声子隧穿：双钙钛矿声子气模型失效

近年来，无铅卤化物双钙钛矿由于其环境友好性以及稳定性，在光电光伏领域得到了极大的关注。同时，由于其强非简谐性可能引起超低热导率，该材料在热电领域也引起了极大的关注。然而，无铅卤化物双钙钛矿的热力学性质具有强烈的温度依赖性，导致传统的晶格动力学和导热模型难以适用。

为解决该问题，香港科技大学机械与航天航空工程系黄宝陵教授团队、山东高等技术研究院郭瑞强教授团队以及美国达特茅斯学院Geoffroy Hautier 教授团队合作，提出了一种综合考虑多种因素的导热模型，该模型不仅精确考虑了有限温度对材料热力学性能的影响，而且在传统导热模型的基础上增加了额外的导热通道，从而能够精准预测无铅卤化物双钙钛矿的热导率，揭示其导热机理。

该研究通过综合考虑多声子温度效应，额外声子热传输通道以及四声子相互作用揭示了Cs₂AgBiBr₆的导热机理，发现了类波性声子隧穿在其导热过程中的主导作用。

Fig. 3 Thermal transport models, theoretically predicted and experimentally measured thermal conductivity.

作者不仅揭示了无铅卤化物双钙钛矿Cs₂AgBiBr₆的导热机理，还准确预测了其相变温度，发现了三声子相互作用对准确预测相变温度的重要性。此外，鉴于双钙钛矿热力学性质的强温度依赖性，四声子相互作用成为了一个关键因素，其纳入考量导致了传统导热模型的失效。

该研究揭示了晶格的非简谐性和类波性声子隧穿对无铅卤化物双钙钛矿热输运的重要影响。相关论文近期发布于npj Computational Materials 10: 30 (2024)。手机阅读原文，请点击本文底部左下角“阅读原文”，进入后亦可下载全文PDF文件。

Editorial Summary

In recent years, lead-free halide double perovskites have gained a lot of attention in the field of solar energy due to their environmental friendliness and stability. Additionally, their potential for extremely low thermal conductivity has sparked interest in the field of thermoelectrics. However, their strong temperature-dependent thermal properties make traditional lattice dynamics and heat transfer prediction models ineffective. This study introduces a heat transfer prediction model that integrates multiple factors. This model not only accurately considers the influence of finite temperatures on the thermal properties of double perovskite materials but also adds extra heat transfer pathways beyond traditional models. This allows for precise predictions and revelations about the heat transfer mechanisms of lead-free halide double perovskites. 

The Professor Huang Baoling’s research team from the Department of Mechanical and Aerospace Engineering at the Hong Kong University of Science and Technology, Professor Guo Ruiqiang’s team from the Shandong Advanced Academy, and Professor Geoffroy Hautier’s team from Dartmouth College in the United States, comprehensively considered the effects of multi-phonon temperature, additional phonon heat transfer pathways, and four-phonon interaction processes. This revealed the heat transfer mechanisms of lead-free halide double perovskite Cs2AgBiBr6 and identified new additional phonon transfer pathways that dominate the heat transfer process. Apart from accurately revealing the heat transfer mechanisms of lead-free halide double perovskite Cs2AgBiBr6, the study also accurately predicted the phase transition temperature of double perovskite Cs2AgBiBr6. It emphasized the importance of three-phonon particle interactions for accurate phase transition temperature predictions. Additionally, considering the strong temperature dependence of double perovskites, four-phonon particle interactions are indispensable, and their involvement renders traditional heat transfer models ineffective. The research underscores the significance of lattice anharmonicity (temperature effects) and phonon wave-like tunnelling heat transfer pathways in the thermal transfer mechanism of lead-free halide double perovskite Cs2AgBiBr6. Thisarticle was recently published in npj Computational Materials 10: 30 (2024).

原文Abstract及其翻译

Unravelling ultralow thermal conductivity in perovskite Cs₂AgBiBr₆: dominant wave-like phonon tunnelling and strong anharmonicity (揭示Cs₂AgBiBr₆钙钛矿的超低热导率：类波性声子隧穿的主导作用和强烈的非简谐性)

Jiongzhi Zheng, Changpeng Lin, Chongjia Lin, Geoffroy Hautier, Ruiqiang Guo & Baoling Huang 

Abstract Understanding the lattice dynamics and heat transport physics in the lead-free halide double perovskites remains an outstanding challenge due to their lattice dynamical instability and strong anharmonicity. In this work, we investigate the microscopic mechanisms of anharmonic lattice dynamics and thermal transport in lead-free halide double perovskite Cs₂AgBiBr₆ from first principles. We combine self-consistent phonon calculations with bubble diagram correction and a unified theory of lattice thermal transport that considers both the particle-like phonon propagation and wave-like tunnelling of phonons. An ultra-low thermal conductivity at room temperature (~0.21 Wm⁻¹K⁻¹) is predicted with weak temperature dependence ( ~ T^−0.34), in sharp contrast to the conventional ~T⁻¹ dependence. Particularly, the vibrational properties of Cs₂AgBiBr₆ are featured by strong anharmonicity and wave-like tunnelling of phonons. Anharmonic phonon renormalization from both the cubic and quartic anharmonicities are found essential in precisely predicting the phase transition temperature in Cs₂AgBiBr₆ while the negative phonon energy shifts induced by cubic anharmonicity has a significant influence on particle-like phonon propagation. Further, the contribution of the wave-like tunnelling to the total thermal conductivity surpasses that of the particle-like propagation above around 310 K, indicating the breakdown of the phonon gas picture conventionally used in the Peierls-Boltzmann Transport Equation. Importantly, further including four-phonon scatterings is required in achieving the dominance of wave-like tunnelling, as compared to the dominant particle-like propagation channel when considering only three-phonon scatterings. Our work highlights the importance of lattice anharmonicity and wave-like tunnelling of phonons in the thermal transport in lead-free halide double perovskites.

摘要无铅卤化物双钙钛矿结构的晶格不稳定性和强非简谐性导致其晶格动力学和导热机理的理解仍然面临极大的挑战。本研究基于第一性原理计算，深入研究了无铅卤化物双钙钛矿Cs₂AgBiBr₆的非简谐晶格动力学及其热输运微观机理。我们采用了带bubble修正的自洽计算对声子进行有限温度重整，并结合统一热传导模型（考虑了类粒子性声子传播通道和类波性声子隧穿传播通道）来预测热导率。在室温下，模型预测Cs₂AgBiBr₆具有超低热导率（约为0.21 Wm⁻¹K⁻¹），并呈现出极弱的温度依赖性（ ~ T^−0.34），与传统的 ~ T⁻¹依赖性形成鲜明对比。值得注意的是，Cs₂AgBiBr₆的表现出强烈的非简谐性和类波性声子隧穿特性。发现准确预测Cs₂AgBiBr₆的相变温必须同时考虑三次和四次非谐声子引起的重整，其中立方非谐性引起的负声子能量偏移对类粒子的声子传播产生了显著的影响。此外，当温度达到310 K左右时，声子隧穿的贡献超越了类粒子声子传播，导致了Peierls-Boltzmann输运方程中传统的声子气模型失效。更重要的是，仅考虑三声子散射时，粒子性传播通道占主导；进一步考虑四声子散射后，类波性声子隧穿效应成为主导。该研究揭示了晶格的强非简谐性和类波性声子隧穿对无铅卤化物双钙钛矿热输运的重要影响。