On the Origin of the Differences in Structure Directing Properties of Polar Metal Oxyfluoride [MO_<i>x</i>F_6–<i>x</i>]^2– (<i>x</i> = 1, 2) Building Units

In oxyfluoride chemistry, the [MO_<i>x</i>F_6–<i>x</i>]^2– anions (M = transition metal) are interesting polar building units that may be used to design polar materials, but their polar vs antipolar orientations in the solid state, which directly depend on the interactions between O^2–/F^– ligands and the extended structure, remain difficult to control. To improve this control, these interactions were assessed through crystallization of five related [MO_<i>x</i>F_6–<i>x</i>]^2– (M = Ti⁴⁺, V⁵⁺, Mo⁶⁺, W⁶⁺) anions with organic molecules. The hybrid organic–inorganic compounds, (4,4′-bpyH₂)­TiF₆ (<b>1</b>), (enH₂)­MoO₂F₄ (<b>2</b>), (4-hpyH)₂­MoO₂F₄·​H₂O (<b>3</b>), (4,4′-bpyH₂)­WO₂F₄ (<b>4</b>), and (4,4′-bpyH₂)­VOF₅ (<b>5</b>), exhibit isolated [MO_<i>x</i>F_6–<i>x</i>]^2– anions in a hydrogen bond network. The analysis of these crystal structures in combination with DFT calculations elucidate how differences in structure directing properties of these anions arise when π-overlap between O 2p orbitals and M d orbitals is weak and significantly affected by an increase of the energy of the d orbitals from 3d to 5d

Evaluation of ⁹⁵Mo Nuclear Shielding and Chemical Shift of [Mo₆X₁₄]^2– Clusters in the Liquid Phase

[Mo₆X₁₄]^2– octahedral molybdenum clusters are the main building blocks of a large range of materials. Although ⁹⁵Mo nuclear magnetic resonance was proposed to be a powerful tool to characterize their structural and dynamical properties in solution, these measurements have never been complemented by theoretical studies which can limit their interpretation for complex systems. In this Article, we use quantum chemical calculations to evaluate the ⁹⁵Mo chemical shift of three clusters: [Mo₆Cl₁₄]^2–, [Mo₆Br₁₄]^2–, and [Mo₆I₁₄]^2–. In particular, we test various computational parameters influencing the quality of the results: size of the basis set, treatment of relativistic and solvent effects. Furthermore, to provide quantum chemical calculations that are directly comparable with experimental data, we evaluate for the first time the ⁹⁵Mo nuclear magnetic shielding of the experimental reference, namely, MoO₄^2– in aqueous solution. This is achieved by combining ab initio molecular dynamics simulations with a periodic approach to evaluate the ⁹⁵Mo nuclear shieldings. The results demonstrate that, despite the difficulty to obtain accurate ⁹⁵Mo chemical shifts, relative values for a cluster series can be fairly well-reproduced by DFT calculations. We also show that performing an explicit solvent treatment for the reference compound improves by ∼50 ppm the agreement between theory and experiment. Finally, the standard deviation of ∼70 ppm that we calculate for the ⁹⁵Mo nuclear shielding of the reference provides an estimation of the accuracy we can achieve for the calculation of the ⁹⁵Mo chemical shifts using a static approach. These results demonstrate the growing ability of quantum chemical calculations to complement and interpret complex experimental measurements

Sb Doping of Metallic CuCr₂S₄ as a Route to Highly Improved Thermoelectric Properties

We report for the first time the thermoelectric properties of CuCr_2–<i>x</i>Sb_<i>x</i>S₄ (0.22 ≤ <i>x</i> ≤ 0.5). Although CuCr₂S₄ has been reported to be a metallic compound, addition of Sb shifts the material toward the semiconductor side. This is confirmed by band structure calculations of CuCr_2–<i>x</i>Sb_<i>x</i>S₄ (<i>x</i> = 0, 0.25, 0.5) models. Increasing Sb content enhances the power factor. However, beyond <i>x</i> = 0.3, further Sb addition lowers the electrical conductivity and power factor. A very interesting point is the simultaneous increase of the Seebeck coefficient as well as the electrical conductivity with increasing temperature, which acts like a variable range hopping (VRH) compounds but possesses much better properties than those having VRH. Samples were annealed for 48 h prior to thermoelectric properties measurements to have a reliable dimensionless figure of merit (ZT). An attractive ZT of 0.43 is obtained at ∼650 °C. The attractive thermoelectric properties we discovered by driving a metal compound into a semiconductor make this compound an interesting thermoelectric material especially because of the cheap constituent elements compared to those of typical state-of-the-art thermoelectric materials. Furthermore, this material is stable up to 650 °C at least, a relatively high temperature for sulfides. Additionally, we discovered a miscibility gap in this solid solution close to an Sb content of 0.15; although a detailed study dedicated entirely to this miscibility gap would be required, it will encourage the researchers to further explore this system

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