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层状快离子晶体是一种独特的材料，既有长程的液相离子扩散率，又有固态的亚晶格。由于其独特的固液二元性，它们在燃料电池、固体充电电池电解质、高效热电材料等领域有着广阔的应用前景。在热电转换中，晶格热导率和载流子热导率直接影响zt值。因此，如何抑制声子的传播是提高zt的关键因素。层状快离子晶体AgCrSe2是高性能热电材料的潜在候选者。尽管人们提出了不同的声子散射框架来解释液体的导热系数，但由于缺乏基于模式的非简谐声子计算和相应的动量分辨非弹性中子散射测量，详细的微观机制尚不完全清楚。
来自香港大学机械工程系的Yue Chen团队提出了一种有效的混合方案来代替三声子散射机制，用从头算分子动力学模拟中的非谐声子准粒子，对于定义良好的声子准粒子，这种方法已经被证明是有效的。基于从头计算的分子动力学模拟表明，主要与Ag贡献有关的声学声子是选择性散射的，而纵向光学声子在快离子状态下保持良好。此外，本研究结果表明，流体静水压可以抵消热诱导类液体Ag流的扩散散射，这可能为进一步的声子工程和新型热电材料设计提供了一种新的选择和思路。该文近期发表于npj Computational Materials 6: 26 (2020)，英文标题与摘要如下，点击左下角“阅读原文”可以自由获取论文PDF。
Highly selective phonon diffusive scattering in superionic layered AgCrSe2
Chen Wang and Yue Chen
Superionic materials that exhibit coexistence of rigid crystalline lattices and liquid-likefluctuating substructures have emerged as promising thermoelectric materials. The inadequate understanding of the phonon behavior in the superionic state, however, still prevents further revealing of the underlying correlation between the thermally induced liquid-like atomic dynamics and anomalous thermal transport properties. Herein, by adopting a hybrid scheme to directly characterize anharmonic phonon quasiparticles from ab-initio molecular dynamics, we manifest that low-energy transverse phonons dominated by Ag atoms totally collapse, whereas longitudinal optical phonons remain largely intact during the superionic transition. The ultralow thermal conductivity originates from the atomic level structural heterogeneity can be ultimately attributed to diffusive phonon dynamics. Our study also reveals that the extremely large selective phonon diffusive scattering can be counteracted by hydrostatic pressure induced deactivation of the liquid-like flow of Ag atoms. These results demonstrate the decisive role of ion superionicity in phonon scattering across superionic transition and may pave the way for new phonon engineering strategies in related superionic materials.
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