layer stacking. Natural natrotantites are typically non-stoichiometric ; Na2-xTa4O11-x(OH,F)x is proposed as a more realistic formula for the mineral.