Zhang Zhen, Ispas Simona, Kob Walter
Laboratoire Charles Coulomb, University of Montpellier, CNRS, F-34095 Montpellier, France.
J Chem Phys. 2020 Sep 28;153(12):124503. doi: 10.1063/5.0019514.
Using molecular dynamics simulations, we investigate how the structural and vibrational properties of the surfaces of sodo-silicate glasses depend on the sodium content as well as the nature of the surface. Two types of glass surfaces are considered: A melt-formed surface (MS) in which a liquid with a free surface has been cooled down into the glass phase and a fracture surface (FS) obtained by tensile loading of a glass sample. We find that the MS is more abundant in Na and non-bridging oxygen atoms than the FS and the bulk glass, whereas the FS has higher concentration of structural defects such as two-membered rings and under-coordinated Si than the MS. We associate these structural differences to the production histories of the glasses and the mobility of the Na ions. It is also found that for Na-poor systems, the fluctuations in composition and local atomic charge density decay with a power-law as a function of distance from the surface, while Na-rich systems show an exponential decay with a typical decay length of ≈2.3 Å. The vibrational density of states shows that the presence of the surfaces leads to a decrease in the characteristic frequencies in the system. The two-membered rings give rise to a pronounce band at ≈880 cm, which is in good agreement with experimental observations.
通过分子动力学模拟，我们研究了钠硅酸盐玻璃表面的结构和振动特性如何取决于钠含量以及表面性质。我们考虑了两种类型的玻璃表面：一种是熔体形成的表面（MS），其中具有自由表面的液体被冷却成玻璃相；另一种是通过对玻璃样品进行拉伸加载获得的断裂表面（FS）。我们发现，与FS和块状玻璃相比，MS中的Na和非桥氧原子含量更高，而FS中诸如二元环和低配位Si等结构缺陷的浓度比MS更高。我们将这些结构差异与玻璃的生产历史以及Na离子的迁移率联系起来。还发现，对于贫钠体系，成分和局部原子电荷密度的波动随距离表面的距离呈幂律衰减，而富钠体系则呈指数衰减，典型衰减长度约为2.3 Å。态密度振动表明，表面的存在导致系统中特征频率降低。二元环在约880 cm处产生一个明显的峰，这与实验观察结果非常吻合。
