Molybdenum cluster chalcogenides Mo6X8: Electrochemical intercalation of closed shell ions Zn2+, Cd2+, and Na

The intercalation ofd¹⁰ ions Zn²⁺ and Cd²⁺ by electron/ion transfer reactions into the Chevrel-type molybdenum cluster chalcogenidesMo₆X₈ (X =S, Se) demonstrates the competitive influence of electronic and steric factors upon these processes. The following rhombohedral phases have been identified: Zn₁Mo₆S₈, Zn₂Mo₆S₈, Zn₁Mo₆Se₈, Zn₂Mo₆Se₈, Cd₁Mo₆S₈, Cd₁Mo₆Se₈, and Cd₂Mo₆Se₈. Thermodynamic data and chemical diffusion coefficients are given. The intercalation of Na⁺, which has an ionic radius close to that of Cd²⁺, exhibits a strong influence of kinetics leading to the partial irreversibility of the reaction and the formation Na₁Mo₆S₈ and Na₁Mo₆Se₈, the first cubic phases among the molybdenum cluster chalcogenidesA_xMo₆X₈.     

Cs2Mo15S19: a novel ternary reduced molybdenum sulfide containing Mo6 and Mo9 clusters

The crystal structure of dicaesium pentadecamolybdenum nonadeca­sulfide, Cs₂Mo₁₅S₁₉, consists of a mixture of Mo₆S₈S₆ and Mo₉S₁₁S₆ cluster units in a 1:1 ratio. Both units are interconnected via inter‐unit Mo—S bonds. The Cs⁺ cations occupy large voids between the different cluster units. The Cs and two inner S atoms lie on sites with 3 symmetry (Wyckoff site 12c) and the Mo and S atoms of the median plane of the Mo₉S₁₁S₆ cluster unit on sites with 2 symmetry (Wyckoff site 18e).     

Cs6Mo27S31: a novel ternary reduced molybdenum sulfide containing Mo9 and Mo18 clusters

The crystal structure of hexacaesium heptacosamolybdenum hentriaconta­sulfide, Cs₆Mo₂₇S₃₁, consists of a mixture of Mo₉S₁₁S₆ and Mo₁₈S₂₀S₆ cluster units in a 1:1 ratio. The units are connected through Mo—S bonds. Cs⁺ cations occupy large voids between the different cluster units.     

One-dimensional condensation of Mo6 octahedral clusters: A new cluster,Mo12, and a new building block,Mo12S14, in M2Mo9S11

Two new Mo compounds, Tl₂Mo₉S₁₁ and K₂Mo₉S₁₁, have been found. The structure of these compounds is characterized by the presence of a completely new building block, Mo₁₂S₁₄, in addition to the well-known Mo₆S₈ unit as in the PbMo₆S₈-type compounds. The new cluster, Mo₁₂, contained in the Mo₁₂S₁₄ unit can be considered as a one-dimensional condensation of three Mo₆ octahedral clusters. These new materials, the structure of which resembles that of PbMo₆S₈, are metallic but are not superconducting above 2.1°K.     

Synthesis of Cu2Mo6S8 powders and thin films from intermediate oxides prepared by polymeric precursor method

Powders and thin films of the copper molybdenum sulfide Cu₂Mo₆S₈ were synthesized from intermediate oxides prepared by polymeric precursor method based on Pechini process. In the case of the thin films, deposition was performed onto R-plane sapphire single crystal by spin coating. The influence of temperature and duration of the 3 step heat treatment cycle (calcination, sulfurization and reduction) were investigated to optimize the synthesis conditions. The first step of calcination under air atmosphere performed for 3 h at 450 °C and 400 °C is suitable to obtain the intermediate oxides powders and thin films, respectively. The sulfurization treatment at 600 °C for 2 h under H₂S/H₂ gas flow followed by reduction at 650 °C for 4 h under H₂ gas flow allowed to obtain Cu₂Mo₆S₈ in powder or thin film form. In the last case, a multilayer process led to dense and homogeneous films. Moreover, the insertion and superconducting behaviour of the final powders allowed to validate the Cu₂Mo₆S₈ synthesis by this moderate temperature process.     

