Ultra-low lattice thermal conductivity driven high thermoelectric figure of merit in Sb/W co-doped GeTe

High thermoelectric performance is a material challenge associated mainly with the manipulation of lattice dynamics to obtain extrinsic phonon transport routes, which can make the lattice thermal conductivity (κ_lat) intrinsically low by introducing multiple scattering mechanisms. The present study shows that the lattice-strain-induced phonon scattering resulting from microstructural distortions in GeTe-based compounds can enable ultralow lattice thermal conductivity. The unusual lattice shrinkage, W interstitials, W nanoprecipitates, and heavy elemental mass, in Ge_0.85Sb_0.1W_0.05Te culminate in an ultralow lattice thermal conductivity of ∼0.2 W m⁻¹ K⁻¹ at 825 K. Microstructural distortions in this Sb/W co-doped GeTe are found to be primarily associated with shorter W-Te bonding owing to the anomalous effect of the higher electronegativity of the W atoms. Furthermore, the increased electrical conductivity (σ) resulting from the enhanced vacancy formation caused by W doping and W interstitials synergistically contributes to optimization of the thermoelectric performance (ZT) to ∼2.93 at 825 K. The thermoelectric efficiency (η) as high as ∼17% has been obtained for a single leg in this composition at an operating temperature of 825 K, with an estimated device ZT value of ∼1.38.