Abstract
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.
Highlights
IntroductionMany efforts were applied for the development of materials that consist of earth-abundant and environmentally friendly elements
The unique ability to convert heat into electrical power makes thermoelectric (TE) materials very promising for improving energy utilization and management.[1,2] The efficiency of this compelling technology is determined by the TE materials’ figure of merit, ZT = σS2T/(κlat + κel), where σ is the electrical conductivity, S is the Seebeck coefficient, κlat and κel are the lattice and electronic components of the thermal conductivity, respectively, and T is the absolute temperature.[3]
A lot of attention was focused on the development of sulfides, especially the copper-based sulfide compounds such as Cu2−xS,[7] ternary Cu−Sn−S semiconductors,[8,9] chalcopyrites,[10] cubanites,[11] colusites,[12] stannoidites,[13] tetrahedrites,[14] argyrodites,[15,16] and some other Cu-based sulfides.[17−20] Despite good TE performance, some of these compounds (e.g., Cu2−xS and argyrodites) are superionic conductors and their practical application is restricted due to low thermal stability accompanied by cation migration, which causes the structure degradation of the material.[21,22]
Summary
Many efforts were applied for the development of materials that consist of earth-abundant and environmentally friendly elements Following this idea, a lot of attention was focused on the development of sulfides, especially the copper-based sulfide compounds such as Cu2−xS,[7] ternary Cu−Sn−S semiconductors,[8,9] chalcopyrites,[10] cubanites,[11] colusites,[12] stannoidites,[13] tetrahedrites,[14] argyrodites,[15,16] and some other Cu-based sulfides.[17−20] Despite good TE performance, some of these compounds (e.g., Cu2−xS and argyrodites) are superionic conductors and their practical application is restricted due to low thermal stability accompanied by cation migration, which causes the structure degradation of the material.[21,22]
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