Formation of novel molybdenum and tungsten sulfides by reduction of MoS2 and WS2: A new route to chevrel phases

Pt-catalyzed hydrogen reduction of MoS₂ and WS₂ at 1000–1050°C yields new metal-rich sulfidesM₂₁S₈ andM₁₄S₅ (M =Mo or W). The reduction of MoS₂ proceeds via the intermediate Mo₆S₈. Chevrel phases, Cu_xMo₆S₈(x < 4.0) and Ni₂Mo₆S₈, are readily prepared by hydrogen reduction of MoS₂ in the presence of the ternary metal.     

Molten salt synthesis (MSS) of Cu2Mo6S8—New way for large-scale production of Chevrel phases

The Chevrel phase (CP), Mo₆S₈, was found to be an excellent cathode material for rechargeable magnesium batteries. Mo₆S₈ is obtained by a leaching process of Cu₂Mo₆S₈, which removes the copper. A new method of Cu₂Mo₆S₈ production was developed. In contrast to the well-known solid-state synthesis of CP, the method is based on the reaction in a molten salt media (KCl). A fast kinetics of this reaction allows using less active, but more convenient precursors (sulfides instead of sulfur), decreasing temperature and synthesis duration, as well as operation in the inert atmosphere instead of dynamic evacuated systems. It was shown that the composition and the electrochemical behavior of the products obtained by MSS and by the solid-state synthesis are identical. Thus, the molten salt method is extremely attractive for the large-scale production of the active materials for Mg batteries.     

Electronic Structure and Molecular Properties of the Mixed Rhenium–Molybdenum [Re6−x Mo x S8(CN)6]q−, (x = 0 to 6) Clusters

Relativistic density functional calculations including scalar and spin-orbit effects via the ZORA approximation and including solvent effects were carried out on the [Re₆S₈(CN)₆]⁴⁻, [Re₅MoS₈(CN)₆]⁵⁻, [Re₄Mo₂S₈(CN)₆]⁵⁻, [Re₃Mo₃S₈(CN)₆]⁵⁻, [Re₂Mo₄S₈(CN)₆]⁵⁻, [ReMo₅S₈(CN)₆]⁵⁻ and [Mo₆S₈(CN)₆]⁶⁻ clusters. By increasing the replacement of each Re atom with Mo atoms we find that for x > 2 the HOMO–LUMO gap decreases significantly. The calculated gap of the [Re₃Mo₃S₈(CN)₆]⁵⁻, [Re₂Mo₄S₈(CN)₆]⁵⁻ and [ReMo₅S₈(CN)₆]⁵⁻ clusters is similar to the calculated and observed gap of the superconducting PbMo₆S₈ Chevrel phases. The current calculations also indicates that the electronic similarities of the lowest excited states of the semiconducting 24e [Re₅MoS₈(CN)₆]⁵⁻ and 23e [Re₄Mo₂S₈(CN)₆]⁵⁻ clusters with the strongly luminescent 24e [Re₆S₈(CN)₆]⁴⁻ cluster, suggest that these mixed metal clusters might be luminescent.     

Sc0.43(2)Rb2Mo15S19, a partially Sc‐filled variant of Rb2Mo15S19

The structure of scandium dirubidium pentadecamolybdenum nonadecasulfide, Sc_{0.43 (2)}Rb₂Mo₁₅S₁₉, constitutes a partially Sc‐filled variant of Rb₂Mo₁₅S₁₉ [Picard, Saillard, Gougeon, Noel & Potel (2000), J. Solid State Chem. 155 , 417–426]. In the two compounds, which both crystallize in the Rc space group, the structural motif is characterized by a mixture of Mo₆Sⁱ₈S^a₆ and Mo₉Sⁱ₁₁S^a₆ cluster units (`i' is inner and `a' is apical) in a 1:1 ratio. The two components are interconnected through interunit Mo—S bonds. The cluster units are centred at Wyckoff positions 6b and 6a (point‐group symmetries and 32, respectively). The Rb⁺ cations occupy large voids between the different cluster units. The Rb and the two inner S atoms lie on sites with 3. symmetry (Wyckoff site 12c), and the Mo and S atoms of the median plane of the Mo₉S₁₁S₆ cluster unit lie on sites with .2 symmetry (Wyckoff site 18e). A unique feature of the structure is a partially filled octahedral Sc site with symmetry. Extended Hückel tight‐binding calculations provide an understanding of the variation in the Mo—Mo distances within the Mo clusters induced by the increase in the cationic charge transfer due to the insertion of Sc.     

