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Cu–Sn-based sulfides are earth-abundant and nontoxic compounds of special interest for low-cost energy harvesting applications. In the present work, we have investigated the effect of grain size on the thermoelectric properties of Cu2SnS3 (CTS). Three dense CTS samples with nanometric grains were produced by mechanical alloying combined with spark plasma sintering, preserving the small size of crystalline domains to 12, 25, and 37 nm, respectively. The experimental results show that the Seebeck coefficient (S) and electrical resistivity (ρ) decrease with decreasing domain sizes, while the thermal conductivity (κ) increases. A smaller domain size correlates with a lower resistivity and a degenerate semiconductor-like behavior due to higher carrier concentration. At the same time, our synthesis method leads to materials with very low lattice thermal conductivity, thanks to the nanometric size of grains and structural disorder. As a result, the sample with the smallest grain size exhibits the highest zT of ∼0.4 at 650 K. First-principles density functional theory (DFT) simulations on various CTS crystallite surfaces revealed localized states near the Fermi level and the absence of band gap, indicating the metallic nature of the surfaces. Various CTS systems were tested by DFT, showing the following order of increasing formation energy: stoichiometric CTS, Cu vacancy, Cu-rich, Sn vacancy, and Sn-rich.

Cu2SnS3 (CTS) is a medium-temperature, ecofriendly, p-type thermoelectric material known for phonon-glass-electron-crystal characteristic. In the present work, ordered and disordered CTS samples were prepared from elemental powders, and their electronic and vibrational properties were systematically investigated by experimental methods and ab initio calculations. The disordered CTS polymorph presents a higher power factor, PF ∼ 1.5 μW/K2 cm, than the ordered and stable phase, PF ∼ 0.5 μW/K2 cm, above 700 K, as an effect of a smaller band gap and higher carrier concentration. Most importantly, the disordered CTS shows an ultralow thermal conductivity, k ∼ 0.4–0.2 W/m K, as compared to ordered, k ∼ 1.0–0.4W/m K, in the temperature range of 323–723 K. The combined effect of a higher PF and lower k results in a higher figure of merit, zT ∼ 0.5 at 723 K, obtained for disordered CTS without resorting to chemical alloying. It turns out that structural disorder contributes to the suppression of thermal conductivity. While group velocity of acoustic phonons, as shown both by experiments and ab initio calculations, is similar in the two polymorphs, a strong anharmonicity characterizes the disordered CTS, resulting in the presence of low-lying optical modes acting as traps for heat transmission. Density functional theory/density functional perturbation theory simulations and nuclear inelastic scattering combined with high-resolution diffraction studies of the lattice parameters reveal details of phonon–phonon interactions in CTS with unprecedented effectiveness.

The parameters governing the thermoelectric efficiency of a material, Seebeck coefficient, electrical, and thermal conductivities, are correlated and their reciprocal interdependence typically prevents a simultaneous optimization. Here, we present the case of disordered cubic kesterite Cu 2 ZnSnS 4 , a phase stabilized by structural disorder at low temperature. With respect to the ordered form, the introduction of disorder improves the three thermoelectric parameters at the same time. The origin of this peculiar behavior lies in the localization of some Sn lone pair electrons, leading to "rattling" Sn ions. On one hand, these rattlers remarkably suppress thermal conductivity, dissipating lattice energy via optical phonons located below 1.5 THz; on the other, they form electron-deficient Sn-S bonds leading to a p-type dopinglike effect and highly localized acceptor levels, simultaneously enhancing electrical conductivity and the Seebeck coefficient. This phenomenon leads to a 3 times reduced thermal conductivity and doubling of both electrical conductivity and the Seebeck coefficient, resulting in a more than 20 times increase in figure of merit, although still moderate in absolute terms.