Local symmetry distortion induced anomalous thermal conduction in thermoelectric layered oxyselenides

Bin Wei, Jiale Li, Yueyang Yang, Wang Li, Zhifang Zhou, Yunpeng Zheng, Wenyu Zhang, Yifan Chai, Zheng Chang, Ce-Wen Nan & Yuan-Hua Lin 

npj Computational Materials  9: 160 (2023)

编辑概述：多面体局域畸变：降低层状硒氧化物热导率

在热电材料领域，提升转换效率核心在于降低晶格热导率。层状硒氧化物，如BiCuSeO和BiAgSeO，因极低热导率而备受瞩目。BiAgSeO的热导率可低至0.7 W/m/K，然而影响该类材料热导率的机制仍不清晰。由清华大学材料学院的林元华院长带领的团队，通过基于第一性原理计算求解玻尔兹曼输运方程，系统研究了层状BiCuSeO和BiAgSeO的电热输运行为。他们发现了一个有趣的现象：尽管从传统观点来看，材料的弹性模量表现出相反的趋势，但BiCuSeO的热导率仍低于BiAgSeO。这种反常的热导率归因于含有弱键的Cu（Ag）-Se不规则四面体的局域对称性畸变。这个结构的特异性使得Cu原子倾向于沿层面方向振动，而Ag则更倾向于垂直层面振动。这些辗转不定的Cu原子，由于其具有较大的三阶原子间力常数（IFC），在层面方向诱导了高度局域化的声子模，导致极低的群速度和与Cu-Se层相关的短寿命声子。该行为显著降低了层状材料的热导率，因为原本垂直于层面的热导率本征较低。该项研究深刻揭示了在层状硒氧化物材料中，多面体畸变在调控热电性质方面所扮演的核心角色，为进一步开发和优化新型热电材料提供了重要的理论依据。 

Editorial Summary: Polyhedral Local Distortion: Reducing the Thermal Conductivity of Layered Oxyselenides

In the field of thermoelectric materials, the key to enhancing conversion efficiency lies in reducing the lattice thermal conductivity. Layered oxyselenides, such as BiCuSeO and BiAgSeO, have garnered attention due to their extremely low thermal conductivity. The thermal conductivity of BiAgSeO can be as low as 0.7 W/m/K. However, the mechanisms affecting the thermal conductivity of such materials remain unclear. A team lead by Prof. Yuan-Hua Lin （林元华教授） from State Key Laboratory of New Ceramics and Fine Processing School of Materials Science and Engineering, Tsinghua University, systematically investigated the thermal and electrical transport behavior in layered BiCuSeO and BiAgSeO by solving the Boltzmann transport equation based on first-principles calculations. Interestingly, the thermal conductivity of BiCuSeO is lower than that of BiAgSeO, although the elastic modulus shows a conventional opposite result. Such anomaly is mainly induced by the local symmetry distortion of the irregular tetrahedron containing weakly bonded Cu (Ag)-Se, where the bonding configuration enforces Cu to be inclined to vibrate along the in-plane direction and yet Ag along the out-of-plane direction. The rattling-like Cu maintains a large third-order interatomic force constant (IFC) and induces the localized phonons along the in-plane direction, leading to ultralow group velocities and a short lifetime corresponding to the Cu-Se layer. This behavior is significantly powerful to reduce the thermal conductivity in layered materials because the thermal conductivity from the out-of-plane direction is originally low. The study unveils the critical role of polyhedral distortion in layered oxyselenides for regulating thermal and electrical transport.