Influence of Cu insertion on the thermoelectric properties of the quaternary cluster compounds Cu3M2Mo15Se19 (M = In, K) and Cu4In2Mo15Se19

Mo-based cluster compounds form a large class of materials with interesting thermoelectric properties primarily due to their very low lattice thermal conductivity. Here, we investigate the influence of the insertion of Cu on the crystal structures and thermoelectric properties of the ternaries In2Mo15Se19 and K2Mo15Se19. Inserting Cu+ cations into the intercluster voids of the rhombohedral lattice enables adjusting the hole concentration, resulting in higher thermopower values with respect to the Cu-free compounds. While Cu+ cations occupy a single crystallographic site in Cu3In2Mo15Se19 as in the Ag-containing analogues, a distinctive crystallographic feature of Cu3K2Mo15Se19 is the different distribution of these cations that reside on split positions. The high degree of structural disorder of these compounds gives rise to very low lattice thermal conductivity kappa(L), which directly depends on the total concentration of inserted cations. The sensitivity of the electronic and thermal properties to the Cu content is evidenced by a Cu-rich sample with nominal composition Cu4In2Mo15Se19 that exhibits enhanced power factor and reduced kappa(L), yielding a peak ZT value of similar to 0.42 at 1145 K. The good thermoelectric performance and high melting point of these compounds make them potential candidates for thermoelectric applications above 1000 K.