标题： 固态到液态行为的铜在超离子Cu₂X（X=S，Se）中的扩散：非弹性中子散射和从头算分子动力学研究 显示英文标题

标题： Solid-like to Liquid-like Behavior of Cu Diffusion in Superionic Cu2X (X=S, Se): An Inelastic Neutron Scattering and Ab-Initio Molecular Dynamics Investigation

摘要： Cu2Se和Cu2S是具有大扩散系数的超离子导体的良好模型体系，据报道它们表现出不同的固-液-like铜离子扩散。 在本文中，我们通过温度依赖的从头算分子动力学（AIMD）模拟和非弹性中子散射（INS）实验澄清了这些化合物的原子动力学。 使用动态结构因子和Van-Hove关联函数，我们研究了跳跃时间、跳跃长度分布和相关的扩散系数。 在500 K的立方Cu2Se中，我们发现类似固体的扩散，铜跳跃长度与晶体中约3{\AA }的第一邻近铜-铜距离相匹配，并且有明确的涉及铜振动的光学声子。 在700 K以上，跳跃长度分布呈现出以4{\AA }为中心的宽峰，覆盖了第一和第二邻近晶格距离，并且铜声子态密度同时展宽。 此外，在900 K以上，铜扩散接近液态-like，铜原子的分布连续连接晶体位点，而涉及铜运动的振动模式高度阻尼，尽管仍未完全阻尼，如液体中那样。 在低温下，类似固体的扩散与之前的X射线衍射和非弹性中子散射实验一致，而高温下液态-like扩散的观察结果与之前的AIMD模拟一致。 我们还报告了在六方和立方超离子相中的Cu2S的AIMD模拟，并分别在低温和高温下观察到类似固体和液态的扩散。 计算的离子电导率与报道的实验值基本一致。 显示英文摘要

摘要： Cu2Se and Cu2S are excellent model systems of superionic conductors with large diffusion coefficients that have been reported to exhibit different solid-liquid-like Cu-ion diffusion. In this paper, we clarify the atomic dynamics of these compounds with temperature-dependent ab-initio molecular dynamics (AIMD) simulations and inelastic neutron scattering (INS) experiments. Using the dynamical structure factor and Van-Hove correlation function, we interrogate the jump-time, hopping length distribution and associated diffusion coefficients. In cubic-Cu2Se at 500 K, we find solid-like diffusion with Cu-jump lengths matching well the first-neighbour Cu-Cu distance of ~3 {\AA} in the crystal, and clearly defined optic phonons involving Cu-vibrations. Above 700 K, the jump-length distribution becomes a broad maximum cantered around 4 {\AA}, spanning the first and second neighbour lattice distances, and a concurrent broadening of the Cu-phonon density of states. Further, above 900 K, the Cu-diffusion becomes close to liquid-like, with distributions of Cu-atoms continuously connecting crystal sites, while the vibrational modes involving Cu motions are highly damped, though still not fully over-damped as in a liquid. At low temperatures, the solid-like diffusion is consistent with previous X-ray diffraction and quasielastic neutron scattering experiments, while the higher-temperature observation of the liquid-like diffusion is in agreement with previous AIMD simulations. We also report AIMD simulations in Cu2S in the hexagonal and cubic superionic phases, and observe similar solid and liquid-like diffusion at low- and high-temperatures, respectively. The calculated ionic-conductivity is in fair agreement with reported experimental values.