From stoichiometric to off-stoichiometric GeTe: Phase diagram reconstruction and thermoelectric performance reassessment

This study investigates the structure, microstructure, and transport properties of off-stoichiometric GeTe (offGeTe). In a narrow range of 50-53 at% Te, both the rhombohedral a-GeTe and orthorhombic g-GeTe phases coexist. Despite their similar chemical composition, GeTe and off-GeTe alloys exhibit distinct microstructural and thermal/electronic properties. Theoretical density functional theory (DFT) calculations were employed to verify that changes in the Ge/Te ratios influence the concentration of Ge vacancies, leading to a significant alteration in transport properties despite minor variations in chemical compositions. The off-GeTe alloy, which is free of Ge precipitates, displays defective domain boundaries, showcasing a non-typical herringbone nanostructure that is unprecedented for GeTe-based materials. Notably, the phase transition temperature of off-GeTe, at 620K, differs from its peak zT temperature of 698K. Moreover, a TE device incorporating off-GeTe demonstrates superior interfacial stability and higher energy conversion efficiency compared to its stoichiometric GeTe counterpart. Consequently, off-GeTe demonstrates superior TE performance and enhanced interfacial stability compared to stoichiometric GeTe. The addition of Sb to off-GeTe further improves its potential for TE applications by lifting the single-leg conversion efficiency greater than 3%.