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

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.

Ultralow Lattice Thermal Conductivity and Improved Thermoelectric Performance in Cl-Doped Bi2Te3–xSex Alloys

“…Understanding and control of the thermal transport in thermoelectric (TE) materials can significantly improve the ability to interconvert heat and electricity by TE devices. , The performance of the TE materials is represented by a dimensionless figure of merit ZT = σ S T /(κ L + κ e + κ B ), where σ is the electrical conductivity; S is the Seebeck coefficient; κ L , κ e , and κ B are the lattice, electronic, and bipolar components of the thermal conductivity, respectively; and T is the absolute temperature. Due to the interlink between electronic thermal conductivity κ e with the electrical conductivity σ through the Wiedemann–Franz law (κ e = L σ T , where L is the Lorenz number), a good way to improve the TE performance of materials is connected with the decrease of κ L and κ B .…”

Structure Evolution and Bonding Inhomogeneity toward High Thermoelectric Performance in Cu2CoSnS4–xSex Materials

“…The reasonable fitting agreement with the experimental points was obtained by employing the four-phonon Umklapp scattering, which is highly probable for the systems with high lattice anharmonicity. The phonon relaxation time contributed to the four-phonon Umklapp scattering can be estimated as follows: τ normalU − 1 = B 1 true( k normalB T normalℏ true) 4 T 2 t 4 where B 1 is the fitting constant that represents the four-phonon Umklapp scattering. As can be seen in Figure a, good agreement with the experimental κ L was achieved by the implementation of only four-phonon Umklapp and grain boundary scattering.…”

“…The reasonable fitting agreement with the experimental points was obtained by employing the fourphonon Umklapp scattering, which is highly probable for the systems with high lattice anharmonicity. The phonon relaxation time contributed to the four-phonon Umklapp scattering can be estimated as follows: 76…”

Amorphous‐Like Ultralow Thermal Transport in Crystalline Argyrodite Cu7PS6

“…These derived large values are comparable to that of other Cu-/Ag-based argyrodites, [4] reflecting its intrinsically soft lattice and strong anharmonicity. [18] To assess the temperature evolution of the optical phonons, we performed Raman spectroscopy measurements on the Cu 7 PS 6 sample from 100 to 300 K (Figure S5, Supporting Information, see the Experimental Section for details). Figure 4a displays the Raman spectrum of the Cu 7 PS 6 sample at 100 K, encompassing Raman shifts from 50 to 700 cm −1 .…”