Cation microsegregation and ionic mobility in mixed alkali glasses
Cation microsegregation and ionic mobility in mixed alkali glasses
复制标题
混合碱玻璃中的阳离子微偏析和离子迁移率
作者：
B. Vessal;G. N. Greaves;G. N. Greaves;P. Marten;Alan V. Chadwick;R. Mole;S. Houde
通过诸如X射线吸收精细结构（XAFS）等局部结构的实验探测手段，人们对氧化物玻璃中短程（<5Å）结构有序性已有很多了解，但对于中程（5 - 20 Å）结构却知之甚少。然而，基于局部原子环境测量参数的计算机模拟能够提供纳米尺度的结构模型，从而可以考虑离子传输等动态特性。在此我们描述了一种关于混合碱金属阳离子对二元硅酸盐玻璃结构影响的分子动力学模拟。众所周知，当存在不止一种碱金属时，碱金属玻璃的离子电导率会显著下降。我们证明碱金属在几埃的距离上从硅酸盐网络中分离出来。尽管预计在这些微观分离区域中离子迁移率比周围的硅酸盐网络中更高，但我们认为碱金属的随机混合仍然阻碍了给定碱金属离子的跳跃过程。这表现为跳跃的平均活化能增加，并导致总离子电导率降低。
MUCH is known about short-range (<5Å) structural order in oxide glasses from experimental probes of local structure such as X-ray absorption fine structure (XAFS)1, but over the medium range (5-20 Å) their structures are poorly understood. Computer simulations based on measured parameters for local atomic environments, however, can provide structural models on the nanometre scale, which enable dynamic properties such as ionic transport to be considered. Here we describe a molecular dynamics simulation of the effects of mixed alkali cations on the structure of binary silicate glasses. It is well known that the ionic conductivity of alkali glasses falls markedly when more than one alkali is present2. We demonstrate that the alkalis segregate from the silicate network over distances of a few ångströms. Although ionic mobility is expected to be higher in these microsegregated regions than in the surrounding silicate network, we suggest that stochastic mixing of alkalis nevertheless impedes the hopping process of a given alkali ion. This is manifest as an increase in the average activation energy for hopping and results in a lowering of the total ionic conductivity.