[en] Ba₂Mo₁₅Se₁₉, M_r=3214.02, trigonal, Ranti 3c, a_R=20.009(3) A, α_R=28.383(4)⁰, V_R=1599.7(4) A³, Z=2, D_x=6.672 g cm^-3, λ(Mo Kα)=0.71073 A, μ=293.6 cm^-1, F(000)=2776, T=295 K, R=0.024 for 960 observed reflections. The title compound is isostructural with In₂Mo₁₅Se₁₉ containing monovalent In cations. Its crystal structure is built up from an equal mixture of Mo₆Se₈ and Mo₉Se₁₁ cluster units linked together through Mo-Se interunit bonds. The increase of the cationic charge transfer towards the Mo clusters leads to a contraction of both types of clusters along the threefold axis. (orig.)

[en] Tin pentamolybdate, SnMo₅O₈, M_r=726.4, monoclinic, P2₁/c, a=7.533 (4), b=9.268 (4), c=9.970 (4) A, β=109.73 (2)deg, V=655.2 (5) A³, Z=4, D_x=7.363 g cm^-3, λ(Mo Kα)=0.71073 A, μ=128.6 cm^-1, F(000)=1296, T=298 K, R=0.033 for 1690 observed reflections. The title compound is isostructural with LaMo₅O₈ and therefore contains infinite chains built up from bioctahedral cluster units Mo₁₀O₁₈, cross-linked to form channels where the Sn atoms are located. (orig.)

[en] Mo₁₅Se₁₉, M_r=2939.34, trigonal, Ranti 3c, a(rh)=20.360(1) A, α(rh)=26.981(1)deg, V(rh)=1532.2(1) A³, Z=2, D_x=6.371 g cm^-3, λ(Mo Kα)=0.71073 A, μ=281.28 cm^-1, F(000)=2552, T=295 K, R=0.029 for 781 observed reflections. The title compound constitutes the host structure of the ternary compounds M₂Mo₁₅Se₁₉ (M=K, Rb, In, Tl, Ba). Its crystal structure is built up from an equal mixture of Mo₆Se₈ and Mo₉Se₁₁ cluster units linked together through Mo-Se interunit bonds. The absence of cationic charge transfer towards the Mo clusters results in an expansion of both types of clusters along the threefold axis and in a decrease of the intercluster distances. (orig.)

[en] The crystal structure data of Mo₂₁Nd₁₆O₅₆ are given including the XRD data, atomic coordinates and thermal displacement parameters

[en] Caesium molybdenum selenide, Cs₃Mo₁₅Se₁₇, M_r=3180.14, hexagonal, P6₃/m, a=9.624(2), c=20.898(8) A, V=1676.4(7) A³, Z=2, D_x=6.276 g cm^-3, λ(Mo Kα)=0.71073 A, μ=266.5 cm^-1, F(000)=2746, T=293 K, R=0.028 for 1028 reflections with I≥2σ(I) and 58 parameters. The title compound is isostructural with Rb₃Mo₁₅Se₁₇ and thus its structure contains as the main building block the Mo₁₅Se₁₇ cluster unit whose Mo core is built up by four face-sharing Mo₆ octahedra. Whereas the intracluster Mo-Mo distances as well as the Mo-Se ones are identical to within ±0.01 A in both compounds, owing to the same cationic charge transfer towards the Mo₁₅S₁₇ unit, a slight lengthening of the intercluster Mo-Mo distance from 3.268 to 3.318 A is observed, as expected, when the size of the cation increases. (orig.)

