Polycrystalline Mo9 cluster chalcogenides AgxMo9Se11 (3.4≤x≤3.9) have been prepared by powder metallurgy techniques, sintered by spark plasma sintering and characterized by x-ray diffraction. Their thermoelectric properties (electrical resistivity, thermopower, thermal conductivity) have been determined in the 300–800 K temperature range. The AgxMo9Se11 compounds show p-type conduction characteristics. The outstanding low lattice thermal conductivities give rise to a rather high value of the dimensionless thermoelectric figure of merit ZT of ∼0.65 at 800 K for x=3.8–3.9, making this family of materials particularly promising for thermoelectric power generation applications.

Polycrystalline samples and single crystals of the new compound Ag 3 In 2 Mo 15 Se 19 were synthesized by solidstate reaction in a sealed molybdenum crucible at 1300 °C. Its crystal structure (space group R3̅ c, a = 9.9755(1) Å, c = 57.2943(9) Å, and Z = 6) was determined from single-crystal X-ray diffraction data and constitutes an Ag-filled variant of the In 2 Mo 15 Se 19 structure-type containing octahedral Mo 6 and bioctahedral Mo 9 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 (ZT) of 0.45 at 1100 K.

We present a detailed study of the evolution of the electrical, galvanomagnetic, and thermodynamic properties of polycrystalline Ag x Mo 9 Se 11 compounds for 3.4 ≤ x ≤ 3.8 at low temperatures (2−350 K). In agreement with density functional theory calculations, the collected data show an overall gradual variation in the transport properties from metallic to semiconducting behavior on going from x = 3.4 to 3.8. The results evidence subtle variations in the electronic properties with the Ag content, typified by both positive and negative phonon-drag effects together with thermopower and Hall coefficient of opposite signs. Analysis of the data suggests that these features may be due to peculiarities of the dispersion of the valence bands in the vicinity of the chemical potential. A drastic influence of the Ag content on the thermal transport was evidenced by a pronounced change in the temperature dependence of the specific heat below 10 K. Nonlinearities in the C p (T 3 ) data are correlated to the concentration of Ag atoms, with an increase in x resulting in a more pronounced departure from a Debye law. The observed behavior mirrors that of ionic conductors, suggesting that Ag x Mo 9 Se 11 for x ≥ 3.6 might belong to this class of compounds.