Orchestrating phase transition in GeTe thermoelectrics: An investigation into the role of electronegativity

Abstract

In thermoelectrics, mastering solid-state phase transition is paramount for enhancing the figure of merit and potentially designing novel thermoelectric materials. Focusing on GeTe, its reversible rhombohedral-to-cubic phase transition poses challenge for practical application. The determinant governing this phase transition remains obscure. Here, we introduce a simple, intuitive, and broadly applicable strategy to facilitate this phase transition by exploiting the large electronegativity disparity between cations and anions. Leveraging this concept, we adeptly identify AgInTe2 as a potent alloying agent and incorporate Sb to enhance the dissolution of AgInTe2 within the matrix, hastening the formation of cubic GeTe thanks to the increased electron transfer. These leads to the enhanced band degeneracy, formed multiscale microstructures, enhanced lattice anharmonicity, and reduced sound velocity, substantially increasing the density-of-states effective mass and decreasing the lattice thermal conductivity. Notably, AgInTe2 alloying engenders impurity bands, significantly narrowing the bandgap and rendering it ideal for low-temperature power generation. Consequently, the cubic (Ge0.94Sb0.06Te)0.9(AgInTe2)0.1 achieves a peak zT ∼ 1.01 at 473 K, resulting in a maximum energy-conversion efficiency ∼ 5.3% (9.2%) in a single-leg device under a 300 K (500 K) temperature differential. These results underscore the critical role of electronegativity difference in managing the rhombohedral-cubic phase transition in chalcogenides.