Faraji Somayeh, Ghasemi S Alireza, Parsaeifard Behnam, Goedecker Stefan
Institute for Advanced Studies in Basic Sciences, P.O. Box 45195-1159, Zanjan, Iran.
Department of Physics, Klingelbergstrasse 82, 4056 Basel, Switzerland.
Phys Chem Chem Phys. 2019 Jul 24;21(29):16270-16281. doi: 10.1039/c9cp02213a.
In this work, surface reconstructions on the (100) surface of CaF2 are comprehensively investigated. The configurations were explored by employing the Minima Hopping Method (MHM) coupled to a machine-learning interatomic potential, that is based on a charge equilibration scheme steered by a neural network (CENT). The combination of these powerful methods revealed about 80 different morphologies for the (100) surface with very similar surface formation energies differing by not more than 0.3 J m-2. To take into account the effect of temperature on the dynamics of this surface as well as to study the solid-liquid transformation, molecular dynamics simulations were carried out in the canonical (NVT) ensemble. By analyzing the atomic mean-square displacements (MSD) of the surface layer in the temperature range of 300-1200 K, it was found that in the surface region the F sublattice is less stable and more diffusive than the Ca sublattice. Based on these results we demonstrate that not only a bulk system, but also a surface can exhibit a sublattice premelting that leads to superionicity. Both the surface sublattice premelting and surface premelting occur at temperatures considerably lower than the bulk values. The complex behaviour of the (100) surface is contrasted with the simpler behavior of other low index crystallographic surfaces.
在这项工作中，对CaF₂（100）表面的表面重构进行了全面研究。通过采用与基于神经网络引导的电荷平衡方案（CENT）的机器学习原子间势相结合的最小跳跃法（MHM）来探索其构型。这些强大方法的结合揭示了（100）表面约80种不同的形态，其表面形成能非常相似，相差不超过0.3 J m⁻²。为了考虑温度对该表面动力学的影响以及研究固液转变，在正则（NVT）系综中进行了分子动力学模拟。通过分析300 - 1200 K温度范围内表面层的原子均方位移（MSD），发现表面区域中F子晶格比Ca子晶格更不稳定且扩散性更强。基于这些结果，我们证明不仅体相系统，而且表面也能表现出导致超离子性的子晶格预熔化。表面子晶格预熔化和表面预熔化均发生在远低于体相值的温度下。（100）表面的复杂行为与其他低指数晶体表面的较简单行为形成对比。
