University Chemical Laboratories, Lensfield Road, Cambridge CB2 1EW, United Kingdom.
Institute of Physical Chemistry, University of Muenster, Corrensstraße 28/30, 48149 Muenster, Germany.
J Chem Phys. 2017 Oct 21;147(15):152726. doi: 10.1063/1.5005924.
We study the dynamical behaviour of a computer model for viscous silica, the archetypal strong glass former, and compare its diffusion mechanism with earlier studies of a fragile binary Lennard-Jones liquid. Three different methods of analysis are employed. First, the temperature and time scale dependence of the diffusion constant is analysed. Negative correlation of particle displacements influences transport properties in silica as well as in fragile liquids. We suggest that the difference between Arrhenius and super-Arrhenius diffusive behaviour results from competition between the correlation time scale and the caging time scale. Second, we analyse the dynamics using a geometrical definition of cage-breaking transitions that was proposed previously for fragile glass formers. We find that this definition accurately captures the bond rearrangement mechanisms that control transport in open network liquids, and reproduces the diffusion constants accurately at low temperatures. As the same method is applicable to both strong and fragile glass formers, we can compare correlation time scales in these two types of systems. We compare the time spent in chains of correlated cage breaks with the characteristic caging time and find that correlations in the fragile binary Lennard-Jones system persist for an order of magnitude longer than those in the strong silica system. We investigate the origin of the correlation behaviour by sampling the potential energy landscape for silica and comparing it with the binary Lennard-Jones model. We find no qualitative difference between the landscapes, but several metrics suggest that the landscape of the fragile liquid is rougher and more frustrated. Metabasins in silica are smaller than those in binary Lennard-Jones and contain fewer high-barrier processes. This difference probably leads to the observed separation of correlation and caging time scales.
我们研究了粘性二氧化硅的计算机模型的动力学行为，这是典型的强玻璃形成体，并将其扩散机制与早期研究的脆弱二元 Lennard-Jones 液体进行了比较。采用了三种不同的分析方法。首先，分析了扩散常数的温度和时间尺度依赖性。粒子位移的负相关性会影响二氧化硅和脆弱液体中的输运性质。我们认为，Arrhenius 和超 Arrhenius 扩散行为之间的差异来自于相关时间尺度和笼时间尺度之间的竞争。其次，我们使用以前为脆弱玻璃形成体提出的笼破坏跃迁的几何定义来分析动力学。我们发现，这个定义准确地捕捉了控制开放网络液体输运的键重排机制，并在低温下准确地再现了扩散常数。由于相同的方法适用于强和脆弱的玻璃形成体，我们可以比较这两种类型的系统中的相关时间尺度。我们将链中相关的笼破坏所花费的时间与特征笼时间进行了比较，发现脆弱的二元 Lennard-Jones 系统中的相关性持续时间比强二氧化硅系统中的长一个数量级。我们通过对二氧化硅的势能景观进行采样并将其与二元 Lennard-Jones 模型进行比较，研究了相关性行为的起源。我们发现景观之间没有定性差异，但几个度量指标表明，脆弱液体的景观更粗糙，更具有挫折感。二氧化硅中的代谢盆地比二元 Lennard-Jones 中的代谢盆地小，并且包含较少的高势垒过程。这种差异可能导致观察到的相关性和笼时间尺度的分离。
