The Cu2FeTi3S8 and Cu2FeZr3S8 compounds: Crystal structure and electroanalytical application

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 κlat over the investigated temperature range. The κlat 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 κlat. 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.

New compounds of the Li2 MSn3S8 type

The substitution of Cu/Ag by lithium in complex thiospinels with the general formula A I 2 B II C IV 3 X VI 8 was achieved by ball milling and a subsequent annealing step in an atmosphere of H2S. Four hitherto unknown compounds Li2 MSn3S8 with M = Mg, Mn, Fe, Ni were obtained without side phases and have been structurally investigated. From X-ray powder diffraction experiments, space group Fd 3 ¯ $\overline{3}$ m and a spinel-type structure are suggested. In these so-called normal spinels, lithium occupies one eighth of the tetrahedral voids (Wyckoff position 8a) of the cubic closest packing of the sulfide ions whereas M and Sn can be found on one half of the octahedral voids (Wyckoff position 16d).