Anisotropic thermoelectric properties of layered compounds in SnX2 (X = S, Se): a promising thermoelectric material

Abstract

Thermoelectrics interconvert heat to electricity and are of great interest in waste heat recovery, solid-state cooling and so on. Here we assessed the potential of SnS₂ and SnSe₂ as thermoelectric materials at the temperature gradient from 300 to 800 K. Reflecting the crystal structure, the transport coefficients are highly anisotropic between a and c directions, in particular for the electrical conductivity. The preferred direction for both materials is the a direction in TE application. Most strikingly, when 800 K is reached, SnS₂ can show a peak power factor (PF) of 15.50 μW cm⁻¹ K⁻² along the a direction, while a relatively low value (11.72 μW cm⁻¹ K⁻²) is obtained in the same direction of SnSe₂. These values are comparable to those observed in thermoelectrics such as SnSe and SnS. At 300 K, the minimum lattice thermal conductivity (κ_min) along the a direction is estimated to be about 0.67 and 0.55 W m⁻¹ K⁻¹ for SnS₂ and SnSe₂, respectively, even lower than the measured lattice thermal conductivity of Bi₂Te₃ (1.28 W m⁻¹ K⁻¹ at 300 K). The reasonable PF and κ_min suggest that both SnS₂ and SnSe₂ are potential thermoelectric materials. Indeed, the estimated peak ZT can approach 0.88 for SnSe₂ and a higher value of 0.96 for SnS₂ along the a direction at a carrier concentration of 1.94 × 10¹⁹ (SnSe₂) vs. 2.87 × 10¹⁹ cm⁻³ (SnS₂). The best ZT values in SnX₂ (X = S, Se) are comparable to that in Bi₂Te₃ (0.8), a typical thermoelectric material. We hope that this theoretical investigation will provide useful information for further experimental and theoretical studies on optimizing the thermoelectric properties of SnX₂ materials.