Crystal structures of some niobium and tantalum oxides. II. The 4Rb 2O:11Nb 2O 5 phase—A tunnel structure

Abstract

Rb 12.20Nb 33.56O 90 crystallizes in the trigonal system with unit-cell dimensions (from single-crystal data) a = 7.527(6), c = 43.17(2)Å and space group R 3m, z = 1 . The structure was solved using three-dimensional Patterson and Fourier techniques. Of the 843 unique reflections measured by counter techniques, 675 with I ⩾ 3 σ( I) were used in the least-squares refinement of the model to a conventional R of 0.072 ( wR = 0.055). The structure of Rb 12.20Nb 33.56O 90 consists of layers of corner-sharing groups of six edge-shared octahedra separated by pyrochlore-like layers of octahedra. The structure can be considered to be derived from that of pyrochlore by the ordered omission of layers of single octahedra.