Mass and strain field mediated low thermal conductivity for enhanced thermoelectric properties in Zn substituted SnS

Abstract

Tin sulfide (SnS) is widely recognized as a promising material for thermoelectrics owing to its layered structure, anharmonicity, earth abundance, and minimal toxicity. This study focuses on controlling the hole concentration of SnS by substituting isovalent Zn through a vacuum melting technique. The presence of point defects, such as mass fluctuations and strain field fluctuations, along with lattice dislocations and stacking faults, results in a drop in thermal conductivity. The mass difference between Zn dopant and Sn host atoms plays a significant role in point defect scattering when Zn is substituted in the SnS lattice. Additionally, variations in size and interatomic coupling forces between Zn and Sn atoms contribute to the amplification of point defect scattering, effectively reducing the lattice thermal conductivity. Furthermore, the diminished lattice thermal conductivity of SnS samples with Zn substitution is ascribed to the decreased phonon mean free path. The synergy of multi-scale scattering results in a low thermal conductivity of 0.88 W m⁻¹ K⁻¹ at 773 K. Further, Zn substitution slightly improved the carrier concentration from 6.88 × 10¹⁵ cm⁻³ to 2.31 × 10¹⁶ cm⁻³ resulting in enhanced electrical conductivity without the drastic decrement in the Seebeck coefficient. This in turn significantly improved the power factor to 42.6 μW m⁻¹ K⁻² for the Sn_0.95Zn_0.05S sample.