Lattice Softening and Defect-Induced Phonon Scattering Drive Near-Minimum Lattice Thermal Conductivity in Sb- and S-Alloyed BiSe

Alloying-induced lattice softening provides an effective route to suppress phonon transport in layered thermoelectric materials. Here, we demonstrate near-minimum lattice thermal conductivity in Sb- and S-alloyed BiSe through coupled mass and strain-field disorder and lattice softening. Sb alloying reduces the room-temperature lattice thermal conductivity from ∼0.90 W/mK in BiSe to ∼0.38 W/mK in Bi0.7Sb0.3Se, corresponding to a 57.8% reduction, while subsequent S alloying further lowers it to ∼0.33 W/mK in Bi0.7Sb0.3Se0.98S0.02, an overall suppression of ∼63.3%. The reduced sound velocity, enhanced Grüneisen parameter, shortened phonon lifetime, lower acoustic cut-off frequency, and weakened interatomic force constant collectively evidence lattice softening and strong anharmonic phonon scattering. In parallel, Sb alloying enhances the Seebeck coefficient and yields a maximum power factor of ∼839 μW/mK2, giving a zT of ∼0.4 at 520 K. These results establish lattice-softening-driven phonon engineering as a viable strategy for Te-free n-type BiSe thermoelectrics.