Crystal structures of some niobium and tantalum oxides. VIII. The 5Rb 2O:14.6Ta 2O 5 phase—A tunnel structure

Abstract

Rb 10Ta 29.20O 78 crystallizes in the hexagonal system with unit-cell dimensions (from single-crystal data) a = 7.503(4)Å, c = 36.348(4)Å, and space group P6 3 mmc , z = 1 . The structure was solved using three-dimensional Patterson and Fourier techniques. Of the 666 unique reflections measured by counter techniques, 515 with I ⩾ 3 σ( I) were used in the least-squares refinement of the model to a conventional R of 0.057 ( R ω = 0.039). The structure of Rb 10Ta 29.20O 78 consists of layers of corner-sharing groups of six edge-shared octahedra separated by layers of single octahedra and double hexagonal tungsten bronze-like layers, these layers being perpendicular to the hexagonal c-axis. Nine-coordinate tricapped trigonal prismatic sites between the hexagonal tungsten bronze-like layers are partially occupied by Ta(V) ions.