Visualization-based analysis of structural and dynamical properties of simulated hydrous silicate melt
Visualization-based analysis of structural and dynamical properties of simulated hydrous silicate melt
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DOI：
10.1007/s00269-009-0315-1
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发表时间：
2010-02-01
影响因子：
1.4
通讯作者：
Stixrude, Lars
中科院分区：
文献类型：
--
作者：
Karki, Bijaya B.;Bhattarai, Dipesh;Stixrude, Lars
我们探索了含水MgSiO3液体（含10 wt%水）的第一性原理分子动力学模拟数据，以深入了解其结构和动力学行为作为压力（0-150 GPa）和温度（2,000-6,000 K）的函数。通过可视化/分析与短期和中期订单相关的一些参数，我们已经表明，熔体结构在压缩时发生了实质性的变化。在低压下，水组分的形态主要是由羟基、水分子、O-H-O桥接和四原子（O-H-O- h和H-O-H-O）组成的孤立结构（90%以上的氢参与），其中每个氧原子可能是多面体的一部分或自由（即仅与镁原子结合）。羟基倾向于多面体位点而不是镁位点，而分子水几乎完全与镁位点结合，并且多面体间桥接（Si-O-H-O-Si）在其他类型的桥接中占主导地位。水的含量分别提高和抑制羟基和分子水的比例。随着压缩的增加，这些孤立的结构越来越多地相互结合，形成扩展结构，涉及总共五个或更多的O和H原子，并含有三倍配位物质，在研究的最高压缩下，它们共同消耗超过80%的氢。结果表明，水降低了包括所有正阴离子环境在内的不同类型的平均配位数。与无水熔体相比，含水熔体更倾向于四面体配位，但硅硅网络更容易被破坏。质子增加了非桥接氧的含量，减少了桥接氧和氧三簇的含量（在压力高于10gpa时存在）。与无水熔体相比，有水存在时计算出的所有原子种类的自扩散系数都提高了。这与水在所有压力下解聚熔体结构的预测是一致的。我们的分析还表明质子扩散涉及两个过程——氢原子的转移（需要断裂并形成O-H键）和作为氢载体的羟基的运动（需要断裂并形成Si-O和/或Mg-O键）。这两个过程都在低压缩下工作，而只有第一个过程在高压缩下工作。
We have explored first-principles molecular dynamics simulation data for hydrous MgSiO3 liquid (with 10 wt% water) to gain insight into its structural and dynamical behavior as a function of pressure (0-150 GPa) and temperature (2,000-6,000 K). By visualizing/analyzing a number of parameters associated with short- and mid-range orders, we have shown that the melt structure changes substantially on compression. The speciation of the water component at low pressures is dominated by the isolated structures (with over 90% hydrogen participated) consisting of hydroxyls, water molecules, O-H-O bridging and four-atom (O-H-O-H and H-O-H-O) groups, where every oxygen atom may be a part of polyhedron or free (i.e., bound to only magnesium atom). Hydroxyls favor polyhedral sites over magnesium sites whereas molecular water is almost entirely bound to magnesium sites, and also interpolyhedral bridging (Si-O-H-O-Si) dominates other types of bridging. Water content is shown to enhance and suppress, respectively, the proportions of hydroxyls and molecular water. As compression increases, these isolated structures increasingly combine with each other to form extended structures involving a total of five or more O and H atoms and also containing threefold coordination species, which together consume over 80% hydrogen at the highest compression studied. Our results show that water lowers the mean coordination numbers of different types including all cation-anion environments. The hydrous melt tends to be more tetrahedrally coordinated but with the Si-Si network being more disrupted compared to the anhydrous melt. Protons increase the content of non-bridging oxygen and decrease the contents of bridging oxygen as well as oxygen triclusters (present at pressures above 10 GPa). The calculated self-diffusion coefficients of all atomic species are enhanced in the presence of water compared to those of the anhydrous melt. This is consistent with the prediction that water depolymerizes the melt structure at all pressures. Our analysis also suggests that proton diffusion involves two processes-the transfer of H atoms (requiring the rupture and formation of O-H bonds) and the motion of hydroxyls as hydrogen carriers (requiring the rupture and formation of Si-O and/or Mg-O bonds). Both the processes are operative at low compression whereas only the first process is operative at high compression.