Is the Presence of Sn²⁺ a Crucial Factor for the Generation of Low Thermal Conductivity in Tin-Based Sulfides?

Abstract

Comprehending the relationship between crystal structures and transport properties is crucial to develop materials with improved electrical and thermal properties for thermoelectric applications. In this article, we report on the complex crystal structure and physical properties of Cr 2 Sn 3 S 7 , a n-type magnetic semiconductor with a low energy band gap and low thermal conductivity. Importantly, we demonstrate that the high level of structural complexity is related to the coexistence of two sublattices: a host lattice, [Cr 4 Sn 2 S 11 ] 2− , characterized by a mixed Sn 4+ /Cr 3+ occupancy of its cationic sites, and a guest lattice characterized by [Sn 4 S 3 ] 2+ chains, containing Sn 2+ cations only, closely related to the ones encountered in the orthorhombic SnS compound. By combining experiments including X-ray diffraction with ab initio calculations of electrons and phonons, we succeeded to elucidate the origin of the low thermal conductivity in Cr 2 Sn 3 S 7. We demonstrate that the low dimensionality of the [Sn 4 S 3 ] 2+ chains, which generates weak Sn•••S interactions with the 3D host lattice, is induced by the lone pair stereochemical activity of Sn 2+ . This lattice softening favors strong anisotropic vibrations at low frequencies, highlighting the primordial role played by Sn 2+ cations in both crystal structure dimensionality and low thermal conductivity in tin-based sulfides. 2 =, where S, σ, κ, and T represent the Seebeck coefficient or thermopower, electrical conductivity, total thermal conductivity, and operating absolute temperature, respectively). This anisotropy in the