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, P63/mmc, image, image, image and Bi9.3Nb16.9Fe1.1O57.8 has rhombohedral symmetry, R-3m, image, image, image. The structures of both phases were determined and refined to image 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 Å, whereas the rhombohedral phase has pyrochlore blocks of two widths, 6 and 12 Å, alternating with HTB blocks. It is proposed that the previously reported binary 4Bi2O3:9Nb2O5 phase has a related structure containing pyrochlore blocks all of width 6 Å. A feature of the structures is partial occupancy (∼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.