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.     

New pseudo-one-dimensional metals: M2Mo6Se6 (M = Na,In, K,TI), M2Mo6S6 (M = K,Rb, Cs), M2Mo6Te6 (M = In,TI)

We present a new series of ternary chalcogenides, derived from divalent molybdenum: M₂Mo₆X₆. These compounds crystallize in a hexagonal lattice with a ≈ 9 Å, c ≈ 4.5 Å, and space group $\text{P6}{}_3\text{m}$. The compounds are characterized by clusters (Mo₃)¹_∞ in the form of linear chains, resulting from a linear condensation of Mo₆ octahedral clusters. The (Mo₃)¹_∞ clusters are well separated from each other, with the shortest MoMo intercluster distance larger than 6 Å. The resulting pseudo-one-dimensional structures show remarkable anisotropy of physical properties.     

Magnetism and structural chemistry of ternary borides RE2MB6 (RE = rare earth, M = Ru,Os)

The magnetic behavior of the ternary borides RE₂RuB₆ and RE₂OsB₆ (RE = Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) was studied in the temperature range 1.5 K < T < 1100 K. All compounds crystallize with the Y₂ReB₆-type structure and are characterized by direct RE-RE contacts and the formation of planar infinite two-dimensional rigid boron nets. The magnetic properties reveal a typical Van Vleck paramagnetism of free RE³⁺-ions at temperatures higher than 200 K with ferromagnetic interaction in the low-temperature range T < 55 K. The ferromagnetic ordering temperatures vary with the De Gennes factor. There is no indication for a magnetic contribution from the Ru(Os)-sublattice. Above 1.8 K none of the samples were found to be superconducting.     

Etude structurale des composés MxTiSe2 (M = Fe,Co, Ni)

New compounds M_xTiSe₂ have been prepared with M = Fe (x ? 0.66), M = Co or Ni (x ? 0.50). The metal M is located in vacant octahedral sites of the TiSe₂ host lattice (hexagonal unit cell a′, c′). An ordering of vacancies occurs if x ? 0.20. With M = Co or Ni (x = 0.50) and with M = Fe (0.25 ? x ? 0.66) isotypic compounds of Ti₃Se₄ can be obtained (M₃ □ X₄ type; monoclinic unit cell a ≈ a′ √3, b ≈ a′, c ≈ 2c′). The compounds Fe_0.38TiSe₂ and Co_0.38TiSe₂ (hexagonal unit cell a ≈ a′ √3, c ≈ 2c′) are of the M₂ □ X₃ type, variety 2c′. The Fe_0.25TiSe₂ and Co_0.25TiSe₂ monoclinic unit cells (a ≈ 2a′ √3, b ≈ 2a′, c ≈ 2c′) allow us to assume, for these two compounds, a structure of the M₅ □ ₃X₈ type, variety 2c′, identical to the Ti₅Se₈ one. The compound Ni_0.25TiSe₂ has an hexagonal unit cell (a ≈ 2a′, c ≈ 3c′); it belongs to a so-called 3c′ variety of the M₅ □ ₃X₈ type.     

Phase diagrams in the quaternary systems M1−xCuxCr2X4 with M = Cd,Mn, and X = S,Se

The phase diagrams of the spinel systems Cd_1?xCu_xCr₂S₄, Cd_1?xCu_xCr₂Se₄, and Mn_1?xCu_xCr₂S₄ have been studied on the basis of X-ray powder photographs of quenched samples and high-temperature X-ray diffraction patterns. At room temperature the mutual solid solubilities of the metallic copper and the semiconducting cadmium and manganese spinels are only small (x < 0.05 and >0.95). The interchangeability, however, increases largely with increasing temperature. Complete series of mixed crystals, as in the Zn_1?xCu_xCr₂X₄ (X = S, Se) systems, however, are not formed. The solid solutions with x > 0.07 and <0.95, x > 0.095 and <0.90, and x > 0.36 and <0.87, respectively, formed at higher temperatures cannot be quenched to room temperature without decomposition. The unit cell dimensions of the spinel solid solutions studied obviously do not obey Vegard's rule.     

