Reduction of thermal conductivity through nanostructuring enhances the thermoelectric figure of merit in Ge1-xBixTe

A promising thermoelectric figure of merit, zT, of similar to 1.3 at 725 K was obtained in high quality crystalline ingots of Ge1-xBixTe. 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 Ge1-xBixTe. More importantly, a low lattice thermal conductivity of similar to 1.1 W m(-1) K-1 for Ge0.90Bi0.10Te 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 Ge1-xBixTe samples. The measured Vickers microhardness value of the high performance sample is similar to 165 H-V, which is comparatively higher than that of state-of-the-art thermoelectric materials, such as PbTe, Bi2Te3, and Cu2Se.