Electrolytic molybdenum sulfides for thin-layer lithium power sources

The active molybdenum sulfide compound Mo₂S₃, which should be considered as a cathode material for thin-layer rechargeable power source, has been produced by electrolysis. Using impedance spectroscopy and potential relaxation method after current interruption, the kinetic parameters of lithium intercalation in electrolytic Mo₂S₃ have been obtained. Activation energy of Li⁺ migration in electrolyte (13.76 kJ/mol), charge transfer through the Mo₂S₃ electrode/electrolyte interface (38.8 kJ/mol), and Li⁺ diffusion in a solid phase (57.3 kJ/mol) have also been established. Taking into account the coefficient data of charge mass transfer in a solid phase and the reaction rate coefficient of charge transfer through the interface electrode/electrolyte within the temperature range 20–50 °C, the stage of Li⁺ transfer in a solid phase has been determined as a limiting stage for lithium intercalation in electrolytic molybdenum sulfide Mo₂S₃.     

Synthesis and structure of a new octahedral molybdenum thiocyanide cluster complex K7[Mo6(μ3-S)8(CN)6]·8H2O

The new octahedral molybdenum thiocyanide cluster complex K₇[Mo₆S₈(CN)₆]·8H₂O was synthesized by excision of the cluster core (the reaction of ZnMo₆S₈ with a melt of KCN). The structure of the complex was established by X-ray diffraction analysis. The reaction of Mo₆Se₈ with a KCN—KSCN mixture afforded the mixed-ligand cluster anions [Mo₆(Se,S)₈(CN)₆]^7–. The salt of composition K_1.5Cs_5.5[Mo₆Se_6.8S_1.2(CN)₆]·8H₂O was obtained. The complexes are isostructural to each other and to the selenium analog described previously. The magnetic properties and the electronic and IR spectra were measured and discussed.     

Stabilization of Mo6S8 by halogens; new superconducting compounds: Mo6S6Br2, Mo6S6I2

We present a study of the properties of the series Mo₆X_8?xY_x (X = S, Se, Te; Y = Br, I) having the hexagonal rhombohedral structure of the PbMo₆S₈ type. For X = S we have found two new superconducting compounds Mo₆S₆Br₂ and Mo₆S₆I₂, having critical temperatures of 13.8 and 14.0°K, respectively. We further find that Mo₆Te₈ becomes superconducting (T_c ≈ 2.6°K) upon substitution of Te by small quantities of iodine, and that in the case of Mo₆Se₈ substitution of a Se atom by a halogen, raises T_c up to about 7.6°K.     

X-ray absorption spectroscopy study of the electronic structure and local coordination of 1st row transition metal-promoted Chevrel-phase sulfides

Abstract

The electronic structures of S and Mo as well as the local coordination of Mo are investigated as a function of metal promotion Chevrel-phase (CP) sulfides. We observe the effect of metal promoter-induced electron donation into the stoichiometric range M_xMo₆S₈ (M?=?Fe, Ni, Cu; x?=?0–2) through analysis of X-ray absorption near-edge structure regions. We further observe the effect of this promotion on the bonding environment of Mo₆ metal centers through extended X-ray absorption fine structure analysis. We monitor expansion and contraction of Mo₆ octahedra with and without metal promotion, as has been predicted by Hückel molecular orbital theory. We further observe a marked tunability in the electronic structure of sulfur upon charge transfer between promoting species and Mo₆S₈ units. Average Mo₆ octahedron Mo–Mo bond contraction from 2.76 Å to as short as 2.69 Å was observed upon incorporation of metal promoters, while intercluster separation displays a pronounced increase for promoter-host lattices compared to un-promoted Mo₆S₈. To corroborate spectroscopically observed phenomena, we performed computational analyses of spin-polarized densities of state for the CP materials investigated herein, where a detectable increase in sulfur-based frontier orbital population is observed in accordance with experimentally validated orbital filling.     

