CRYSTAL-STRUCTURES OF SOME NIOBIUM AND TANTALUM OXIDES .5. CETA7O19

CeTa7O19 crystallizes in the hexagonal system with unit-cell dimensions (from single crystal data) a = 6.226(3), c = 19.976(8) Å, and space group P63 mcm, z = 2. The structure was solved by conventional methods with 290 counter-measured reflections (MoKα radiation), and refined using Fourier and least-squares techniques to a conventional R of 0.067 (Rw = 0.057). CeTa7O19 has a structure that consists of double layers of tantalum polyhedra that may be described either as edge-shared pentagonal bipyramids or as corner-shared octahedra with a further long (2.48 Å) contact through one edge in the plane, separated by a layer of distorted bicapped trigonal antiprismatic sites. The latter polyhedra are edge-shared with the double layers and are fully occupied by two Ce(III) and two Ta(V) ions distributed randomly over the 4-fold site. The average TaO distance in the double layers is 2.06 Å and in the (Ce, Ta) layer is 2.34 Å. © 1979.