Chemical twinning of the pyrochlore structure in the system Bi2O3-Fe2O3-Nb2O5

New ternary bismuth iron niobates having structures based on chemical twinning of pyrochlore are described. Bi5.67Nb10FeO35 has hexagonal symmetry, P6(3)/mmc, a = 7.432(1) angstrom, c = 31.881(2) angstrom, Z = 2 and Bi9.3Nb16.9Fe1.1O57.8 has rhombohedral symmetry, R-3m, a = 7.433(1) angstrom, c = 77.488(2) angstrom, Z = 3. The structures of both phases were determined and refined to R-1 = 0.04 using single-crystal X-ray data. They can be described as being derived from the pyrochlore structure by chemical twinning on (111)(py) oxygen planes. The chemical twin operation produces pairs of corner-connected hexagonal tungsten bronze (HTB) layers as in the HTB structure, so the structures may alternatively be described as pyrochlore:HTB unit-cell intergrowth structures. In the hexagonal phase the pyrochlore blocks have a width of 12 angstrom, whereas the rhombohedral phase has pyrochlore blocks of two widths, 6 and 12 angstrom, alternating with HTB blocks. It is proposed that the previously reported binary 4Bi(2)O(3):9Nb(2)O(5) phase has a related structure containing pyrochlore blocks all of width 6 angstrom. A feature of the structures is partial occupancy (similar to 65%) of the Bi sites and displacement of the Bi atoms from the ideal pyrochlore A sites towards the surrounding oxygen atoms, as observed in Bi-containing pyrochlores. (c) 2006 Elsevier Inc. All rights reserved.