Synthesis, Crystal Structure, and Electrical and Magnetic Characterizations of the Monoclinic Compounds Ln3Mo4+xSi1-xO14 (Ln = La, Ce, Pr, and Nd; x = 0 and 0.35) Containing Mo3 Clusters and Mo2 Dimers.

Powder samples of the new monoclinic compounds Ln3Mo4SiO14 (Ln = La, Ce, Pr, and Nd) and single crystals of Pr3Mo4.35Si0.65O14 were obtained by the solid-state reaction. The crystal structure of Pr3Mo4.35Si0.65O14 was determined by single-crystal X-ray diffraction. Pr3Mo4.35Si0.65O14 crystallizes in the monoclinic space group P21/n with unit-cell parameters a = 5.6361 (2) Å, b = 17.5814 (8) Å, c = 10.9883 (4) Å, and Z = 4. Full-matrix least-squares refinement on F2 using 7544 independent reflections for 203 refinable parameters results in R1 = 0.0359 and wR2 = 0.0831. The structure contains chains of Mo3O13 clusters and chains of edge-sharing MoO6 octahedra with alternately short (2.508 Å) and long (3.161 Å) Mo-Mo distances running parallel to the a axis that are separated by 8- or 10-coordinate Pr-O polyhedra. Magnetic susceptibility measurements on the Ln3Mo4SiO14 (Ln = La, Ce, Pr, and Nd) are in agreement with a trivalent state of the rare earths for the Ce, Pr, and Nd compounds, while that on the lanthanum compound confirms the presence of one unpaired electron per Mo3 as expected. Resistivity measurements on a single crystal show that Pr3Mo4.35Si0.65O14 is a small band gap semi-conductor.

LiMo8O10: Polar Crystal Structure with Infinite Edge-Sharing Molybdenum Octahedra.

Polycrystalline LiMo8O10 was prepared in a sealed Mo crucible at 1380 °C for 48 h using the conventional high-temperature solid-state method. The polar tetragonal crystal structure (space group I41md) is confirmed based on the Rietveld refinement of powder neutron diffraction and 7Li/6Li solid-state NMR. The crystal structure features infinite chains of Mo4O5 (i.e., Mo2Mo4/2O6/2O6/3) as a repeat unit containing edge-sharing Mo6 octahedra with strong Mo-Mo metal bonding along the chain. X-ray absorption near-edge spectroscopy of the Mo-L3 edge is consistent with the formal Mo valence/configuration. Magnetic measurements reveal that LiMo8O10 is paramagnetic down to 1.8 K. Temperature-dependent resistivity [ρ(T)] measurement indicates a semiconducting behavior that can be fitted with Mott's variable range hopping conduction mechanism in the temperature range of 215 and 45 K. The ρ(T) curve exhibits an exponential increase below 5 K with a large ratio of ρ1.8/ρ300 = 435. LiMo8O10 shows a negative field-dependent magnetoresistance between 2 and 25 K. Heat capacity measurement fitted with the modified Debye model yields the Debye temperature of 365 K.

Synthesis, Crystal and Electronic Structures, and Electrical Properties of the Fifth Member of the Rb2(Mo9S11)(Mo6nS6n+2) Series: Rb10Mo39S43, an Atypical Reduced Molybdenum Sulfide Containing Mo9 and Mo30 Clusters.

The new compound Rb10Mo39S43 has been synthesized as single crystals by a solid-state reaction in a sealed molybdenum crucible at 1500 °C. It constitutes the fifth member of the homologous series of the trigonal (space group R3̅c) compounds Rb2n(Mo9S11)(Mo6nS6n+2). Consequently, its crystal structure is based on an equal mixture of Mo9S11 and Mo30S32 cluster units interlinked through Mo-S bonds with the Rb+ cations localized in the voids between the Mo9S11 and Mo30S32 units. The coexistence of the two high-nuclearity Mo9 and Mo30 clusters in the crystal structure leads to an unusual c parameter of 163.96(1) Å, and thus, Rb10Mo39S43 is the first ternary and multinary compound in solid-state chemistry to have such a large parameter. Contrary to the first four members, Rb10Mo39S43 was not found to be superconducting down to 2 K. First-principles calculations showed that the electronic structures of this series of compounds can be determined from those of the Mo9 and Mo6n clusters and that fewer interactions between clusters in Rb2n(Mo9S11)(Mo6nS6n+2) occur when n increases.

Electronic Band Structure and Transport Properties of the Cluster Compound Ag3Tl2Mo15Se19.

