Yedukondalu N, Shafique Aamir, Rakesh Roshan S C, Barhoumi Mohamed, Muthaiah Rajmohan, Ehm Lars, Parise John B, Schwingenschlögl Udo
Department of Geosciences, Stony Brook University, Stony Brook, New York 11794-2100, United States.
Joint Photon Sciences Institute, Stony Brook University, Stony Brook, New York 11790-2100, United States.
ACS Appl Mater Interfaces. 2022 Sep 14;14(36):40738-40748. doi: 10.1021/acsami.2c01135. Epub 2022 Sep 2.
Understanding the interplay between various design strategies (for instance, bonding heterogeneity and lone pair induced anharmonicity) to achieve ultralow lattice thermal conductivity (κ) is indispensable for discovering novel functional materials for thermal energy applications. In the present study, we investigate layered PbXF (X = Cl, Br, I), which offers bonding heterogeneity through the layered crystal structure, anharmonicity through the Pb 6s lone pair, and phonon softening through the mass difference between F and Pb/X. The weak interlayer van der Waals bonding and the strong intralayer ionic bonding with partial covalent bonding result in a significant bonding heterogeneity and a poor phonon transport in the out-of-plane direction. Large average Grüneisen parameters (≥2.5) demonstrate strong anharmonicity. The computed phonon dispersions show flat bands, which suggest short phonon lifetimes, especially for PbIF. Enhanced Born effective charges are due to cross-band-gap hybridization. PbIF shows lattice instability at a small volume expansion of 0.1%. The κ values obtained by the two channel transport model are 20-50% higher than those obtained by solving the Boltzmann transport equation. Overall, ultralow κ values are found at 300 K, especially for PbIF. We propose that the interplay of bonding heterogeneity, lone pair induced anharmonicity, and constituent elements with high mass difference aids the design of low κ materials for thermal energy applications.
理解各种设计策略（例如，键合不均匀性和孤对诱导非谐性）之间的相互作用以实现超低晶格热导率（κ），对于发现用于热能应用的新型功能材料至关重要。在本研究中，我们研究了层状PbXF（X = Cl、Br、I），它通过层状晶体结构提供键合不均匀性，通过Pb 6s孤对提供非谐性，并通过F与Pb/X之间的质量差异实现声子软化。层间弱范德华键和具有部分共价键的强层内离子键导致显著的键合不均匀性和平面外方向上较差的声子输运。大的平均格林爱森参数（≥2.5）表明存在强非谐性。计算得到的声子色散显示出平带，这表明声子寿命较短，尤其是对于PbIF。增强的玻恩有效电荷归因于跨带隙杂化。PbIF在0.1%的小体积膨胀下表现出晶格不稳定性。通过双声子输运模型获得的κ值比通过求解玻尔兹曼输运方程获得的值高20 - 50%。总体而言，在300 K时发现了超低的κ值，尤其是对于PbIF。我们提出，键合不均匀性、孤对诱导非谐性以及具有高质量差异的组成元素之间的相互作用有助于设计用于热能应用的低κ材料。
