From Rashba Splitting to Ultralow Lattice Thermal Conductivity: A First-Principles Study of Asymmetric Layered Ga Chalcogenides

Abstract

Mixed-chalcogen engineering provides an effective route for tuning lattice dynamics, spin−orbit coupling, and carrier transport in layered semiconductors. Here, first-principles density functional theory combined with Boltzmann transport calculations is employed to investigate the structural stability, phonon transport, Rashba splitting, and thermoelectric properties of layered GaSSe, GaSTe, and GaSeTe systems. The asymmetric chalcogen environment breaks inversion symmetry and induces sizable Rashba spin splitting, with GaSTe exhibiting the largest Rashba coefficient of 1.73 eV Å. Phonon dispersion and ab initio molecular dynamics calculations confirm the dynamical and thermal stability of all compounds. Te incorporation strongly alters the lattice dynamics, producing pronounced phonon softening, enhanced anharmonicity, and large positive and negative Grüneisen parameters, indicative of strong phonon scattering. Enhanced optical phonon scattering in Te-containing systems further suppresses heat transport, resulting in a low room-temperature lattice thermal conductivity of 4.1 W/mK for GaSTe. Electronic structure analysis reveals significant band convergence near the band edges, where GaSSe favors p-type transport through valence-band convergence, while GaSTe exhibits superior conduction-band convergence beneficial for n-type transport. Crystal orbital Hamilton population analysis demonstrates the formation of extended conductive pathways in Te-containing systems arising from strong Ga−Te orbital hybridization and weakened local bonding interactions. Carrier mobility calculations additionally include the Fröhlich interaction to account for polar optical phonon scattering neglected in conventional deformation potential theory. The combined effects of reduced lattice thermal conductivity, Rashba-induced band degeneracy, and enhanced electronic transport yield excellent thermoelectric performance, with the n-type ZT of GaSTe reaching ∼2.63 at 600 K, substantially exceeding those of GaSSe and GaSeTe. These results establish mixed-chalcogen gallium chalcogenides as promising candidates for thermoelectric and spintronic applications.