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 degrees C. Its crystal structure (space group R (3) over barc, a = 9.9755(1) angstrom, c = 57.2943(9) angstrom, and Z = 6) was determined from single-crystal X-ray diffraction data and constitutes an Ag-filled variant of the In2Mo15Se19 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