Chemical twinning of the pyrochlore structure in the system Bi{sub 2}O{sub 3}-Fe{sub 2}O{sub 3}-Nb{sub 2}O{sub 5}

Journal Article · Mon Jan 15 04:00:00 EST 2007 · Journal of Solid State Chemistry

DOI:https://doi.org/10.1016/j.jssc.2006.10.004· OSTI ID:21015632

Mumme, W G ^[1]; Vanderah, T A ^[2]; Roth, R S ^[2]; Bougerol, C ^[3]

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1. CSIRO Minerals, Box 312, Clayton South, Vic., 3169 (Australia)
2. National Institute of Standards and Technology, Materials Science, and Engineering Laboratory, Gaithersburg, MD 20899 (United States)
3. Equipe CEA-CNRS NPSC SP2M/DRFMC/CEA, 17 rue des Martyrs, 38054 Grenoble (France)

New ternary bismuth iron niobates having structures based on chemical twinning of pyrochlore are described. Bi{sub 5.67}Nb{sub 10}FeO{sub 35} has hexagonal symmetry, P6{sub 3}/mmc, a=7.432(1)A, c=31.881(2)A, Z=2 and Bi{sub 9.3}Nb{sub 16.9}Fe{sub 1.1}O{sub 57.8} has rhombohedral symmetry, R-3m, a=7.433(1)A, c=77.488(2)A, Z=3. The structures of both phases were determined and refined to R{sub 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){sub 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 12A, whereas the rhombohedral phase has pyrochlore blocks of two widths, 6 and 12A, alternating with HTB blocks. It is proposed that the previously reported binary 4Bi{sub 2}O{sub 3}:9Nb{sub 2}O{sub 5} phase has a related structure containing pyrochlore blocks all of width 6A. A feature of the structures is partial occupancy ({approx}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.

• OSTI ID: 21015632
• Journal Information: Journal of Solid State Chemistry, Journal Name: Journal of Solid State Chemistry Journal Issue: 1 Vol. 180; ISSN 0022-4596; ISSN JSSCBI
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
BISMUTH OXIDES
FERRITES
HCP LATTICES
IRON OXIDES
LAYERS
MONOCRYSTALS
NIOBATES
NIOBIUM OXIDES
PYROCHLORE
SYMMETRY
TUNGSTEN BRONZE
TWINNING