Mo-based cluster compounds are a large class of materials with complex crystal structures that give rise to very low lattice thermal conductivity. Here, we report on the crystal structure and transport property measurements (5-800 K) of the novel Tl-filled compound Ag3Tl2Mo15Se19. This compound adopts a crystal structure described in the rhombohedral R3 c space group [ a = 9.9601(1) Å, c = 57.3025(8) Å, and Z = 6] built by the covalent arrangement of octahedral Mo6 and bioctahedral Mo9 clusters in a 1:1 ratio, with the Ag and Tl atoms filling the large cavities between them. Transport property measurements performed on polycrystalline samples indicate that this compound behaves as a heavily doped semiconductor with mixed electrical conduction. Electronic band structure calculations combined with a semiclassical approach using the Boltzmann transport equation are in good agreement with these measurements. This compound exhibits a lattice thermal conductivity as low as 0.4 W m-1 K-1 because of highly disordered Ag and Tl atoms. Because of the low thermopower values induced by the mixed electrical conduction, the dimensionless thermoelectric figure of merit ZT remains moderate with a peak value of 0.18 at 750 K.

Synthesis, Crystal Structure, and Transport Properties of the Hexagonal Mo9 Cluster Compound Ag3RbMo9Se11.

Mo-based cluster compounds are promising candidates for thermoelectric applications at high temperatures due to their very low lattice thermal conductivity values. Here, we report on a detailed investigation of the crystal structure and transport properties measured in a wide range of temperatures (2-800 K) of polycrystalline Ag3RbMo9Se11. Single-crystal X-ray diffraction shows that this compound crystallizes in the hexagonal space group P63/m. The crystal structure is formed by stacked Mo9Se11 units leaving channels that are randomly filled by Rb+ cations, while Ag+ cations are located between the Mo9Se11 units. The high disorder in the unit cell induced by these atoms and their large anisotropic thermal displacement parameters are two key characteristics that lead to very low lattice thermal conductivity as low as 0.6 W m-1 K-1 at 800 K. The combination of semiconducting-like electrical properties and low ability to transport heat leads to a maximum dimensionless thermoelectric figure of merit ZT of 0.4 at 800 K.

Synthesis, Structure, and Electrical Properties of the Mo12 Cluster Sulfide Hg∼2.8KMo12S14 Containing Mercury Chains.

The new compound Hg∼2.8KMo12S14 was synthesized by diffusing mercury into the metastable KMo12S14 compound at 350 °C. Its crystallographic structure, solved from single-crystal X-ray diffraction, shows that the Mo-S framework is maintained during the synthesis. It is based on Mo12S14S6 units interlinked via Mo-S bonds as in the parent compound. The mercury forms linear chains with Hg-Hg distances of about 2.72 Å in the tunnels delimited by the Mo12S14S6 units. Superconductivity was observed below 2.5 K by electrical resistivity and magnetic susceptibility measurements.

Li2GeMo3O8: a novel reduced molybdenum oxide containing Mo3O13 cluster units.

The crystal structure of the title compound, dilithium germanium trimolybdenum octa-oxide, consists of distorted hexa-gonal-close-packed oxygen layers with stacking sequence ABAC along [001] that are held together by alternating lithium-germanium and molybdenum layers. The two Li(+) and Ge(4+) ions all have site symmetry 3m. and occupy, respectively, tetra-hedral and octa-hedral sites in the ratio 2:1. The Mo atom has a formal oxidation state of +3.3 and occupies an octa-hedral site (site symmetry .m.) and forms strongly bonded triangular cluster units [Mo-Mo distance = 2.4728 (8) Å] involving three MoO6 octa-hedra that are each shared along two edges, constituting an Mo3O13 unit.

Cu Insertion Into the Mo12 Cluster Compound Cs2Mo12Se14: Synthesis, Crystal and Electronic Structures, and Physical Properties.

Mo-based cluster compounds are promising materials for high-temperature thermoelectric applications due to their intrinsic, extremely low thermal conductivity values. In this study, polycrystalline cluster compounds Cs2CuxMo12Se14 were prepared for a wide range of Cu contents (0 ≤ x ≤ 2). All samples crystallize isostructurally in the trigonal space group R3̅. The position of the Cu atoms in the unit cell was determined by X-ray diffraction on a single-crystalline specimen indicating that these atoms fill the empty space between the Mo-Se clusters. Density functional theory calculations predict a metallic ground state for all compositions, in good agreement with the experimental findings. Magnetization measurements indicate a rapid suppression of the superconducting state that develops in the x = 0.0 sample upon Cu insertion. Transport properties measurements, performed in a wide temperature range (2-630 K) on the two end-member compounds x = 0 and x = 2, revealed a multiband electrical conduction as shown by sign reversal of the thermopower as a function of temperature.

