Chemical Twinning of the Pyrochlore Structure in the System Bi2O3-Nb2O5
Ian E. Grey, W G. Mumme, Terrell A. Vanderah, Robert S. Roth, C Bougerol-Chaillout
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.
Journal of Solid State Chemistry
Bi5.67Nb10FeO35, Bi9.3Nb16.9Fe1.1O57.8, bismuth iron niobates, crystal structure, x-ray diffraction
, Mumme, W.
, Vanderah, T.
, Roth, R.
and Bougerol-Chaillout, C.
Chemical Twinning of the Pyrochlore Structure in the System Bi<sub>2</sub>O<sub>3-Nb</sub>2O<sub>5</sub>, Journal of Solid State Chemistry
(Accessed June 9, 2023)