Acta Crystallographica Section B

Structural Science

Volume 55, Part 4 (August 1999)

Acta Cryst. (1999). B55, 467-483    [doi:10.1107/S0108768199000452]

Cluster configurations in modulated EuV_xMo_8±yO₁₄ crystals

H. Leligny, D. Grebille, P. Roussel, Ph. Labbé, M. Hervieu, B. Raveau, J. Tortelier and P. Gougeon

Abstract: Three orthorhombic crystals of chemical formula Eu_xV_yMo_8±zO₁₄ were investigated by X-ray diffraction (Mo K radiation, = 0.71073  Å). They have nearly the same lattice parameters (a 11.3, b 10.0, c 9.2  Å), display one-dimensional incommensurate modulations of wavevector q* = c* and are characterized by the same superspace group Cmca(00)s00. The crystals differ both in their compositions (namely Eu_0.976(6)V_1.13(5)Mo_7.10(5)O₁₄, Eu_0.986(4)V_1.10(3)Mo_7.30(1)O₁₄ and EuMo_7.96(1)O₁₄) and in their components [0.195  (2), 0.245  (2) and 0.286  (3), respectively]. The average structures of these crystals appear closely related to the structures of LaMo_7.7O₁₄ (not modulated) and LaMo₈O₁₄ (modulated); however, two main differences are outlined: first, the modulation direction is c in the Eu-containing crystals but b in the modulated La-containing crystal [q* = (1/3)b*], second, the Eu-containing crystals have centrosymmetric structures while the La-containing crystals have polar structures (space group C2ca). The Mo (or Mo and V) atoms are stacked to form (001) layers of metallic clusters. The density modulation of these structures implies the existence of the new types of clusters Mo₉, Mo₁₀, Mo₆V₄, Mo₇V₃ and Mo₈V₂ besides the clusters M₈ (Mo₈, Mo₆V₂ and Mo₇V) and M₇ (Mo₇ and Mo₆V) which are already known. Mo₈ units with cis and trans configurations and Mo₆V₂ units with a trans configuration appear as the main cluster types in these crystals. The nature of the metallic clusters changes along c, but inside one (001) layer it is likely that only one cluster type with a given configuration is present. The main structural result is the formation, in some unit cells, of strong intercluster Mo-Mo, Mo-V or V-V bonds with distances close to 2.6  Å within a layer as well as between two neighbouring layers.

Online 1 August 1999

	Structure factor file (CIF format) (98.4 kbytes) Contains datablock Crystal1
	Structure factor file (CIF format) (86.0 kbytes) Contains datablock Crystal2
	Structure factor file (CIF format) (94.9 kbytes) Contains datablock Crystal3

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