Seeking solids with low lattice thermal conductivity (κ_l) for energy conversion is of great significance. However, the extrinsic mechanisms for low-κ_l solids like endotaxial precipitates or grain boundary engineering cannot arbitrarily scatter the low-frequency phonons, since there is a limit of scattering length, below which the vibrations no longer carry the phonon heat. Alternatively, the intrinsic mechanisms provide approaches to reduce κ_l at the atomistic level by manipulating the structure and compositions, such as phonon dispersion engineering and enhanced phonon–phonon scattering, which can decrease the phonon group velocities and the phonon scattering length, respectively. In this Account, we discuss the κ_l in crystalline solids and outline the role of bonding and lattice dynamics in heat transport. Then, we focus our research interest in the roles of subunits and the interactions between sublayers in low-dimensional solids and propose multilayer approaches for ultralow κ_l. First, we develop the strategy of multilayer stacking with mixed-valence for bonding heterogeneity, which can further reduce the κ_l. The mixed-valence can enhance the bond variety, such as covalent, ionic, metavalent, metallic, and resonant bonding. The wide variety of chemical bonding and asymmetric geometry of building blocks are responsible for the strong phonon–phonon scattering. Second, we consider that the weak bonds in a multilayered superlattice leading to phonon softening can be a potential route to achieve intrinsic low κ_l, which is closely related to the low-frequency phonon modes. According to the kinetic theory, a weak chemical bond is suggestive of the small bond force constant, which is conductive to the low-energy phonons. Several natural superlattices depict the soft phonon modes with low group velocities, providing compelling evidence for this strategy to realize low κ_l. Third, we propose that the hierarchical design of sublayers can effectively block phonon propagation. From the structure chemistry point of view, the bonding strengths are different along the different crystallographic directions, and such anisotropic bonding can enhance the phonon scattering along the layer-stacking direction. Our experimental investigation reveals that the layered solids tend to possess lower κ_l in comparison with the three-dimentional binary derivative, or related compunds with the same chemical compositions. Furthermore, we emphasize the hierarchical architecture of sublayers to boost the phonon scattering process, which leads to low κ_l. Overall, we hope to develop low-dimensional or layered solids with ultralow κ_l and understand the roles of sublayers and interlayer interactions in the thermal transport property, and these multilayer approaches for low κ_l will provide useful and atomistic-level insights into thermal transport in solids.

中文翻译：

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低维固体中超低晶格热导率的多层方法

_{寻找低晶格热导率（κ l} ）的固体用于能量转换具有重要意义。然而，低κ _l固体的外在机制，如内轴析出物或晶界工程，不能任意散射低频声子，因为散射长度是有限的，低于该长度，振动不再携带声子热量。或者，内在机制提供了通过操纵结构和成分在原子水平上降低 κ _l的方法，例如声子色散工程和增强声子-声子散射，这可以分别降低声子群速度和声子散射长度。在这篇文章中，我们讨论了结晶固体中的 κ _l并概述了键合和晶格动力学在热传输中的作用。然后，我们将研究兴趣集中在低维固体中亚基的作用和亚层之间的相互作用，并提出了超低 κ _l的多层方法。首先，我们开发了混合价多层堆叠策略来实现键合异质性，这可以进一步降低 κ _l。混合价可以增强键的多样性，例如共价键、离子键、偏价键、金属键和共振键。结构单元的各种化学键和不对称几何形状是造成强烈声子-声子散射的原因。其次，我们认为多层超晶格中的弱键导致声子软化可能是实现固有低 κ _l的潜在途径，这与低频声子模式密切相关。根据动力学理论，弱化学键意味着键力常数小，有利于低能声子。几种天然超晶格描绘了低群速度的软声子模式，为该策略实现低 κ _l提供了令人信服的证据。第三，我们提出子层的分层设计可以有效地阻止声子传播。从结构化学的角度来看，沿不同晶体方向的键合强度不同，这种各向异性键合可以增强沿层堆叠方向的声子散射。我们的实验研究表明，与三维二元衍生物或具有相同化学成分的相关化合物相比，层状固体往往具有较低的 κ _l 。此外，我们强调子层的分层架构以促进声子散射过程，从而导致低 κ _l。总的来说，我们希望开发具有超低κ _{l的低维或层状固体}并了解子层和层间相互作用在热传输特性中的作用，这些低 κ _l的多层方法将为固体热传输提供有用的原子级见解。