Molecular dynamics study of heterogeneous dynamics in lithium disilicate crystal
Molecular dynamics study of heterogeneous dynamics in lithium disilicate crystal
复制标题
二硅酸锂晶体非均相动力学的分子动力学研究
DOI：
10.1007/s10832-014-9897-1
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发表时间：
2015
影响因子：
1.7
通讯作者：
K. L. Ngai
中科院分区：
文献类型：
--
作者：
J. Habasaki;K. L. Ngai
采用分子动力学（MD）方法研究了锂离子在二硅酸锂晶体中的非均匀各向异性动力学行为。锂离子的均方位移在几个时间区域内显示出不断变化的动力学，然后才达到终端扩散制度。即使在系统熔点附近，也需要几十ns的模拟来获得扩散系数。在模拟中观察到离子的局域运动和扩散运动，发现动力学主要由具有长程运动的稀有事件决定。这种情况类似于具有相同组成的玻璃态对应物的缓慢动力学，尽管离子的迁移率非常小（即，由于跳跃等待时间较长，1400 K时晶体中Li离子的扩散率与650 K时玻璃中Li离子的扩散率相当。沿着c轴观察到一种锯齿形和长尺度的跳跃运动，而在短时间尺度上沿着a轴和b轴观察到更多的局部运动。在长时间内，这些运动趋于混合，并且沿沿着a轴和B轴的运动对扩散的贡献不可忽略。非均相动力学与玻璃非常相似，并且显然是独立于结构无序且依赖于移动的离子之间的相互作用的普遍性质。晶体中离子动力学的非高斯性和非指数性的幅度明显小于玻璃中的。然而，当晶体中引入缺陷时，离子的迁移率和动力学如预期的那样发生了相当大的变化。
Molecular dynamics (MD) simulations were performed to study the heterogeneous and anisotropic dynamics of lithium ions in lithium disilicate crystal. Mean squared displacements of Li ions show changing dynamics over several time regions before the terminal diffusive regime is attained. Even near the melting point of the system, simulation of several tens of ns was necessary to obtain the diffusion coefficient. Both localized and diffusive motions of ions were observed in the simulations and the dynamics were found to be dominated by the rare event with a long range motion. This situation is similar to the slow dynamics of the glassy counterpart having the same composition, although the mobility of ions is remarkably small (i.e., the diffusivity of Li ions in the crystal at 1400 K is comparable to that in the glass at a much lower temperature of ~650 K) due to longer waiting time of jumps. A kind of zig-zag and long length-scale jump motions were observed along the c-axis, while more localized motions are observed along the a- and b-axes at short time scales. At long times these motions tend to mix and the contribution of the motion along a- and b- axes to the diffusion are not negligible. The heterogeneous dynamics is quite similar to the glass and is apparently a universal property independent of structural disorder and dependent on interaction between the mobile ions. The magnitudes of the non-Gaussanity and non-exponentiality of ion dynamics in the crystal is significantly less than in the glass. However, when defects are introduced in the crystal, mobility and dynamics of ions changed considerably as expected.