Phase-transition temperature suppression to achieve cubic GeTe and high thermoelectric performance by Bi and Mn codoping.

ABSTRACT

Germanium telluride (GeTe)-based materials, which display intriguing functionalities, have been intensively studied from both fundamental and technological perspectives. As a thermoelectric material, though, the phase transition in GeTe from a rhombohedral structure to a cubic structure at ∼700 K is a major obstacle impeding applications for energy harvesting. In this work, we discovered that the phase-transition temperature can be suppressed to below 300 K by a simple Bi and Mn codoping, resulting in the high performance of cubic GeTe from 300 to 773 K. Bi doping on the Ge site was found to reduce the hole concentration and thus to enhance the thermoelectric properties. Mn alloying on the Ge site simultaneously increased the hole effective mass and the Seebeck coefficient through modification of the valence bands. With the Bi and Mn codoping, the lattice thermal conductivity was also largely reduced due to the strong point-defect scattering for phonons, resulting in a peak thermoelectric figure of merit (ZT) of ∼1.5 at 773 K and an average ZT of ∼1.1 from 300 to 773 K in cubic Ge0.81Mn0.15Bi0.04Te. Our results open the door for further studies of this exciting material for thermoelectric and other applications.