[en] Within the last decade, the ternary molybdenum chalcogenides have played a major role in superconductivity: opening of the ternary superconductor range and competition between superconductivity and magnetism. It is mainly due to the pseudomolecular nature of the Mo₆X₈ unit. In these materials, the Mo₆ cluster plays the electron-acceptor role and the number of valence electrons per molybdenum atom is related to the Mo-Mo intracluster bonding. The increase of this valence electron concentration (VEC) per molybdenum in new ternary chalcogenides corresponds to larger and larger clusters. These new extended clusters belong to a very large family containing the general Mo/sub 3n/X/sub 3n + 2/ (n greater than or equal to 2) block unit. The tridimensional stacking of these building blocks provides more and more large channels and leads to a more and more anisotropic character of their physical properties. 23 references, 6 figures, 3 tables

[en] X-ray analyses were performed at room temperature on single crystals of Eu/sub x/Mo₆S₈. Refinement of its crystal structure shows an upper stoichiometry of x = 1.0. Atomic positions and crystallographic data are given in full. Crystals were also characterized through their susceptibility and transport properties. All europium atoms are divalent as confirmed by the magnetization at low temperatures. A well-defined anomaly on the electrical resistivity at T/sub s/ = 107 K separates the metallic high-temperature rhombohedral phase from the low-temperature nonmetallic (triclinic) phase. Very large residual resistivity ratios obtained for stoichiometric crystals favor the interpretation that the Eu/sub 1.0/Mo₆S₈ low-temperature phase must be essentially semiconducting

[en] The temperature dependence of the reflectance of the spin glass materials R₂Mo₂O_7-δ (R:Sm, Gd, and Ho) has been measured for frequencies from 40 to 40 000 cm^-1. The AC conductivity of Sm₂Mo₂O_7-δ derived from Kramers-Kronig analysis, indicates a Drude-like behaviour as the temperature is lowered. Between 150 and 40 K the scattering rate shows a sharp drop which is attributed to the scattering of conduction electrons by short-range ordered moments of the Mo ions. Below the spin glass transition temperature T_f (≅40 K) the scattering rate saturates due to the freezing of the moments. Gd₂Mo₂O_7-δ behaves like a poor metal at room temperature, but at low temperatures shows a linear increase in the conductivity for frequencies up to 400 cm¹ suggesting a localized hopping conductivity. The localization remains in the spin glass state for T< T_f (≅25 K). The conductivity of Ho₂Mo₂O_7-δ is semiconductor-like with a small and slightly temperature-dependent gap around 0.25 eV. Our optical results support a qualitative band model proposed by Sleight and Bouchard for the pyrochlore oxides. (author)

[en] Gadolinium pentamolybdate, GdMo₅O₈, M_r = 764x95, monoclinic, P2₁/c, a = 7x559 (3), b = 8x9926 (9), c = 9x923 (3) A, β = 109x81 (2)⁰, V = 634x6 (3) A³, Z = 4, D_x = 8x006 g cm^-3, λ(Mo Kα) = 0x71073 A, μ = 197x4 cm^-1, F(000) = 1352, T = 298 K, R = 0x028 for 2098 observed reflections. The title compound is isostructural with LaMo₅O₈ and contains bioctahedral Mo₁₀O₁₈ cluster units strongly linked by Mo(1)-Mo(1) intercluster bonds [2x674(1) A] to form infinite molybdenum oxide chains. These are interconnected through Mo-O-Mo bonds to create channels where the Gd³⁺ ions are located. The Mo-Mo distances in the Mo₁₀ cluster lie between 2x598 (1) and 2x832 (1) A (average 2x746 A). The Mo-O distances range between 1x994 (5) and 2x142 (5) A (average 2x074 A). The Gd³⁺ ions are ten coordinated to O atoms with Gd-O distances ranging from 2x315 (5) to 2x959 (5) A (average 2x614 A). (orig.)

[en] Rb₃Mo₁₅Se₁₇, M_r=3037.83, hexagonal, P6₃/m, a=9.4842(8), c=21.006(7) A, V=1636.3(5) A³, Z=2, D_x=6.165 g cm^-3, λ(Mo Kα)=0.71073 A, μ=284.5 cm^-1, F(000)=2638, T=295 K, R=0.022 for 824 observed reflections. The title compound contains for the first time the new condensed cluster unit Mo₁₅Se₁₇. The cluster units are linked together through Mo-Se interunit bonds and delimit large tunnels. The Rb ions are located either in these tunnels or in cavities situated between two consecutive units along the threefold axis. (orig.)