Partial substitution induced symmetry breaking in GaSe monolayers for enhanced thermoelectric performance

Guided by experimental synthesis and theoretical studies, we investigate noncentrosymmetric GaSe monolayers, in which strategic anionic, cationic, and co-substitution deliberately breaks mirror symmetry, systematically reducing the elastic moduli and reflecting weakened lattice rigidity and enhanced structural flexibility. Pristine GaSe exhibits an inverted "Mexican-hat" (Caldera-type) valence band topology with a characteristic DOS singularity, most pronounced in Ga_0.5Tl_0.5Se_0.5Te_0.5, while substituted systems show progressively weaker band coupling and orbital hybridization, favoring more coherent charge transport. This symmetry breaking further drives pronounced Rashba-type spin splitting in the conduction band of GaSe_0.5Te_0.5 and Ga_0.5Tl_0.5Se_0.5Te_0.5, and a marked reduction in the lattice thermal conductivity of Ga_0.5Tl_0.5Se_0.5Te_0.5, arising from phonon softening, band flattening, acoustic band bunching, and restricted Tl vibrations, reinforced by enhanced anharmonicity and a lowered Debye temperature. Together, these effects drive a monotonic improvement in thermoelectric performance across the series, establishing mirror-symmetry breaking via concurrent anionic and cationic substitution as an effective strategy for simultaneously enhancing electrical transport and suppressing lattice thermal conductivity.