Reentrant Phase Coherence in Superconducting Nanowire Composites.

The short coherence lengths characteristic of low-dimensional superconductors are associated with usefully high critical fields or temperatures. Unfortunately, such materials are often sensitive to disorder and suffer from phase fluctuations in the superconducting order parameter which diverge with temperature T, magnetic field H, or current I. We propose an approach to overcome synthesis and fluctuation problems: building superconductors from inhomogeneous composites of nanofilaments. Macroscopic crystals of quasi-one-dimensional Na2-δMo6Se6 featuring Na vacancy disorder (δ ≈ 0.2) are shown to behave as percolative networks of superconducting nanowires. Long-range order is established via transverse coupling between individual one-dimensional filaments, yet phase coherence remains unstable to fluctuations and localization in the zero (T,H,I) limit. However, a region of reentrant phase coherence develops upon raising (T,H,I). We attribute this phenomenon to an enhancement of the transverse coupling due to electron delocalization. Our observations of reentrant phase coherence coincide with a peak in the Josephson energy EJ at nonzero (T,H,I), which we estimate using a simple analytical model for a disordered anisotropic superconductor. Na2-δMo6Se6 is therefore a blueprint for a future generation of nanofilamentary superconductors with inbuilt resilience to phase fluctuations at elevated (T,H,I).

Crystal structure of Sc1.91In1.39Mo15Se19, containing Mo6 and Mo9 clusters.

The structure of scandium indium penta-deca-molybdenum nona-deca-selenide, Sc1.91In1.39Mo15Se19, is isotypic with In2.9Mo15Se19 [Grüttner et al. (1979 ▸). Acta Cryst. B35, 285-292]. It is characterized by two cluster units Mo6Se (i) 8Se (a) 6 and Mo9Se (i) 11Se (a) 6 (where i represents inner and a apical atoms) that are present in a 1:1 ratio. The cluster units are centered at Wyckoff positions 2b and 2c and have point-group symmetry -3 and -6, respectively. The clusters are inter-connected through additional Mo-Se bonds. Sc-Se and In-Se bonds complete the structural set-up. In the title compound, the Sc(3+) cations replace the trivalent indium atoms present in In2.9Mo15Se19, and a deficiency is observed at the monovalent indium site. One Mo, one Se and the Sc atom are situated on mirror planes, whereas two other Se atoms and the In atom are situated on threefold rotation axes.

X-ray characterization, electronic band structure, and thermoelectric properties of the cluster compound Ag2Tl2Mo9Se11.

We report on a detailed investigation of the crystal and electronic band structures and of the transport and thermodynamic properties of the Mo-based cluster compound Ag2Tl2Mo9Se11. This novel structure type crystallizes in the trigonal space group R3̅c and is built of a three-dimensional network of interconnected Mo9Se11 units. Single-crystal X-ray diffraction indicates that the Ag and Tl atoms are distributed in the voids of the cluster framework, both of which show unusually large anisotropic thermal ellipsoids indicative of strong local disorder. First-principles calculations show a weakly dispersive band structure around the Fermi level as well as a semiconducting ground state. The former feature naturally explains the presence of both hole-like and electron-like signals observed in Hall effect. Of particular interest is the very low thermal conductivity that remains quasi-constant between 150 and 800 K at a value of approximately 0.6 W·m(-1)·K(-1). The lattice thermal conductivity is close to its minimum possible value, that is, in a regime where the phonon mean free path nears the mean interatomic distance. Such extremely low values likely originate from the disorder induced by the Ag and Tl atoms giving rise to strong anharmonicity of the lattice vibrations. The strongly limited ability of this compound to transport heat is the key feature that leads to a dimensionless thermoelectric figure of merit ZT of 0.6 at 800 K.

Na3.88Mo15Se19: a novel ternary reduced molybdenum selenide containing Mo6 and Mo9 clusters.