Defects and Luminescence Behavior of Cubic F23 [(NH4(18-Crown-6))4MnX4] [TlX4]2 (X=Cl, Br) Crystals

Luminescence from [(NH₄(18-Crown-6))₄MnBr₄][TlBr₄]₂ (1), [(NH₄(18-Crown-6))₄MnCl₄][TlCl₄]₂ (2), [(NH₄(18-Crown-6))₂MnBr₄] (3), and [(NH₄(18-Crown-6))₂MnCl₄] (4) was studied in search of new insights regarding crystal defects in 2. Emission from 3 and 4 is normal Mn²⁺(⁴T₁(⁴G)→⁶A₁); that of 2 (λ_max≈520 nm at ca. 300 K and 560 nm at 77 K) is unusual and temperature dependent. Thermal barriers (kJ/mol, assignment): green emission of 1 and 2, T<150 K (1-2, NH⁺₄ rotations), 150<T<250 K (7-14, energy migration among [MnX₄]²⁻), 250<T<300 K (26-35, rotations of 18-Crown-6)); yellow emission of 2: T;<250 K (7-8, energy migration among [MnX₄]²⁻), T>250 K (29 kJ/mol, defect-to-Mn²⁺(⁴T₁(⁴G)) back energy transfer). Crystal data for 4: Space group P2₁/c; Z=4; a=20.173(1) Å; b=9.0144(8) Å; c=20.821(1) Å; β=98.782(5)°; V=3741.9(8) ų; R_w=0.059; R=0.054.     

Structural refinement of T2Mo3O8 (T=Mg, Co, Zn and Mn) and anomalous valence of trinuclear molybdenum clusters in Mn2Mo3O8

Crystal structure of a series of mixed-metal oxides, T₂Mo₃O₈ (T=Mg, Co, Zn and Mn; P6₃mc; a=5.7628(1) Å, c=9.8770(3) Å for Mg₂Mo₃O₈; a=5.7693(3) Å, c=9.9070(7) Å for Co₂Mo₃O₈; a=5.7835(2) Å, c=9.8996(5) Å for Zn₂Mo₃O₈; a=5.8003(2) Å, c=10.2425(5) Å for Mn₂Mo₃O₈) was investigated by X-ray diffraction on single crystals. Structural analysis, magnetization measurements, X-ray photoemission spectroscopy and cyclic voltammetry showed that the Mn ions at the tetrahedral and octahedral sites in Mn₂Mo₃O₈ adopt different valences of +2 and 2+δ (δ>0), respectively. The formal valence of the Mo₃ in Mn₂Mo₃O₈ is 12−δ to retain electric neutrality of the compound. In contrast, the T ions and Mo₃ in T₂Mo₃O₈ (T=Mg, Co and Zn) adopt the valences of +2 and +12, respectively.     

Layered Intergrowth Phases Bi4MO8X (X=Cl, M=Ta and X=Br, M=Ta or Nb): Structural and Electrophysical Characterization

The high-temperature structural behavior of the layered intergrowth phase Bi₄TaO₈Cl, belonging to the Sillén-Aurivillius family, has been studied by powder neutron diffraction. This material is ferroelectric, space group P2₁cn, at T_C<640 K. An order-disorder transition to centrosymmetric space group Pmcn is found around 640 K, which involves disordering of TaO₆ octahedral tilts. A second phase transition, of a first-order nature, to space group P4/mmm occurs at a temperature of ∼1038 K. The crystal structures of the bromide analogs Bi₄MO₈Br (M=Nb, Ta) have also been determined at room temperature; both are isomorphous with Bi₄TaO₈Cl and exhibit maxima in dielectric constant at temperatures of approximately 588 and 450 K, respectively.     

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.     

Synthesis and Crystal Structure of M11X10 Compounds, M=Sr, Ba and X=Bi, Sb. Electronic Requirements and Chemical Bonding

