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

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 Cs₂Cu_<i>x</i>Mo₁₂Se₁₄ were prepared for a wide range of Cu contents (0 ≤ <i>x</i> ≤ 2). All samples crystallize isostructurally in the trigonal space group <i>R</i>3̅. 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 <i>x</i> = 0.0 sample upon Cu insertion. Transport properties measurements, performed in a wide temperature range (2–630 K) on the two end-member compounds <i>x</i> = 0 and <i>x</i> = 2, revealed a multiband electrical conduction as shown by sign reversal of the thermopower as a function of temperature

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 Cs₂Cu_<i>x</i>Mo₁₂Se₁₄ were prepared for a wide range of Cu contents (0 ≤ <i>x</i> ≤ 2). All samples crystallize isostructurally in the trigonal space group <i>R</i>3̅. 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 <i>x</i> = 0.0 sample upon Cu insertion. Transport properties measurements, performed in a wide temperature range (2–630 K) on the two end-member compounds <i>x</i> = 0 and <i>x</i> = 2, revealed a multiband electrical conduction as shown by sign reversal of the thermopower as a function of temperature

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 <i>R</i>3̅<i>c</i> 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 <i>ZT</i> of 0.6 at 800 K

Synthesis, Crystal and Electronic Structures, and Thermoelectric Properties of the Novel Cluster Compound Ag₃In₂Mo₁₅Se₁₉

Polycrystalline samples and single crystals of the new compound Ag₃In₂Mo₁₅Se₁₉ were synthesized by solid-state reaction in a sealed molybdenum crucible at 1300 °C. Its crystal structure (space group <i>R</i>3̅<i>c</i>, <i>a</i> = 9.9755(1) Å, <i>c</i> = 57.2943(9) Å, and <i>Z</i> = 6) was determined from single-crystal X-ray diffraction data and constitutes an Ag-filled variant of the In₂Mo₁₅Se₁₉ structure-type containing octahedral Mo₆ and bioctahedral Mo₉ clusters in a 1:1 ratio. The increase of the cationic charge transfer due to the Ag insertion induces a modification of the Mo–Mo distances within the Mo clusters that is discussed with regard to the electronic structure. Transport properties were measured in a broad temperature range (2–1000 K) to assess the thermoelectric potential of this compound. The transport data indicate an electrical conduction dominated by electrons below 25 K and by holes above this temperature. The metallic character of the transport properties in this material is consistent with electronic band structure calculations carried out using the linear muffin-tin orbital (LMTO) method. The complex unit cell, together with the cagelike structure of this material, results in very low thermal conductivity values (0.9 W m^–1 K^–1 at 300 K), leading to a maximum estimated thermoelectric figure of merit (<i>ZT</i>) of 0.45 at 1100 K