The structure of tetrasodium penta-deca-molybdenum nona-deca-selenide, Na3.88Mo15Se19, is isotypic with the In3+x Mo15Se19 compounds [Grüttner et al. (1979 ▶). Acta Cryst. B35, 285-292]. It is characterized by two cluster units, Mo6Se (i) 8Se (a) 6 and Mo9Se (i) 11Se (a) 6 (where i represents inner and a apical atoms), that are present in a 1:1 ratio. The cluster units are centered at Wyckoff positions 2b and 2c and have point-group symmetry -3 and -6, respectively. The clusters are inter-connected through additional Mo-Se bonds. In the title compound, the Na(+) cations replace the trivalent as well as the monovalent indium atoms present in In3.9Mo15Se19. One Mo, one Se and one Na atom are situated on mirror planes, and two other Se atoms and one Na atom [occupancy 0.628 (14)] are situated on threefold rotation axes. The crystal studied was twinned by merohedry with refined components of 0.4216 (12) and 0.5784 (12).

Na2.9KMo12S14: a novel quaternary reduced molybdenum sulfide containing Mo12 clusters with a channel structure.

The crystal structure of tris-odium potassium dodeca-molybdenum tetra-deca-sulfide, Na2.9 (2)KMo12S14, consists of Mo12S14S6 cluster units inter-connected through inter-unit Mo-S bonds and delimiting channels in which the Na(+) cations are disordered. The cluster units are centered at Wyckoff positions 2d and have point-group symmetry 3.2. The K atom lies on sites with 3.2 symmetry (Wyckoff site 2c) between two consecutive Mo12S14S6 units. One of the three independent S atoms and one Na atom lie on sites with 3.. symmetry (Wyckoff sites 4e and 4f). The other Na atom occupies a 2b position with -3.. symmetry. The crystal studied was a merohedral twin with refined components of 0.4951 (13) and 0.5049 (13).

Sc(0.43(2))Rb2Mo15S19, a partially Sc-filled variant of Rb2Mo15S19.

The structure of scandium dirubidium pentadecamolybdenum nonadecasulfide, Sc(0.43(2))Rb(2)Mo(15)S(19), constitutes a partially Sc-filled variant of Rb(2)Mo(15)S(19) [Picard, Saillard, Gougeon, Noel & Potel (2000), J. Solid State Chem. 155, 417-426]. In the two compounds, which both crystallize in the R ̅3c space group, the structural motif is characterized by a mixture of Mo(6)S(i)(8)S(a)(6) and Mo(9)S(i)(11)S(a)(6) 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 ̅3. 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 Mo(9)S(11)S(6) 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 ̅1 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.

LiCe(9)Mo(16)O(35).

The structure of lithium nona-cerium hexa-deca-molybdenum penta-trideca-oxide, LiCe(9)Mo(16)O(35), is isotypic with LiNd(9)Mo(16)O(35) [Gougeon Gall, Cuny, Gautier, Le Polles, Delevoye & Trebosc (2011 ▶). Chem. Eur. J.17, 13806-13813]. It is characterized by Mo(16)O(26) (i)O(10) (a) units (where i = inner and a = apical) containing Mo(16) clusters that share some of their O atoms to form infinite molybdenum cluster chains running parallel to the b axis and separated by Li(+) and Ce(3+) cations. The Mo(16) cluster units are centred at Wyckoff positions 2c and have point-group symmetry 2/m. The Li(+) atom, in a flattened octa-hedron of O atoms, is in a 2a Wyckoff position with 2/m symmetry. The Ce(3+) cations have coordination numbers to the O atoms of 6, 9 or 10. Two Ce, two Mo and five O atoms lie on sites with m symmetry (Wyckoff site 4i), and one Ce and one O atom on sites with 2/m symmetry (Wyckoff sites 2b and 2d, respectively).

The pyrochlore-type molybdate Pr2Mo1.73Sc0.27O7.

Dipraseodymium molybdenum scandium hepta-oxide, Pr2Mo1.73Sc0.27O7, crystallizes in the cubic pyrochlore-type structure. In the crystal, (Mo,Sc)O6 octa-hedra are linked together by common corners, forming a three-dimensional [(Mo,Sc)2O6] framework. The Pr atom and another O atom atom are located in the voids of this framework. The Mo and the Sc atom are distributed statistically over the same 16d crystallographic position, with site-occupancy factors of 0.867 (3) and 0.133 (3), respectively. The Pr(3+) ions are surrounded by six O atoms from the MoO6 octa-hedra and by two other O atoms, forming a ditrigonal scalenohedron. All atoms lie on special positions. The Pr and the statistically distributed (Mo,Sc) sites are in the 16c and 16d positions with .-3m symmetry, and two O atoms are in 48f and 8a positions with 2.mm and -43m site symmetry, respectively.