The intermetallic compounds Sr₁₁Bi₁₀, Ba₁₁Bi₁₀, and (Sr₅Ba₆)Sb₁₀ have been obtained from melts of mixtures of the elements. They crystallize in the tetragonal system, space group I4/mmm, Ho₁₁Ge₁₀ structure type, tI84 Pearson symbol, Z=4, with cell parameters a=12.765(3), 13.230(3), 12.748(2) Å and c=18.407(3), 19.365(3), 18.761(2) Å, respectively. The structures were solved from single-crystal X-ray data and refined by full-matrix least-squares to R1=6.71, 5.44, and 5.73%. The structure of M₁₁X₁₀ contains three discrete anionic moieties: square rings X⁴⁻₄, dumbbells X⁴⁻₂, and isolated X³⁻. Using formal charges the unit cell of M₁₁X₁₀ may be described as containing 44 M²⁺, 2X⁴⁻₄, 8X⁴⁻₂, and 16X³⁻ ions. This structure is discussed in comparison with other Bi or Sb pnictide compounds. Bonding is analyzed therein using molecular orbital (EHMO) calculations for the anions (dumbbell and square units) and also the periodic tight-binding method. Lone pair repulsions inside and between the anionic units are evidenced; they are compensated by strong bonding cation-to-anion interactions. Interatomic distances along the series appear to be more dependent on packing than on electronic effects.     

Studies of microstructure and ruthenium valence in the ruthenocuprates Pb2RuSr2Cu2O8Cl and (Ru, M)Sr2GdCu2O8 (M=Sn, Nb)

Ruthenocuprate microstructures and Ru valences have been studied. Electron microscopy reveals short-range order of the RuO₆ octahedra rotations into a √2a×√2a×c supercell in Pb₂RuSr₂Cu₂O₈Cl. However, reanalysis of neutron diffraction data gives no significant difference between the populations of the rotation states, showing that the coherence length is very short (<100 Å). The Ru valence estimated from the XANES spectrum of Pb₂RuSr₂Cu₂O₈Cl is ∼5, in keeping with the physical properties of this material which show that there is essentially no Ru-Cu charge transfer. The Ru valence in doped Ru_1−xM_xSr₂GdCu₂O₈ (M=Sn, Nb) is ∼4.8 in all samples, verifying a previous rigid band analysis of the charge distribution in these materials.     

A novel route to synthesize cubic ZrW2−xMoxO8 (x=0-1.3) solid solutions and their negative thermal expansion properties

Cubic ZrW_2−xMo_xO₈ (c-ZrW_2−xMo_xO₈) (x=0-1.3) solid solutions were prepared by a novel polymorphous precursor transition route. X-ray diffraction (XRD) analysis reveals that the solid solutions are single phase with α- and β-ZrW₂O₈ structure for 0?x?0.8 and 0.9?x?1.3, respectively. The optimum synthesis conditions of ZrWMoO₈ are obtained from differential scanning calorimetry-thermal gravimetric analysis (DSC-TGA), XRD and mass loss-temperature/time curves. Following the above experience, the stoichiometric solid solutions of c-ZrW_2−xMo_xO₈ (x=0-1) are obtained within 1 wt% of mass loss. The relationships of lattice parameters (a), phase transition temperatures (T_c) and instantaneous coefficients of thermal expansion (α_i) against the content x of Mo are discussed based on the variation of order degree parameters of ZrW_2−xMo_xO₈.     

New Ternary Compounds MxTa11−xGe8 (M=Ti, Zr, Hf): Structure and Stabilization

The title compounds M_xTa_11−xGe₈ (M=Ti, Zr, Hf) were prepared from the pure elements by arc-melting and subsequent induction heating at temperatures between 1200°C and 1400°C. X-ray powder diffraction studies of the samples were performed using the Guinier technique and the respective powder patterns were refined with a structure model based on the orthorhombic Cr₁₁Ge₈-structure type (oP76, Pnma). The homogeneity ranges of the compounds were determined to be 0.9<x<1.3 (M=Ti), 0.7<x<1.3 (M=Zr) and 0.7<x<2.4> (M=Hf) by means of electron probe microanalysis. Chemical bonding, electronic structure and site preferences are discussed based on extended Hückel calculations performed on hypothetical binary Ta₁₁Ge₈.     

