The Role of Cation Vacancies in GeSe: Stabilizing High-Symmetric Phase Structure and Enhancing Thermoelectric Performance

Enhancing crystal symmetry is perceived as the most efficacious strategy to improve the thermoelectric performance of GeSe. Although multicomponent alloying is commonly employed to increase crystal symmetry, the excessive point defects inevitably degrade the carrier mobility. Herein, a new and promising strategy is proposed to stabilize the high-symmetric rhombohedral GeSe by intentionally introducing cation vacancies. Specially, three compositional series of GeSe(InSe)(u), GeSe(InTe)(v), and GeSe(InTe3/2)(x) are studied to highlight the pivotal role of cation vacancy in upgrading crystal symmetry. It is demonstrated that sole In substitution cannot change the crystal structure, InTe alloying obtains orthorhombic and rhombohedral composite phases, and adding InTe3/2 with cation vacancies enables exclusively rhombohedral GeSe. Regarding the electrical properties, rhombohedral GeSe has higher carrier concentration, carrier mobility, density-of-state effective mass, and hence superior power factor compared to orthorhombic GeSe. Meanwhile, cation vacancies hinder the heat propagation and significantly reduce the lattice thermal conductivity. Further doping the GeSe(InTe3/2)(0.15) with trace Pb to eliminate the residual orthorhombic phase results in a peak zT approximate to 0.76 at 773 K in rhombohedral Ge0.97Pb0.03Se(InTe3/2)(0.15). These results confirm the validity of cation vacancies in stabilizing high-symmetric crystal structures of GeSe system and similar strategy can be extrapolated to other low-symmetry materials.