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MTi0.7Mo0.3Mo5O10 (M = Sr, Eu), First Evidence of Mono- and Bicapped Bioctahedral Mo11 and Mo12 Clusters:  Synthesis, Crystal Structures, and Physical Properties

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posted on 12.04.2001, 00:00 by J. Tortelier, P. Gougeon, R. Gautier, R. Berjoan
The novel quaternary reduced molybdenum oxides MTi0.7Mo0.3Mo5O10 (M = Sr, Eu) have been synthesized by solid-state reaction at 1400 °C for 48 h in sealed molybdenum crucibles. Their crystal structures were determined on single crystals by X-ray diffraction. Both compounds crystallize in the orthorhombic space group Pbca with 8 formula units per cell and the following lattice parameters: aSr = 9.1085 (7), bSr = 11.418 (1), and cSr = 15.092 (3) Å; aEu = 9.1069 (7), bEu = 11.421 (2), and cEu = 15.075 (1) Å. The Mo network is dominated by bioctahedral Mo10 clusters, which coexist randomly with Mo11 and Mo12 clusters (monocapped and bicapped Mo10 clusters). The Mo−Mo distances within the clusters range from 2.62 to 2.92 Å and the Mo−O distances from 1.99 to 2.17 Å as usually observed in the reduced molybdenum oxides. The Sr2+ and Eu2+ ions occupy large cavities, which result from the fusion of two cubooctahedra and thus are surrounded by 11 oxygen atoms. The M−O distances range from 2.50 to 3.23 Å for the Sr compound and from 2.49 to 3.24 Å for the Eu analogue. Single-crystal resistivity measurements indicate that both materials are poor metals with transitions to semiconducting states below 50 and 40 K and room temperature resistivity values of 9 × 10-3 and 5 × 10-3 Ω·cm for the Sr and Eu compounds, respectively. The magnetic susceptibility data indicate paramagnetic behavior due to the Eu2+ moment at high temperatures for the Eu compound and do not reveal the existence of localized moments on the Mo and Ti sublattice in the Sr compound. An XPS study clearly suggests that the isolated Ti ions are tetravalent. Theoretical considerations preclude the existence of heterometallic Mo−Ti clusters.