Crystal Structure and Thermoelectric Properties of Novel Quaternary Cu₂MHf₃S₈ (M─Mn, Fe, Co, and Ni) Thiospinels with Low Thermal Conductivity

Uncovering of the origin of intrinsically low thermal conductivity in novel crystalline solids is among the main streams in modern thermoelectricity. Because of their earth-abundant nature and environmentally friendly content, Cu-based thiospinels are attractive functional semiconductors, including thermoelectric (TE) materials. Herein, we report the crystal structure, as well as electronic and TE properties of four new Cu₂MHf₃S₈ (M-Mn, Fe, Co, and Ni) thiospinels. The performed density functional theory calculations predicted the decrease of the band gap and transition from p- to n-type conductivity in the Mn-Fe-Co-Ni series, which was confirmed experimentally. The best TE performance in this work was observed for the Cu₂NiHf₃S₈ thiospinel due to its highest power factor and low thermal conductivity. Moreover, all the discovered compounds possess very low lattice thermal conductivity κ_lat over the investigated temperature range. The κ_lat for Cu₂CoHf₃S₈ has been found to be as low as 0.8 W m^-1 K^-1 at 298 K and 0.5 W m^-1 K^-1 at 673 K, which are significantly lower values compared to the other Cu-based thiospinels reported up to date. The strongly disturbed phonon transport of the investigated alloys mainly comes from the peculiar crystal structure where the large cubic unit cells contain many vacant octahedral voids. As it was evaluated from the Callaway approach and confirmed by the speed of sound measurements, such a crystal structure promotes the increase in lattice anharmonicity, which is the main reason for the low κ_lat. This work provides a guideline for the engineering of thermal transport in thiospinels and offers the discovered Cu₂MHf₃S₈ (M-Mn, Fe, Co, and Ni) compounds, as new promising functional materials with low lattice thermal conductivity.