Ultralow thermal conductivity and high thermoelectric figure of merit in mixed valence In5X5Br (X = S, and Se) compounds†
Discovering a material with a combination of high electrical conductivity, high thermopower, and low lattice thermal conductivity is crucial for designing efficient thermoelectric materials. Here, we show that a perfect balance of these properties can be realized in In5X5Br (X = S and Se) compounds, where indium simultaneously exists in three different oxidation states (In1+, In2+ and In3+). The presence of multiple charge carrier pockets near the band edge results in a high thermopower of 250–300 μV K−1 for both p- and n-type doping over a wide range of carrier concentrations and temperatures. Furthermore, our calculations find an exceptionally low lattice thermal conductivity of 0.55 W m−1 K−1 and 1.1 W m−1 K−1 for In5Se5Br and In5S5Br at room temperature, respectively. This ultralow lattice thermal conductivity is attributed to In1+, which is very weakly bound in the lattice. These In1+ atoms are located in a flat potential well and exhibit large mean square displacements demonstrating the strong anharmonic behavior. The strong anharmonicity was further confirmed by the large negative Grüneisen parameters. Remarkably, these compounds possess a rare combination of low thermal conductivity and high thermopower leading to an exceptionally high ZT of ∼2.4 and 2.7 for n-type In5S5Br and In5Se5Br, respectively, at high temperatures. The insights obtained from this work suggest an opportunity to discover efficient thermoelectric materials among other unexplored mixed valence compounds.