arXiv 2012.001542020-12cond-mat.mtrl-sciphysics.comp-ph

靶向声子激发增强离子扩散

Enhancement of Ion Diffusion by Targeted Phonon Excitation

Kiarash Gordiz, Sokseiha Muy, Wolfgang G. Zeier, Yang Shao-Horn, Asegun Henry

备注：: 50 pages, 17 figures

摘要：

离子扩散在各种应用中都很重要，但对固体中扩散过程的基本理解仍然缺失，特别是考虑到晶格振动（声子）和移动物种的相互作用。在这项工作中，我们介绍了两种形式主义，确定通过固体的离子扩散的正常模式的振动（声子）的个人贡献，基于（i）轻推弹性带（NEB）计算和（ii）分子动力学（MD）模拟。$\rm{Ge}$取代的$\rm{Li_3PO_4}$（$\rm{Li_{3.042}Ge_{0.042}P_{0.958}O_4}$）的模型离子导体的结果显示，晶格中超过87%的$\rm{Li^+}$离子扩散源自小于10%的频率在8和20 THz之间的振动模式的子集。通过故意将这些贡献模式的小的目标子集（小于1%）激发到更高的温度并且仍然将晶格保持在低温下，我们观察到扩散率增加了几个数量级，这与如果整个材料（即，所有模式）被激发到相同的高温。该观察结果表明，整个材料不需要被加热到升高的温度以增加扩散率，而是只有有助于扩散的模式，或者更一般地说，反应/过渡路径，需要被激发到升高的温度。这种新的理解确定了通过设计材料中的振动来增加扩散率和/或通过外部刺激/声子激发（例如，通过光子或其它相互作用），而不必改变化合物的化学性质。

英文摘要：

Ion diffusion is important in a variety of applications, yet fundamental understanding of the diffusive process in solids is still missing, especially considering the interaction of lattice vibrations (phonons) and the mobile species. In this work, we introduce two formalisms that determine the individual contributions of normal modes of vibration (phonons) to the diffusion of ions through a solid, based on (i) Nudged Elastic Band (NEB) calculations and (ii) molecular dynamics (MD) simulations. The results for a model ion conductor of $\rm{Ge}$-substituted $\rm{Li_3PO_4}$ ($\rm{Li_{3.042}Ge_{0.042}P_{0.958}O_4}$) revealed that more than 87% of the $\rm{Li^+}$ ion diffusion in the lattice originated from a subset of less than 10% of the vibrational modes with frequencies between 8 and 20 THz. By deliberately exciting a small targeted subset of these contributing modes (less than 1%) to a higher temperature and still keeping the lattice at low temperature, we observed an increase in diffusivity by several orders of magnitude, consistent with what would be observed if the entire material (i.e., all modes) were excited to the same high temperature. This observation suggests that an entire material need not be heated to elevated temperatures to increase diffusivity, but instead only the modes that contribute to diffusion, or more generally a reaction/transition pathway, need to be excited to elevated temperatures. This new understanding identifies new avenues for increasing diffusivity by engineering the vibrations in a material, and/or increasing diffusivity by external stimuli/excitation of phonons (e.g., via photons or other interactions) without necessarily changing the compound chemistry.

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