Cluster configurations in modulated EuVxMo8+/-yO14 crystals.
Three orthorhombic crystals of chemical formula Eu(x)V(y)Mo(8+/-z)O(14) were investigated by X-ray diffraction (Mo Kalpha radiation, lambda = 0.71073 Å). They have nearly the same lattice parameters (a approximately 11.3, b approximately 10.0, c approximately 9.2 Å), display one-dimensional incommensurate modulations of wavevector q* = gammac* and are characterized by the same superspace group Cmca(00gamma)s00. The crystals differ both in their compositions (namely Eu(0.976(6))V(1.13(5))Mo(7.10(5))O(14), Eu(0.986(4))V(1.10(3))Mo(7.30(1))O(14) and EuMo(7.96(1))O(14)) and in their gamma 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(7.7)O(14) (not modulated) and LaMo(8)O(14) (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 Mo(9), Mo(10), Mo(6)V(4), Mo(7)V(3) and Mo(8)V(2) besides the clusters M(8) (Mo(8), Mo(6)V(2) and Mo(7)V) and M(7) (Mo(7) and Mo(6)V) which are already known. Mo(8) units with cis and trans configurations and Mo(6)V(2) 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.
Laboratoire CRISMAT (UMR CNRS 6508), ISMRA, 14050 Caen CEDEX, France. h.leligny@crismat.ismra.fr
H
Leligny
D
Grebille
P
Roussel
P
Labbé
M
Hervieu
B
Raveau
J
Tortelier
P
Gougeon