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A Molecular Dynamics Study of Grain Boundary Diffusion in MgO
Geochimica et Cosmochimica Acta ( IF 5 ) Pub Date : 2021-01-01 , DOI: 10.1016/j.gca.2020.09.012
Adriaan A. Riet , James A. Van Orman , Daniel J. Lacks

Abstract Molecular dynamics simulations are carried out on polycystalline periclase (MgO) to determine the structure and diffusivity at grain boundaries for pressures and temperatures relevant to Earth’s mantle. As temperature increases, the grain boundary structure becomes more disordered, with more ions having incomplete coordination and the system occupying regions of the energy landscape with shallower energy mimima. In contrast, as pressure increases the grain boundary structure becomes more ordered. The grain boundary diffusivity as a function of temperature and pressure can be understood in terms of these structural changes. At atmospheric pressure, the grain boundary diffusion coefficients for Mg and O extrapolate with increasing temperature to the values for the melt, indicating that the dynamics in the grain boundary are similar to those of a supercooled liquid. Just as in a supercooled liquid, diffusion in the grain boundary slows down with decreasing temperature for two reasons: there is less energy to surmount energy barriers, and the barriers are larger due to the more ordered structure. As pressure increases from zero pressure, the diffusivities first decrease sharply, due to the increase in energy barriers associated with the more ordered system, and then more gradually as pressure increases beyond ∼4 GPa. At conditions relevant to Earth’s core-mantle boundary region (135 GPa, 3500 °C), no diffusion is observed on the timescale of ∼50 ns, and the diffusion coefficients are thus constrained to be

中文翻译:

MgO中晶界扩散的分子动力学研究

摘要 对多晶方镁石 (MgO) 进行了分子动力学模拟,以确定与地幔相关的压力和温度下晶界的结构和扩散率。随着温度的升高,晶界结构变得更加无序,更多的离子具有不完全配位,并且系统占据能量图谱中具有较浅能量最低值的区域。相反,随着压力的增加,晶界结构变得更加有序。作为温度和压力的函数的晶界扩散率可以根据这些结构变化来理解。在大气压下,Mg 和 O 的晶界扩散系数随着温度的升高外推到熔体的值,表明晶界的动力学类似于过冷液体的动力学。就像在过冷液体中一样,晶界中的扩散随着温度的降低而减慢,原因有两个:克服能垒的能量较少,并且由于结构更有序,势垒更大。随着压力从零压力增加,扩散率首先急剧下降,这是由于与更有序的系统相关的能量势垒的增加,然后随着压力增加超过~4 GPa 逐渐下降。在与地球核心-地幔边界区域(135 GPa,3500 °C)相关的条件下,在~50 ns 的时间尺度上没有观察到扩散,因此扩散系数被限制为 晶界中的扩散随着温度的降低而减慢,有两个原因:克服能垒的能量较少,并且由于结构更有序,势垒更大。随着压力从零压力增加,扩散率首先急剧下降,这是由于与更有序的系统相关的能量势垒的增加,然后随着压力增加超过~4 GPa 逐渐下降。在与地球核心-地幔边界区域(135 GPa,3500 °C)相关的条件下,在~50 ns 的时间尺度上没有观察到扩散,因此扩散系数被限制为 晶界中的扩散随着温度的降低而减慢,有两个原因:克服能垒的能量较少,并且由于结构更有序,势垒更大。随着压力从零压力增加,扩散率首先急剧下降,这是由于与更有序的系统相关的能量势垒的增加,然后随着压力增加超过~4 GPa 逐渐下降。在与地球核心-地幔边界区域(135 GPa,3500 °C)相关的条件下,在~50 ns 的时间尺度上没有观察到扩散,因此扩散系数被限制为 由于与更有序的系统相关的能垒的增加,然后随着压力增加超过~4 GPa 会逐渐增加。在与地球核心-地幔边界区域(135 GPa,3500 °C)相关的条件下,在~50 ns 的时间尺度上没有观察到扩散,因此扩散系数被限制为 由于与更有序的系统相关的能垒的增加,然后随着压力增加超过~4 GPa 会逐渐增加。在与地球核心-地幔边界区域(135 GPa,3500 °C)相关的条件下,在~50 ns 的时间尺度上没有观察到扩散,因此扩散系数被限制为
更新日期:2021-01-01
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