First-Principles Study of Ultrathin Molybdenum Sul des Nanowires: Electronic and Catalytic Hydrogen Evolution Properties

Molybdenum sulfides nanomaterials, such as one-dimensional (1D) nanotubes, nanoribbons, and two-dimensional (2D) nanosheets, have attracted intensive research interests for their novel electronic, optical, and catalytic properties. On the basis of first-principles calculation, here, we report a new series of 1D ultrathin molybdenum sulfides nanowires, including Mo₂S₆、Mo₃S₆ and Mo₆S₁₀ nanowires. Our results demonstrate that these ultrathin nanowires are both thermal and lattices dynamically stable, confirmed with the calculated phonon spectrum and Born-Oppenheimer molecular dynamic simulation at the temperature up to 600 K. The calculated elastic constant is 21.33, 103.22, and 163.00 eV/? for Mo₂S₆, Mo₃S₆, and Mo₆S₁₀ nanowires, respectively. Mo₂S₆ and Mo₃S₆ nanowires are semiconductors with band gap of 1.55 and 0.46 eV, while Mo₆S₁₀ nanowires is metal, implying their potential applications in electronics and optoelectronics. In particular, ultrathin molybdenum sulfides nanowires can be used as catalysts for hydrogen evolution reaction. The calculated Gibbs free energy change for hydrogen evolution is about -0.05 eV for Mo₂S₆ nanowire, comparable with those of Pt and H-MoS₂. The prediction of these 1D molybdenum sulfides nanowires may enrich the 1D family molybdenum sulfides and make a supplement to understand the high performance of hydrogen evolution reaction in transition-metal dichalcogenides.     

An Eicosanuclear Heterothiometallic Mo/S/Cu Cluster: Synthesis and Third-Order Nonlinear Optical Property

A potassium-crownether host–guest cation-templated synthetic method was used to build a heterothiometallic Mo/S/Cu cluster {[(K ? dibenzo-18-crown-6)(NMP)₂]₂[(K ? dibenzo-18-crown-6)(NMP)]₂[Mo₈S₃₂Cu₁₂]·H₂O} (1). 1 was structurally determined by X-ray single crystal and powder diffractions. The anionic Mo/S/Cu cluster [Mo₈S₃₂Cu₁₂]^4? exhibits a unique octameric eicosanuclear supra-cubane-like architecture. Potassium-crownether cations show “satellite-receiver”-shaped and hexagonal-pyramidal configurations. Moreover, the third-order nonlinear optical (NLO) property was studied through Z-scan method (532 nm, 4 ns pulses), which reveals that 1 possesses effective NLO absorptive and refractive properties.     

Investigation of vortex dynamics close to Bc2 in Cu2Mo6S8 quasi epitaxial thin films

The Ginzburg number of superconducting Chevrel phases M_xMo₆S₈ with small coherence length (10⁻³ to 10−5) is intermediate between those obtained for conventional low T_c materials (10⁻⁸) and those of high T_c (10⁻¹) indicating that these phases may display features in the dynamics of the vortices similar to those observed in high T_c superconductors. In this work we present a detailed study of I–V measurements close to the B_c2 line carried out on quasi epitaxial thin films of Cu₂Mo₆S₈. The non-linear I–V curves show a scaling behaviour making possible to determine a transition temperature between an unpinned vortex state and a vortex glass state. However, the temperature range of the unpinned vortex state is much wider than expected.     

