Promising cubic MnGeTe_2 thermoelectrics

立方相MnGeTe_2:一种有前景的热电材料（英文）

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 highconcentration 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 HV, strongly indicating MnGeTe2 as a promising thermoelectric material.

具有立方结构的IV族碲化物半导体(PbTe和SnTe)已经引领了热电领域的诸多革新.近年来,非立方相化合物GeTe与MnTe也表现出很好的热电前景.基于此,本文对GeTe与MnTe的衍生化合物(MnGeTe_2)的热电性能进行了探究.在本工作中,本征态MnGeTe_2因单质锗的析出而存在高浓度的阳离子空位,载流子浓度高达～3.6×10^(21)cm^(-3),远高于热电应用所需,通过Bi的掺杂可使得载流子显著降低(室温下MnGe_(0.9)Bi_(0.)1Te_2载流子约为～9×10^(20)cm^(-3)).在这样大的载流子浓度范围内,一方面可以基于声学声子散射机制下的单抛物带模型,实现对载流子输运性质进行全面的评估;另一方面还可以实现热电功率因子的优化.此外,由于材料中存在高度无序的阳离子和阳离子空位,可在测试温度范围内获得1.2 W m^(-1)K^(-1)甚至更低的晶格热导率.当载流子浓度达到优化值～9×10^(20)cm^(-3)时,在850 K各向同性的立方相下可获得接近1.0的zT值以及高于200 HV的维氏硬度值,进一步证实MnGeTe_2是一个很有前景的热电材料.