Weak-Bonding Elements Lead to High Thermoelectric Performance in BaSnS₃and SrSnS₃: A First-Principles Study

SnS2, an earth-abundant and ecofriendly material, is limited as a thermoelectric material because of the high lattice thermal conductivity κL and low carrier mobility μ. By introducing weak-bonding elements Ba or Sr into the SnS2 framework, we discovered two SnS2-based materials BaSnS3 and SrSnS3 with the calculated low κL values of 0.15 and 0.17 W m-1 K-1, respectively, along the a-axis. The low group velocity and high lattice anharmonicity originating from the weakened and distorted Sn-S bonding network are found in both systems. Moreover, the vibrations of Ba and Sr induce low-lying optical phonons, which strongly couple with the acoustic phonons and strengthen the phonon scattering rates. Compared to SnS2, both compounds present lower single-band effective masses, smaller deformation potential constants, and better band convergence, which enhance μ with an insignificantly reduced effective mass. By solving the linearized Boltzmann transport equation with a nonempirical carrier lifetime, we predict excellent ZT values of 2.89 and 2.77 along the a-axis at 900 K in BaSnS3 and SrSnS3, respectively. Further phase diagram calculations of Ba1-xSrxSnS3 solid solutions propose a new compound, Ba0.5Sr0.5SnS3, with an even higher ZT of 3.0. Our work analyzes explicitly how weak-bonding elements enhance μ and suppress κL simultaneously in SnS2-analogous systems with a series of compounds nominated as potential high-performance thermoelectric materials.