Chemical twinning of the pyrochlore structure in the system Bi 2O 3–Fe 2O 3–Nb 2O 5

Abstract

New ternary bismuth iron niobates having structures based on chemical twinning of pyrochlore are described. Bi 5.67Nb 10FeO 35 has hexagonal symmetry, P6 3 /mmc, a = 7.432 ( 1 ) Å , c = 31.881 ( 2 ) Å , Z = 2 and Bi 9.3Nb 16.9Fe 1.1O 57.8 has rhombohedral symmetry, R-3 m, a = 7.433 ( 1 ) Å , c = 77.488 ( 2 ) Å , 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 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 4Bi 2O 3:9Nb 2O 5 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.