Sarkar Debattam, Ghosh Tanmoy, Roychowdhury Subhajit, Arora Raagya, Sajan Sandra, Sheet Goutam, Waghmare Umesh V, Biswas Kanishka
Department of Physical Sciences, Indian Institute of Science Education and Research Mohali, Sector 81, S. A. S. Nagar, Manauli 140306, India.
J Am Chem Soc. 2020 Jul 15;142(28):12237-12244. doi: 10.1021/jacs.0c03696. Epub 2020 Jun 30.
The orthorhombic phase of GeSe, a structural analogue of layered SnSe (space group: ), has recently attracted attention after a theoretical prediction of high thermoelectric figure of merit, zT > 2. The experimental realization of such high performance in orthorhombic GeSe, however, is still elusive (zT ≈ 0.2). The rhombohedral phase of GeSe, a structural analogue of GeTe (space group: 3), previously stabilized at high pressure (2 GPa) and high temperature (1600 K), is promising due to its theoretically predicted ferroelectric instability and the higher earth abundance of Se compared to Te. Here, we demonstrate high thermoelectric performance in the rhombohedral crystals of GeSe, which is stabilized at ambient conditions by alloying with 10 mol % AgBiSe. We show ultralow lattice thermal conductivity (κ) of 0.74-0.47 W/mK in the 300-723 K range and high zT ≈ 1.25 at 723 K in the -type rhombohedral (GeSe)(AgBiSe) crystals grown using Bridgman method. First-principles density functional theoretical analysis reveals its vicinity to a ferroelectric instability which generates large anomalous Born effective charges and strong coupling of low energy polar optical phonons with acoustic phonons. The presence of soft optical phonons and incipient ferroelectric instability in (GeSe)(AgBiSe) are directly evident in the low temperature heat capacity () and switching spectroscopy piezoresponse force microscopy (SS-PFM) experiments, respectively. Effective scattering of heat carrying acoustic phonons by ferroelectric instability induced soft transverse optical phonons significantly reduces the κ and enhances the thermoelectric performance in rhombohedral (GeSe)(AgBiSe) crystals.
GeSe的正交相是层状SnSe的结构类似物（空间群： ），在理论预测其具有高热电优值（zT > 2）之后，最近引起了关注。然而，在正交相GeSe中实现如此高性能的实验仍难以捉摸（zT ≈ 0.2）。GeSe的菱方相是GeTe的结构类似物（空间群：3），先前在高压（2 GPa）和高温（1600 K）下稳定，由于其理论预测的铁电不稳定性以及与Te相比Se在地球上的丰度更高，因此很有前景。在这里，我们展示了GeSe菱方晶体中的高热电性能，通过与10 mol%的AgBiSe合金化，其在环境条件下得以稳定。我们发现在300 - 723 K范围内，使用布里奇曼法生长的 - 型菱方（GeSe）（AgBiSe）晶体的晶格热导率（κ）超低，为0.74 - 0.47 W/mK，在723 K时zT ≈ 1.25。第一性原理密度泛函理论分析表明其接近铁电不稳定性，这会产生大的反常玻恩有效电荷以及低能极性光学声子与声学声子的强耦合。在低温热容（ ）和开关光谱压电响应力显微镜（SS - PFM）实验中，分别直接证明了（GeSe）（AgBiSe）中存在软光学声子和初始铁电不稳定性。铁电不稳定性诱导的软横向光学声子对携带热量的声学声子的有效散射显著降低了κ，并提高了菱方（GeSe）（AgBiSe）晶体的热电性能。
