Li Juan, Zhang Xinyue, Wang Xiao, Bu Zhonglin, Zheng Liangtao, Zhou Binqiang, Xiong Fen, Chen Yue, Pei Yanzhong
Interdisciplinary Materials Research Center, School of Materials Science and Engineering , Tongji University , Shanghai 201804 , China.
Department of Mechanical Engineering , The University of Hong Kong , Pokfulam Road , Hong Kong SAR , China.
J Am Chem Soc. 2018 Nov 28;140(47):16190-16197. doi: 10.1021/jacs.8b09147. Epub 2018 Nov 2.
GeTe experiences phase transition between cubic and rhombohedral through distortion along the [111] direction. Cubic GeTe shares the similarity of a two-valence-band structure (high-energy L and low-energy Σ bands) with other cubic IV-VI semiconductors such as PbTe, SnTe, and PbSe, and all show a high thermoelectric performance due to a high band degeneracy. Very recently, the two valence bands were found to switch in energy in rhombohedral GeTe and to be split due to symmetry-breaking of the crystal structure. This enables the overall band degeneracy to be manipulated either by the control of symmetry-induced degeneracy or by the design of energy-aligned orbital degeneracy. Here, we show Sb-doping for optimizing carrier concentration and manipulating the degree of rhombohedral lattice distortion to maximize the band degeneracy and then electronic performance. In addition, Sb-doping significantly promotes the solubility of PbTe, enhancing the scattering of phonons by Ge/Pb substitutional defects for minimizing the lattice thermal conductivity. This successfully realizes a superior thermoelectric figure of merit, zT of >2 in both rhombohedral and cubic GeTe, demonstrating these alloys as top candidates for thermoelectric applications at T < 800 K. This work further sheds light on the importance of crystal structure symmetry manipulation for advancing thermoelectrics.
GeTe通过沿[111]方向的畸变在立方相和菱方相之间经历相变。立方相GeTe与其他立方IV-VI族半导体（如PbTe、SnTe和PbSe）具有相似的双价带结构（高能L带和低能Σ带），并且由于高的能带简并性，它们都表现出高的热电性能。最近，发现菱方相GeTe中的两个价带在能量上发生了切换，并且由于晶体结构的对称性破缺而分裂。这使得可以通过控制对称性诱导的简并性或设计能量对齐的轨道简并性来操纵整体能带简并性。在此，我们展示了通过Sb掺杂来优化载流子浓度并操纵菱方晶格畸变程度，以最大化能带简并性进而提高电子性能。此外，Sb掺杂显著提高了PbTe的溶解度，通过Ge/Pb替代缺陷增强了声子散射，从而最小化晶格热导率。这成功实现了优异的热电优值，菱方相和立方相GeTe的zT均大于2，表明这些合金是T < 800 K时热电应用的顶级候选材料。这项工作进一步揭示了晶体结构对称性操纵对推进热电学的重要性。
