Philippe Gall

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.

Redetermination of Zn2Mo3O8

The crystal structure of dizinc trimolybdenum(IV) octaoxide, Zn2Mo3O8, has been redetermined from single-crystal X-ray data. The structure has been reported previously based on neutron powder diffraction data [Hibble et al. (1999 ▶). Acta Cryst. B55, 683-697] and single-crystal data [McCarroll et al. (1957 ▶). J. Am. Chem. Soc. 79, 5410–5414; Ansell & Katz (1966 ▶) Acta Cryst. 21, 482–485]. The results of the current redetermination show an improvement in the precision of the structural and geometric parameters with all atoms refined with anisotropic displacement parameters. The crystal structure consists of distorted hexagonal-close-packed oxygen layers with stacking sequence abac along [001] and is held together by alternating zinc and molybdenum layers. The Zn atoms occupy both tetrahedral and octahedral interstices with a ratio of 1:1. The Mo atoms occupy octahedral sites and form strongly bonded triangular clusters involving three MoO6 octahedra that are each shared along two edges, forming a Mo3O13 unit. All atoms lie on special positions. The Zn atoms are in 2b Wyckoff positions with 3m. site symmetry, the Mo atoms are in 6c Wyckoff positions with . m. site symmetry and the O atoms are in 2a, 2b and 6c Wyckoff positions with 3m. and . m. site symmetries, respectively.

Preparation and magnetic properties of rare-earth pentamolybdenum octaoxides [REMo5O8 (RE = La, Ce, Pr, Nd and Sm)]

Abstract We present the synthesis of the REMo 5 O 8 compounds with RE = La, Ce, Pr, Nd and Sm. These compounds which crystallize in the space group P2 1 /c are characterized by infinite chains of bioctahedral Mo 10 O 18 cluster units. First magnetic dc measurements performed on these compounds are also reported. The La compound shows a temperature independent susceptibility with an anomaly at 13 K which could be associated to a phase transition. The Ce, Pr and Nd members have a nearly RE 3+ free ion behaviour at high temperature. Finally, the temperature dependence of the magnetic susceptibility of SmMo 5 O 8 depends on the cooling mode (zero-field cooled or field cooled). This behaviour seems to be due a spin glass transition.

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.

The scheelite‐type europium molybdate Eu0.96MoO4

Eu0.96MoO4 crystallizes with the scheelite-type structure in the space group I41/a. The Eu and Mo atoms are in 4b and 4a positions with \overline{4} symmetry, and the O atoms are in general 16f positions.

La5Mo6O21: a novel ternary reduced molybdenum oxide containing {MoIV}3 clusters and isolated MoV centres.

The crystal structure of La5Mo6O21 (pentalanthanum hexamolybdenum henicosaoxide) is made up of Mo3O13 units containing triangular {MoIV}3 clusters, three distorted MoVO6 octahedral units and six interstitial LaIII atoms. The Mo3O13 unit consists of three edge-sharing MoIVO6 units involving Mo-Mo bonding. The three MoVO6 octahedra share their corners or edges with each other and with the Mo3O13 units.

Rare earth site splitting in Sm3MoO7

Trisamarium molybdenum heptaoxide, Sm(3)MoO(7), is isomorphous with Ln(3)MoO(7) (Ln = La and Pr). The crystal structure consists of chains of corner-linked MoO(6) octahedra running parallel to the b axis and separated from each other by seven- or eight-coordinate Sm-O polyhedra. In contrast to La(3)MoO(7) and Pr(3)MoO(7), a splitting of one Sm site into two positions is observed.

V1.42In1.83Mo15Se19

The structure of the title compound, vanadium indium pentadecamolybdenum nonadecaselenide, V1.42In1.83Mo15Se19, is isotypic with In2.9Mo15Se19 [Grüttner et al. (1979 ▶). Acta Cryst. B35, 285–292]. It is characterized by two cluster units Mo6Sei 8Sea 6 and Mo9Sei 11Sea 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 interconnected through additional Mo—Se bonds. In the title compound, the V3+ cations replace the trivalent indium atoms present in In2.9Mo15Se19, 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.

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

The crystal structure of trisodium potassium dodecamolybdenum tetradecasulfide, Na2.9 (2)KMo12S14, consists of Mo12S14S6 cluster units interconnected through interunit 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).

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.