在熱電材料領(lǐng)域，提升轉(zhuǎn)換效率核心在于降低晶格熱導率。層狀硒氧化物，如BiCuSeO和BiAgSeO，因極低熱導率而備受矚目。BiAgSeO的熱導率可低至0.7 W/m/K，然而影響該類材料熱導率的機制仍不清晰。

由清華大學材料學院的林元華院長帶領(lǐng)的團隊，通過基于第一性原理計算求解玻爾茲曼輸運方程，系統(tǒng)研究了層狀BiCuSeO和BiAgSeO的電熱輸運行為。他們發(fā)現(xiàn)了一個有趣的現(xiàn)象：盡管從傳統(tǒng)觀點來看，材料的彈性模量表現(xiàn)出相反的趨勢，但BiCuSeO的熱導率仍低于BiAgSeO。

這種反常的熱導率歸因于含有弱鍵的Cu（Ag）-Se不規(guī)則四面體的局域?qū)ΨQ性畸變。這個結(jié)構(gòu)的特異性使得Cu原子傾向于沿層面方向振動，而Ag則更傾向于垂直層面振動。這些輾轉(zhuǎn)不定的Cu原子，

由于其具有較大的三階原子間力常數(shù)（IFC），在層面方向誘導了高度局域化的聲子模，導致極低的群速度和與Cu-Se層相關(guān)的短壽命聲子。該行為顯著降低了層狀材料的熱導率，因為原本垂直于層面的熱導率本征較低。

該項研究深刻揭示了在層狀硒氧化物材料中，多面體畸變在調(diào)控熱電性質(zhì)方面所扮演的核心角色，為進一步開發(fā)和優(yōu)化新型熱電材料提供了重要的理論依據(jù)。該文近期發(fā)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 （林元華教授） 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及其翻譯

Local symmetry distortion induced anomalous thermal conduction in thermoelectric layered oxyselenides (熱電層狀硒氧化物中局部對稱性畸變誘導的異常熱導)

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 多面體畸變，與原子排列和原子間相互作用密切相關(guān)，是固體中眾多獨特行為的驅(qū)動力，例如相變和負熱膨脹。在熱電異質(zhì)陰離子氧化物中，陰離子多面體廣泛存在，但其對熱輸運的影響鮮有研究。在這里，我們基于第一性原理計算求解玻爾茲曼輸運方程，報告了層狀硒氧化物中由局部對稱性畸變引發(fā)的異常熱導率現(xiàn)象。有趣的是，我們發(fā)現(xiàn)更輕的BiCuSeO的熱導率比更重的BiAgSeO低。由于CuSe₄和AgSe₄四面體畸變程度不同，Cu更傾向于面內(nèi)振動，而Ag更可能進行面外振動。因此，由Cu的類共振主導的傳熱聲子被顯著抑制，導致BiCuSeO的熱導率降低。這項研究突出了多面體畸變在調(diào)控層狀異質(zhì)陰離子材料熱輸運過程中的重要性。