Substitutional Effects of 3d Transition Metals on the Magnetic and Structural Properties of Quasi-Two-Dimensional La5Mo4O16

Substituted phases with the composition La₅Mo_4−xT_xO_16−δ (T=Co, Fe, Mn, and Mg and x∼0.7) were prepared by fused-salt electrolysis and/or conventional solid-state methods. The crystal structure of the parent compound, La₅Mo₄O₁₆, contains perovskite-like corner-sharing MoO₆ octahedral units in the ab plane separated by Mo₂O₁₀ bioctahedral units along the c direction. Detailed single-crystal X-ray diffraction studies on the Co-substituted phase, La₅Mo_3.31Co_0.69O_16−δ, indicated that the unit cell is triclinic (space group C-1) with Co exclusively replacing Mo atoms in the perovskite layers. X-ray absorption measurements revealed that the transition metal ions are divalent, consistent with the crystal structure analysis. The anomalous magnetic transition observed at 180 K in the parent compound shifts to lower temperatures upon substitution with transition metal ions. No long-range magnetic order was evident in the Mg²⁺-substituted compositions. The electrical resistivity of all the substituted phases was at least 3 orders of magnitude higher than that of the parent compound. Variations in the magnetic and electrical properties have been ascribed to the disruption of exchange correlations caused by substitutional disorder at the Mo sites.     

Expanded polycationic mercury-chalcogen networks in the layered compounds Hg3E2[MX6] (E=S, Se; M=Zr, Hf; X=Cl, Br)

The reactions of HgE (E=S, Se) with HgX₂ and MX₄ (M=Zr, Hf; X=Cl, Br) in evacuated glass ampoules lead to a series of isotypic compounds of the general formula Hg₃E₂[MX₆] in the form of colorless (X=Cl) and light-yellow (X=Br) air-sensitive crystals. The crystal structures of Hg₃S₂[ZrCl₆] (I), Hg₃S₂[HfCl₆] (II), Hg₃Se₂[ZrCl₆] (III), Hg₃Se₂[HfCl₆] (IV), Hg₃S₂[ZrBr₆] (V), and Hg₃Se₂[ZrBr₆] (VI) were refined based on single-crystal data. All compounds crystallize in the monoclinic space group P2₁/a with the lattice parameters a=662.18(2) pm, b=734.97(3) pm, c=1290.83(5) pm, β=91.755(2)° for (I) and and a=701.97(3) pm, b=756.79(3) pm, c=1350.99(6) pm, β=92.164(3)° for (VI). The structures are built of (Hg₃E₂)²⁺ layers stacked perpendicular to the c-axis. The polycationic layers consist of two-dimensionally linked 12-membered Hg₆E₆ rings in the chair conformation with linear coordinated Hg and trigonal pyramidal coordinated chalcogen atoms. Almost regular octahedral [MX₆]²⁻ ions are embedded between the layers. This arrangement is closely related to the structure of Hg₃S₂[SiF₆], which represents a higher symmetric congener. The structure relation is discussed using the supergroup-subgroup relation between space groups.     

Transition metal tetramolybdate dihydrates MMo4O13·2H2O (M=Co,Ni) having a novel pillared layer structure

Hydrothermal synthesis in the M/Mo/O (M=Co,Ni) system was investigated. Novel transition metal tetramolybdate dihydrates MMo₄O₁₃·2H₂O (M=Co,Ni), having an interesting pillared layer structure, were found. The molybdates crystallize in the triclinic system with space group P−1, Z=1 with unit cell parameters of a=5.525(3) Å, b=7.058(4) Å, c=7.551(5) Å, α=90.019(10)°, β=105.230(10)°, γ=90.286(10)° for CoMo₄O₁₃·2H₂O, and a=5.508(2) Å, b=7.017(3) Å, c=7.533(3) Å, α=90.152(6)°, β=105.216(6)°, γ=90.161(6)° for NiMo₄O₁₃·2H₂O The structure is composed of two-dimensional molybdenum-oxide (2D Mo-O) sheets pillared with CoO₆ octahedra. The 2D Mo-O sheet is made up of infinite straight ribbons built up by corner-sharing of four molybdenum octahedra (two MoO₆ and two MoO₅OH₂) sharing edges. These infinite ribbons are similar to the straight ones in triclinic-K₂Mo₄O₁₃ having 1D chain structure, but are linked one after another by corner-sharing to form a 2D sheet structure, like the twisted ribbons in BaMo₄O₁₃·2H₂O (or in orthorhombic-K₂Mo₄O₁₃) are.     

