Rhombohedral to Cubic Conversion of GeTe via MnTe Alloying Leads to Ultralow Thermal Conductivity, Electronic Band Convergence, and High Thermoelectric Performance

Abstract

In this study, a series of Ge 1– x Mn x Te ( x = 0–0.21) compounds were prepared by a melting–quenching–annealing process combined with spark plasma sintering (SPS). The effect of alloying MnTe into GeTe on the structure and thermoelectric properties of Ge 1– x Mn x Te is profound. With increasing content of MnTe, the structure of the Ge 1– x Mn x Te compounds gradually changes from rhombohedral to cubic, and the known R 3 m to Fm -3 m phase transition temperature of GeTe moves from 700 K closer to room temperature. First-principles density functional theory calculations show that alloying MnTe into GeTe decreases the energy difference between the light and heavy valence bands in both the R 3 m and Fm -3 m structures, enhancing a multiband character of the valence band edge that increases the hole carrier effective mass. The effect of this band convergence is a significant enhancement in the carrier effective mass from 1.44 m 0 (GeTe) to 6.15 m 0 (Ge 0.85 Mn 0.15 Te). In addition, alloying with MnTe decreases the phonon relaxation time by enhancing alloy scattering, reduces the phonon velocity, and increases Ge vacancies all of which result in an ultralow lattice thermal conductivity of 0.13 W m –1 K –1 at 823 K. Subsequent doping of the Ge 0.9 Mn 0.1 Te compositions with Sb lowers the typical very high hole carrier concentration and brings it closer to its optimal value enhancing the power factor, which combined with the ultralow thermal conductivity yields a maximum ZT value of 1.61 at 823 K (for Ge 0.86 Mn 0.10 Sb 0.04 Te). The average ZT value of the compound over the temperature range 400–800 K is 1.09, making it the best GeTe-based thermoelectric material.