Synthesis, crystal and electronic structures, and magnetic properties of LiLn9Mo16O35 (Ln = La, Ce, Pr, and Nd) compounds containing the original cluster Mo16O36.

The new compounds LiLn(9)Mo(16)O(35) (Ln=La, Ce, Pr, and Nd) were synthesized from stoichiometric mixtures of Li(2)MoO(4), Ln(2)O(3), Pr(6)O(11) or CeO(2), MoO(3), and Mo heated at 1600 °C for 48 h in a molybdenum crucible sealed under a low argon pressure. The crystal structure, determined from a single crystal of the Nd member, showed that the main building block is the Mo(16)O(36) unit, the Mo(16) core of which is totally new and results from the fusion of two bioctahedral Mo(10) clusters. It can also be viewed as a fragment of an infinite twin chain of edge-sharing Mo(6) octahedra. The Mo(16)O(36) cluster units share some oxygen atoms to form infinite chains running parallel to the b axis, which are separated by the rare-earth and lithium cations. (7)Li-NMR experiments, carried out at high field on the nonmagnetic LiLa(9)Mo(16)O(35), provided insights into the local environment of the lithium ions. Magnetic susceptibility measurements confirmed the trivalent oxidation state of the magnetic rare-earth cations and indicated the absence of localized moments on the Mo(16) clusters. The electronic structure of the LiLn(9)Mo(16)O(35) compounds was analyzed using molecular and periodic quantum calculations. The study of the molecular orbital diagrams of isolated Mo(16)O(36) models allowed the understanding of this unique metallic architecture. Periodic density functional theory calculations demonstrated that few interactions occur between the Mo(16) clusters, and predicted semiconducting properties for LiLn(9)Mo(16)O(35) as a band gap of 0.57 eV was computed for the lanthanum phase.

Pr(16)Mo(21)O(56).

The structure of hexa-deca-praseodymium henicosa-molybden-um hexa-penta-contaoxide, Pr(16)Mo(21)O(56), is isotypic with other rare earth representatives of formula type RE(16)Mo(21)O(56) (RE = La, Ce, Nd). It is characterized by Mo(10)O(18) (i)O(8) (a) units (where i = inner and a = apical O atoms) containing biocta-hedral Mo(10) clusters and octa-hedral MoO(6) units that share some of their O atoms to form the Mo-O framework. The two independent Mo(10) cluster units are centred at Wyckoff positions 2b and 2c and have point-group symmetry [Formula: see text]. The Mo atom of the MoO(6) unit is likewise situated at an inversion centre (2d). The eight crystallographically different Pr(3+) cations occupy irregular voids in the framework with coordination numbers to the O atoms varying between 8 and 11.

V(1.42)in(1.83)mo(15)se(19).

The structure of the title compound, vanadium indium penta-deca-molybdenum nona-deca-selenide, V(1.42)In(1.83)Mo(15)Se(19), is isotypic with In(2.9)Mo(15)Se(19) [Grüttner et al. (1979 ▶). Acta Cryst. B35, 285-292]. It is characterized by two cluster units Mo(6)Se(i) (8)Se(a) (6) and Mo(9)Se(i) (11)Se(a) (6) (where i represents inner and a apical atoms) that are present in a 1:1 ratio. The cluster units are centered at Wyckoff positions 2b and 2c and have point-group symmetry and , respectively. The clusters are inter-connected through additional Mo-Se bonds. In the title compound, the V(3+) cations replace the trivalent indium atoms present in In(2.9)Mo(15)Se(19), and a deficiency is observed on the monovalent indium site. One Mo, one Se and the V atom are situated on mirror planes, and two other Se atoms and the In atom are situated on threefold rotation axes.

Tl(2)Mo(9)Se(11).

The structure of Tl(2)Mo(9)Se(11), dithallium nona-molybdenum undeca-selenide, is isotypic with Tl(2)Mo(9)S(11) [Potel et al. (1980 ▶). Acta Cryst. B36, 1319-1322]. The structural set-up is characterized by a mixture of Mo(6)Se(i) (8)Se(a) (6) and Mo(12)Se(i) (14)Se(a) (6) cluster units in a 1:1 ratio. Both components are inter-connected through inter-unit Mo-Se bonds. The cluster units are centered at Wyckoff positions 3a and 3b (point-group symmetry .). The two Tl(I) atoms are situated in the voids of the three-dimensional arrangement. Two of the five independent Se atoms and the Tl atoms lie on sites with 3. symmetry (Wyckoff site 6c).