The crystal structure of the inorganic surface films formed on Mg and Li intercalation compounds and the electrode performance

A crystallographic approach was applied to elucidate the influence of the nature of the surface films on the electrochemical behavior of Li and Mg intercalation compounds. This paper presents two examples: (1) protection of graphite electrodes by Li₂CO₃ surface films, and (2) the unique electrochemical behavior of Mg-containing Chevrel phases (MgCP) obtained by different synthetic routes. In the former case, the elucidation of the protection mechanism and the explanation of the high performance of such protected electrodes are based on the analysis of possible Li-ion motion in the carbonate crystal structure. In the latter case, a combination of synthesis, electrochemistry and XRD analysis was used to explain an unusual phenomenon: the difference between the excellent electrochemical behavior of the Chevrel phase (CP) based on Cu-leached Cu₂Mo₆S₈ (CuCP), and the poor electrochemical activity of the high-temperature synthesized MgCP, with the same phase composition. It is shown that this phenomenon is caused by MgO formation on the surface of the latter material. The different surface chemistry of the MgCPs obtained by the two different synthetic routes was substantiated by revealing the correlation between the electrochemical activity and the chemical stability of these materials under ambient atmosphere conditions. Dedicated to Prof. Mikhail A. Vorotyntsev on the occasion of his 60th birthday.     

The crystal structure of (NH4)2[Mo2(S2)6].8/3H2

The black crystal of (NH₄)[Mo₂(S₂)₆]* 8/3 H₂O belongs to the orthorhombic system, space group D³₂-P22₁2₁, with a = 12.064(6), b = 12.534(4), c = 19.558(9)Å, V =2957(3)ų, Z = 4 and D_c = 2.23g.cm^?3. The intensity data were collected on a Syntex R3 four-circle diffractometer. The structure was solved by Patterson method and direct method, the light atoms (except H atoms) were obtained from ΔF syntheses. The structure was refined by least-squares with anisotropic thermal parameters. The values of R and R_w were 0.092 and 0.072 respectively. The crystal structure contains discrete dimeric cluster [Mo₂(S₂)₆]^2? ions, NH₄⁺ cations and H₂O molecules. There are two crystallographically independent [Mo₂S₂)₆]^2? ions in the crystal, one locates on general position [Figure 1(a)], the other locates on two-fold axis [Figure 1(b)]. It contains one and a half [Mo₂S₂)₆]^2? ions in an asymmetric unit. In [Mo₂S₂)₆]^2? each Mo is coordinated side on by four S₂^2? groups in a distorted dodecahedral arrangement, two of which are bridging and the other two are terminal. The Mo? S bond length is 2.441 Å (mean), and S? S is 2.049 Å (mean). The Mo? Mo distance is 2.784 Å (mean), which is to be regarded as a single bond length. The formal oxidation state of Mo is five, it is probably a mixed valence Mo^IV? Mo^VI, and so shows a remarkable deep colour.     

EXAFS Study of the Structure of Desolvated Cluster Compounds of Re and Mo with Cyanide and Thiocyanate Ligands

This paper reports on EXAFS and XANES studies of structural changes in complex compounds (H₃O)₂Co₃[Re₆Se₈(CN)₆]₂14.5H₂O, Cs₂Co[Re₆S₈(CN)₆]2H₂O, and Co(DMF)₆[Mo₆Br₈(NCS)₆] containing the octahedral cluster anions of rhenium [Re₆X₈(CN)₆]^4– (X = S, Se) or molybdenum [Mo₆Br₈(NCS)₆]^2–, induced by vacuum annealing at temperatures of up to 250°C. According to EXAFS and XANES data, the complex cluster anions do not undergo any pronounced changes during annealing. The parameters of the local environment of the cobalt cations have been determined. Structural models of the environment of the cations in various compounds after desolvation have been constructed. For Co(DMF)₆[Mo₆Br₈(NCS)₆], vacuum annealing at 250°C leads to a complete removal of DMF and formation of a new compound, Co[Mo₆Br₈(NCS)₆], in which the cobalt atoms are coordinated by four sulfur atoms of the SCN groups.