First-principles study of diffusion and viscosity of anorthite (CaAl2Si2O8) liquid at high pressure
First-principles study of diffusion and viscosity of anorthite (CaAl2Si2O8) liquid at high pressure
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DOI：
10.2138/am.2011.3646
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发表时间：
2011-05-01
影响因子：
3.1
通讯作者：
Stixrude, Lars
中科院分区：
文献类型：
--
作者：
Karki, Bijaya B.;Bohara, Bidur;Stixrude, Lars
我们进行了平衡分子动力学模拟的CaAl 2Si 2 O 8（钙长石）液体作为压力（高达160 GPa）和温度（2500至6000 K）的函数在密度泛函理论。沿着3000 K等温线，Ca的自扩散系数变化最大（在0和50 GPa之间减小两个数量级），而Al、Si和O2的自扩散系数变化很大-它们最初随压力增加，达到宽的最大值（约5 GPa），然后在进一步压缩时减小。计算的熔体粘度也显示出弱的反常行为，在类似的压力附近具有局部最小值。温度对动力学异常和总气压变化都有抑制作用。因此，扩散率（粘度）等温线的曲率从3000 K时的凹（凸）形变为6000 K时的凸（凹）形。研究发现钙长石液体由于含有高含量的非桥氧原子（NBO）和氧三簇（O3），比二氧化硅液体具有更高的移动的性能。预测的压力变化可以与结构变化组成的压力诱导的最大值的丰度的五面体状态（五倍Al/Si-O协调）和快速增加的O3丰度。最后，我们预测的第一原理的结果与现有的实验数据相比，毫不逊色。
We have carried out equilibrium molecular dynamics simulations of CaAl2Si2O8 (anorthite) liquid as a function of pressure (up to 160 GPa) and temperature (2500 to 6000 K) within density functional theory. Along the 3000 K isotherm, the Ca self-diffusivity varies most (decreasing by two orders of magnitude between 0 and 50 GPa), whereas the self-diffusion coefficients of Al, Si, and Ovary anomalously-they initially increase with pressure, reach the broad maxima (around 5 GPa), and then decrease upon further compression. The calculated melt viscosity also shows a weak anomalous behavior with a local minimum around a similar pressure. Temperature suppresses the dynamical anomalies as well as the overall pressure variations. Therefore, the curvatures of the diffusivity (viscosity) isotherms change from a concave (convex) shape at 3000 K to a convex (concave) shape at 6000 K. We find that anorthite liquid is much more mobile than silica liquid because of its high content of non-bridging oxygen atoms (NBO) and oxygen triclusters (O3). The predicted pressure variations can be associated with structural changes consisting of the pressure-induced maximum in the abundance of pentahedral states (fivefold Al/Si-O coordination) and rapid increase in the O3 abundance. Finally, our predicted first-principles results compare favorably with the available experimental data.