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 6s 2 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, in optimal electronic content, n-type BiSeBr achieves a peak zT of 1.00 along the c-axis at 600 K.