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。该文近期发表于npj Computational Materials 9: 160 (2023).

Fig. 5 Temperature-dependent properties of BiCuSeO and BiAgSeO.

Editorial Summary

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. This article was recently published in npj Computational Materials 9: 160 (2023).

原文Abstract及其翻译

Local symmetry distortion induced anomalous thermal conduction in thermoelectric layered oxyselenides (热电层状硒氧化物中局部对称性畸变诱导的异常热导)

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

Abstract Polyhedral distortion, associated closely with the atomic arrangement and interatomic interactions, drives many unique behaviors in solids, such as phase transition and negative thermal expansion. In thermoelectric heteroanionic oxides, the anionic polyhedra are widely present, but their effect on thermal transport is rarely investigated. Here, we report an anomalous thermal conduction induced by local symmetry distortion in layered oxyselenides via solving the Boltzmann transport equation based on first-principles calculations. We found interestingly that lighter BiCuSeO exhibits lower thermal conductivity than heavier BiAgSeO. Due to the different distorted degrees of CuSe4 and AgSe4 tetrahedrons, Cu prefers the in-plane vibration, while Ag has more tendency of out-of-plane vibration. Thus, the heat-carrying phonons dominated by the rattling-like vibration of Cu are significantly suppressed, resulting in lower thermal conductivity of BiCuSeO. This study highlights the importance of polyhedral distortion in regulating thermal conduction in layered heteroanionic materials.

摘要 多面体畸变，与原子排列和原子间相互作用密切相关，是固体中众多独特行为的驱动力，例如相变和负热膨胀。在热电异质阴离子氧化物中，阴离子多面体广泛存在，但其对热输运的影响鲜有研究。在这里，我们基于第一性原理计算求解玻尔兹曼输运方程，报告了层状硒氧化物中由局部对称性畸变引发的异常热导率现象。有趣的是，我们发现更轻的BiCuSeO的热导率比更重的BiAgSeO低。由于CuSe₄和AgSe₄四面体畸变程度不同，Cu更倾向于面内振动，而Ag更可能进行面外振动。因此，由Cu的类共振主导的传热声子被显著抑制，导致BiCuSeO的热导率降低。这项研究突出了多面体畸变在调控层状异质阴离子材料热输运过程中的重要性。