Cluster configurations in modulated EuVxMo8 +/- yO14 crystals

Three orthorhombic crystals of chemical formula EuxVyMo8+/-zO14 were invest igated by X-ray diffraction (Mo K alpha radiation, lambda = 0.71073 Angstro m). They have nearly the same lattice parameters (a similar or equal to 11. 3, b similar or equal to 10.0, c similar or equal to 9.2 A), display one-di mensional incommensurate modulations of wavevector q* = gamma c* and are ch aracterized by the same superspace group Cmca(00 gamma)s00. The crystals di ffer both in their compositions (namely Eu0.976(6)V1.13(5)Mo7.10(5)O14, Eu0 .986(4)V1.10(3)Mo7.30(1)O14 and EuMo7.96(1)O14) and in their gamma componen ts [0.195 (2), 0.245 (2) and 0.286 (3), respectively]. The average structur es of these crystals appear closely related to the structures of LaMo7.7O14 (not modulated) and LaMo8O14 (modulated); however, two main differences ar e outlined: first, the modulation direction is c in the Eu-containing cryst als but b in the modulated La-containing crystal [q* = (1/3)b*], second, th e Eu-containing crystals have centrosymmetric structures while the La-conta ining 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 c lusters Meg, Mole, Mo6V4, Mo7V3 and Mo8V2 besides the clusters M-8 (Mo-8, M o6V2 and Mo7V) and M-7 (Mo-7 and Mo6V) which are already known. Mo-8 units with cis and trans configurations and Mo6V2 units with a trans configuratio n appear as the main cluster types in these crystals. The nature of the met allic clusters changes along c, but inside one (001) layer it is likely tha t only one cluster type with a given configuration is present. The main str uctural 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 Angstrom within a lay er as well as between two neighbouring layers.