Screening of promising thermoelectric materials from MnTe-GeTe alloying

Abstract

GeTe-MnTe-based materials have been proposed as promising thermoelectric materials due to the ineluctable phase transition of GeTe materials. However, the previously reported Ge_1-xMn_xTe alloys with low MnTe content may mix with the rhombohedral phase of GeTe, making the sample unstable during thermal cycling. Moreover, the exact phase of GeTe-MnTe-based alloys have not been thoroughly studied. In this work, a series of Ge_1-xMn_xTe (0 ≤ x ≤ 1) compounds were synthesized, and the changes in crystal structure and thermoelectric properties were investigated across the composition range from GeTe to MnTe. At the phase boundary, both rhombohedral and cubic phases GeTe coexist in the range of x = 0.15 to 0.3, while cubic phase GeTe and hexagonal MnTe phases coexist for x = 0.5 to 0.7. The single cubic phase is observed only near x = 0.4. Alloying with MnTe significantly increase the effective mass of the electronic band, which negatively impacts the electrical performance, suggesting that the thermoelectric properties are compromised in favor of enhancing the stability of the GeTe structure. Although the resistivity of the alloy increases with Mn content, the Seebeck coefficient rises significantly, and thermal conductivity decreases. These changes are likely due to enhanced phonon scattering caused by the two phases and the Mn solute atoms. As a result, relatively high figure-of-merit (zT) values are achieved in the rhombohedral-to-cubic phase transition region (x = 0.15, 0.2, 0.3) for Ge_1-xMn_xTe.