Promising cubic MnGeTe2 thermoelectrics

Abstract

Semiconducting cubic group IV monotellurides, including PbTe and SnTe, have historically led most of the advancements in thermoelectrics. Recently, noncubic ones such as GeTe and MnTe have also shown to be promising, which motivates the current work focusing on the thermoelectric properties of MnGeTe2, a derivative compound of noncubic GeTe and MnTe but crystalizing in a cubic structure. This compound intrinsically comes with a carrier concentration as high as ~3.6 ´ 1021 cm−3, indicating the existence of high-concentration cation vacancies due to Ge-precipitation. This intrinsic carrier concentration is much higher than that needed for thermoelectric applications but can be successfully decreased to ~9 ´ 1020 cm−3 for MnGe0.9Bi0.1Te2 at room temperature. Such a broad carrier concentration not only offers a full assessment of its electronic transport properties according to a single parabolic band model with acoustic scattering, but also enables an optimization for thermoelectric power factor. The low lattice thermal conductivity of ~1.2 W m−1 K−1 or lower in the entire temperature range, can be understood by the highly disordered cations and cation vacancies. A peak zT approaching 1.0 at 850 K was achieved in materials at an optimal carrier concentration of ~9 ´ 1020 cm−3, an isotropic cubic structure as well as a Vickers hardness of >200 H V, strongly indicating MnGeTe2 as a promising thermoelectric material.