Electronic Band Structure and Transport Properties of the Cluster Compound Ag3Tl2Mo15Se19

Mo-based cluster compounds are a large class of materials with complex crystal structures that give rise to very low lattice thermal conductivity. Here, we report on the crystal structure and transport property measurements (5–800 K) of the novel Tl-filled compound Ag3Tl2Mo15Se19. This compound adopts a crystal structure described in the rhombohedral R 3 c space group [a = 9.9601(1) Å, c = 57.3025(8) Å, and Z = 6] built by the covalent arrangement of octahedral Mo6 and bioctahedral Mo9 clusters in a 1:1 ratio, with the Ag and Tl atoms filling the large cavities between them. Transport property measurements performed on polycrystalline samples indicate that this compound behaves as a heavily doped semiconductor with mixed electrical conduction. Electronic band structure calculations combined with a semiclassical approach using the Boltzmann transport equation are in good agreement with these measurements. This compound exhibits a lattice thermal conductivity as low as 0.4 W m–1 K–1 because of highly disordered Ag and Tl atoms. Because of the low thermopower values induced by the mixed electrical conduction, the dimensionless thermoelectric figure of merit ZT remains moderate with a peak value of 0.18 at 750 K.