Ultralow Lattice Thermal Conductivity and High Thermoelectric Performance in Ge1-x-yBixCayTe with Ultrafine Ferroelectric Domain Structure

GeTe and its derivatives emerging as a promising lead-free thermoelectric candidate have received extensive attention. Here, a new route was proposed that the minimization of kappa L in GeTe through considerable enhancement of acoustic phonon scattering by introducing ultrafine ferroelectric domain structure. We found that Bi and Ca dopants induce strong atomic strain disturbance in the GeTe matrix because of large differences in atom radius with host elements, leading to the formation of ultrafine ferroelectric domain structure. Furthermore, large strain field and mass fluctuation induced by Bi and Ca codoping result in further reduced kappa L by effectively shortening the phonon relaxation time. The co-existence of ultrafine ferroelectric domain structure, large strain field, and mass fluctuation contribute to an ultralow lattice thermal conductivity of 0.48 W m(-1) K-1 at 823 K. Bi and Ca codoping significantly enhances the Seebeck coefficient and power factor through reducing the energy offset between light and heavy valence bands of GeTe. The modified band structure boosts the power factor up to 47 mu W cm-1 K-2 in Ge0.85Bi0.09Ca0.06Te. Ultimately, a high ZT of similar to 2.2 can be attained. This work demonstrates a new design paradigm for developing high-performance thermoelectric materials.

Automated summary

GeTe and its derivatives have been widely studied as promising lead-free thermoelectric materials. In this study, a new approach to minimize the thermal conductivity of GeTe was proposed by introducing an ultrafine ferroelectric domain structure and enhancing acoustic phonon scattering. Bi and Ca dopants were found to induce atomic strain disturbance and create an ultrafine ferroelectric domain structure, leading to a significantly reduced thermal conductivity. The coexistence of the ultrafine ferroelectric domain structure, large strain field, and mass fluctuation contributed to the ultralow lattice thermal conductivity and enhanced power factor in Ge0.85Bi0.09Ca0.06Te. A high ZT value of approximately 2.2 was achieved. This work demonstrates a new design paradigm for developing high-performance thermoelectric materials.