Reduction of thermal conductivity through nanostructuring enhances the thermoelectric figure of merit in Ge_1−xBi_xTe

A promising thermoelectric figure of merit, zT, of ∼1.3 at 725 K was obtained in high quality crystalline ingots of Ge_1−xBi_xTe. The substitution of Bi³⁺ in a Ge²⁺ sublattice of GeTe significantly reduces the excess hole concentration due to the aliovalent donor dopant nature of Bi³⁺. Reduction in carrier density optimizes electrical conductivity, and subsequently enhances the Seebeck coefficient in Ge_1−xBi_xTe. More importantly, a low lattice thermal conductivity of ∼1.1 W m⁻¹ K⁻¹ for Ge_0.90Bi_0.10Te was achieved, which is due to the collective phonon scattering from meso-structured grain boundaries, nano-structured precipitates, nano-scale defect layers, and solid solution point defects. We have obtained a reasonably high mechanical stability for the Ge_1−xBi_xTe samples. The measured Vickers microhardness value of the high performance sample is ∼165 H_V, which is comparatively higher than that of state-of-the-art thermoelectric materials, such as PbTe, Bi₂Te₃, and Cu₂Se.