Anisotropic thermal transport and thermoelectric properties of hexagonal LiSr <i>X</i> ( <i>X</i>  = As, Sb, Bi)

Abstract

Hexagonal LiSrX (X = As, Sb, Bi) compounds exhibit intrinsically low lattice thermal conductivity and promising thermoelectric transport properties due to their layered crystal structures and strong phonon anharmonicity. In this work, we systematically investigate their thermal and electronic transport behaviors using first-principles calculations combined with anharmonic phonon and Boltzmann transport theories. The results demonstrate that all three compounds possess excellent dynamical, thermodynamic, and mechanical stability over a wide temperature range. The lattice thermal conductivity is dominated by low- to mid-frequency acoustic phonons and exhibits pronounced crystallographic anisotropy. As the X atom evolves from As to Bi, phonon softening and enhanced fourth-order anharmonicity significantly suppress the lattice thermal conductivity. Electronic structure calculations reveal that all compounds are direct-bandgap semiconductors with favorable n-type transport characteristics, including high carrier mobility and enhanced power factors. Among them, LiSrBi achieves the best thermoelectric performance with a maximum ZT value of approximately 1.0 at 800 K. Our results further show that the synergistic interplay among weak interatomic bonding, layered structural anisotropy, and anharmonic phonon dynamics governs the thermal transport behavior and thermoelectric performance of this material family. These findings provide useful insights for the design of high-performance layered thermoelectric materials.