Chemical Twinning of the Pyrochlore Structure in the System Bi2O3-Nb2O5

Published

January 28, 2007

Author(s)

Ian E. Grey, W G. Mumme, Terrell A. Vanderah, Robert S. Roth, C Bougerol-Chaillout

Abstract

New ternary bismuth iron niobates having structures based on chemical twinning of pyrochlore are described. Bi5.67Nb10FeO35 has hexagonal symmetry, P63/mmc, a = 7.432(1) , c = 31.881(2) , Z = 2 and Bi9.3Nb16.9Fe1.1O57.8 has rhombohedral symmetry, R-3m, a = 7.433(1) , c = 77.488(2) , Z = 3. The structures of both phases were determined and refined to R1 = 0.04 using single-crystal X-ray data. They can be described as being derived from the pyrochlore structure by chemical twinning on (1 1 1)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 , whereas the rhombohedral phase has pyrochlore blocks of two widths, 6 and 12 , alternating with HTB blocks. It is proposed that the previously reported binary 4Bi2O3:9Ta2O5 phase has a related structure containing pyrochlore blocks all of width 6 . 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.

Citation

Journal of Solid State Chemistry

Volume

180

Issue

1

Pub Type

Journals

Keywords

Bi5.67Nb10FeO35, Bi9.3Nb16.9Fe1.1O57.8, bismuth iron niobates, crystal structure, x-ray diffraction

Citation

Grey, I. , Mumme, W. , Vanderah, T. , Roth, R. and Bougerol-Chaillout, C. (2007), Chemical Twinning of the Pyrochlore Structure in the System Bi₂O_3-Nb2O₅, Journal of Solid State Chemistry (Accessed October 31, 2024)