Crystal structure refinement of Mg{sub 5}Nb{sub 4}O{sub 15} and Mg{sub 5}Ta{sub 4}O{sub 15} by Rietveld analysis of neutron powder diffraction data
- Univ. Nacional de Cordoba (Argentina)
- Inst. Laue-Langevin, Grenoble (France)
- C.S.I.C., Madrid (Spain). Inst. de Ciencia de Materiales de Madrid
The crystal structure of the isomorphous phases Mg{sub 5}Nb{sub 4}O{sub 15} and Mg{sub 5}Ta{sub 4}O{sub 15} was refined from neutron powder diffraction data. These compounds are isostructural with pseudobrookite, Fe{sub 2}TiO{sub 5}. The two kinds of metal sites of this structure are randomly occupied by Mg(II) and Nb(V) or Ta(V). The structure consists of double chains of (Mg,M)O{sub 6} units (where M = Nb or Ta), sharing edges on the bc plane, interconnected through common oxygens along the a axis to give a three-dimensional array. The (Mg, M)O{sub 6} polyhedra at both metal positions can be described as very distorted octahedra, with (Mg, M)-O distances ranging from 1.93 to 2.22 {angstrom}. The crystallographic formulas can be written as (Mg{sub 0.73(2)}Nb{sub 0.27(2)}){sub 4c} (Mg{sub 0.94(2)}Nb{sub 1.06(2)}){sub 8f}O{sub 5} and (Mg{sub 0.66(2)}Ta{sub 0.34(2)}){sub 4c}(Mg{sub 1.01(2)}Ta{sub 0.99(2)}){sub 8f}O{sub 5} respectively, where 4c and 8f are the Wyckoff sites of the two metal positions of the structure. The space group is Cmcm (orthorhombic) and Z = 4. Unit cell parameters, cell volume, R{sub wp} and R{sub I} values obtained were a = 3.8068(1) {angstrom}, b = 10.0561(1) {angstrom}, c = 10.2566(1) {angstrom}, V = 392.64(2) {angstrom}{sup 3}, and 6.72% and 2.94% for the niobium compound; and a = 3.81884(6) {angstrom}, b = 10.0574(2) {angstrom}, c = 10.2343(2) {angstrom}, V = 393.07(2) {angstrom}{sup 3}, and 4.46%, and 2.36% for the tantalum compound.
- OSTI ID:
- 642217
- Journal Information:
- Journal of Solid State Chemistry, Vol. 137, Issue 2; Other Information: PBD: May 1998
- Country of Publication:
- United States
- Language:
- English
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