Dielectric properties of some MM′O4 and MTiM′O6 (M=Cr, Fe, Ga; M′=Nb, Ta, Sb) rutile-type oxides

We describe an investigation of the structure and dielectric properties of MM′O₄ and MTiM′O₆ rutile-type oxides for M=Cr, Fe, Ga and M′=Nb, Ta and Sb. All the oxides adopt a disordered rutile structure (P4₂/mnm) at ambient temperature. A partial ordered trirutile-type structure is confirmed for FeTaO₄ from the low temperature (17 K) neutron diffraction studies. While both the MM′O₄ oxides (CrTaO₄ and FeTaO₄) investigated show a normal dielectric property MTiM′O₆ oxides for M=Fe, Cr and M′=Nb/Ta/Sb display a distinct relaxor/relaxor-like response. Significantly the corresponding gallium analogs, GaTiNbO₆ and GaTiTaO₆, do not show a relaxor response at T<500 K.     

Structure and physical properties of ternary W5Si3-type antimonides and bismuthides Zr5M1-xPn2+x (M=Cr, Mn; Pn=Sb, Bi)

Four new ternary compounds Zr₅M_1-xPn_2+x (M=Cr, Mn; Pn=Sb, Bi) were synthesized by arc-melting and annealing at 800 °C. They crystallize in the tetragonal W₅Si₃-type structure. The crystal structure of Zr₅Cr_0.49(2)Sb_2.51(2) was refined from powder X-ray diffraction data by the Rietveld method (Pearson symbol tI32, tetragonal, space group I4/mcm, Z=4, a=11.1027(6) Å, c=5.5600(3) Å). Four-probe electrical resistivity measurements on sintered polycrystalline samples indicated metallic behavior. Magnetic susceptibility measurements between 2 and 300 K revealed temperature-independent Pauli paramagnetism for Zr₅Cr_1-xSb_2+x and Zr₅Cr_1-xBi_2+x, but a strong temperature dependence for Zr₅Mn_1-xSb_2+x and Zr₅Mn_1-xBi_2+x which was fit to the Curie-Weiss law for the latter with θ=-11.3 K and μ_eff=1.81(1) μ_B. Band structure calculations for Zr₅Cr_0.5Sb_2.5 support a structural model in which Cr and Sb atoms alternate within the chain of interstitial sites formed at the centers of square antiprismatic Zr₈ clusters.     

Crystal structure and electronic properties of the new compounds U3Co12−xX4 with X=Si, Ge

The new compounds U₃Co_12−xX₄ with X=Si, Ge were prepared by direct solidification of the corresponding liquid phase, followed by subsequent annealing at 1173 K. Single crystal X-ray diffraction carried out at room temperature showed that they crystallize with the hexagonal space group P6₃/mmc (no.194) and the unit-cell parameters a=8.130(5), c=8.537(5) Å and a=8.256(1), c=8.608(1) Å for the silicide and germanide, respectively. Their crystal structure derives from the EuMg_5.2 structure type, and is closely related to the Sc₃Ni₁₁Si₄ and Gd₃Ru_4−xAl_12+x types. For the present compounds, no substitution mechanisms have been observed, the partial occupancy of one Co site results from the presence of vacancies, only. The homogeneity ranges, evaluated by energy dispersive spectroscopy analysis, extend from x=0.0(2) to 0.3(2) and from x=0.0(2) to 1.0(2) for U₃Co_12−xSi₄ and U₃Co_12−xGe₄, respectively. The electronic properties of both compounds were investigated by means of DC magnetic susceptibility and DC electrical resistivity measurements. The U₃Co_12−xX₄ compounds are both Pauli paramagnets with their electrical resistivity best described as poor metallic or dirty metallic behavior.     

Magnetic order in AVX3 (A = Rb,Cs, (CD3)4 N; X = Cl,Br, I): A neutron diffraction study

AVX₃ (A = Rb, Cs, (CD₃)₄ N; X = Cl, Br, I) crystallize in the hexagonal system, space group $\text{P6}{}_3\text{mmc}$, with chains of face-sharing VX₆ octahedra along the c-axis. This leads to a pronounced one-dimensional character of their magnetic properties with a strong antiferromagnetic exchange interaction J between nearest neighbor V²⁺ ions along these chains. All compounds except [(CD₃)₄N]VCl₃ order three-dimensionally with ordering temperatures T_c between 13 and 32 K. In the ordered phase the magnetic moments, μ, lie in the basal plane in a triangular arrangement typical for antiferromagnetic interchain interaction J′.