Quasi-one-dimensional BiSeX (X = Br, I) semiconductors as promising thermoelectric materials with anisotropic transport properties

Abstract

Quasi-one-dimensional (quasi-1D) textured chalcohalides have garnered significant attention in thermoelectrics due to their distinct structural anisotropy and favorable transport properties. In this work, we use first-principles calculations combined with self-consistent transport theory to investigate the electronic structure, lattice dynamics, and electrical and thermal transport properties of the heavy chalcohalides BiSeX (X = Br, I). Our results reveal that the stereochemically active 6s² lone pair of Bi³⁺ induces strong lattice anharmonicity, acting as the primary driver for the observed ultralow lattice thermal conductivity (κ_l). Crucially, we demonstrate that incorporating four-phonon (4ph) scattering is indispensable for accurate thermal transport prediction, leading to a marked reduction in calculated κ_l. We uncover a distinct anisotropy governing the performance landscape of BiSeX (X = Br, I). Comparative analysis identifies BiSeBr as the superior candidate, attributed to its stronger bonding interactions and a narrower optical phonon frequency gap that facilitates acoustic–optical scattering. While the power factor is maximized along the covalent chain direction (b-axis), reaching ∼1.40 mW m⁻¹ K⁻², this direction is limited by a relatively high lattice thermal conductivity. In contrast, the transverse directions (specifically the a- and c-axes) exhibit significantly suppressed thermal conductivities of 0.33 and 0.32 W m⁻¹ K⁻¹, respectively, which effectively compensates for the reduced electronic transport. Consequently, the optimal figure of merit (zT) emerges along the transverse axes rather than the chain direction. Ultimately, at the optimal carrier concentration, n-type BiSeBr achieves a peak zT of 1.00 along the c-axis at 600 K.