第一性原理分子动力学计算核幔边界条件下Ni的结构和动力学性质

采用基于密度泛函理论的第一性原理分子动力学计算了在地幔与外地核(核幔)边界条件下Ni的结构和动力学, 发现在核幔条件(4000 K, 139 GPa)下, Ni是一种液态结构. 常压下液态Ni的原子排列主要是二十面体序, 在压强的作用下, 二十面体序有一个增加的过程, 当压强大于68 GPa后, 二十面体序的增强过程发生逆转, 大量的完整二十面体结构受到破坏, 缺陷二十面体急剧增加. 核幔边界条件下的液态Ni形成了一种由完整二十面体、缺陷二十面体、面心以及密排六方等其他原子排列方式并存的复杂结构. 我们计算了液态Ni的扩散系数, 其数量级大约为10^-9 m²/s, 与相应条件下Fe的扩散系数的数量级相同. 由于核幔边界条件下的高压作用, 液态Ni原子比常压和低压下的Ni原子扩散得更慢, 且在弛豫过程中出现了β弛豫.

Structure and relaxation dynamics of liquid nickel of Earth’s core-mantle by first-principles molecular dynamics

First-principles molecular-dynamics simulations based on density-functional theory are used to study the structural and dynamical properties of liquid nickel under boundary conditions between Earth’s core and mantle. We present the pressure-induced structure transformations when pressure is up to 139 GPa. At 139 GPa, the icosahedron order is partly destroyed, and transforms to defected icosahedron and other more complex orders. A remarkable feature of dynamic properties of nickel is that β-relaxation regime appeared under the pressure of Earth’s core-mantle. The diffusion coefficient of nickel atom is measured as 10⁹ m²/s roughly in earth’s core-mantle, as similar as that of iron atoms under the same conditions. The movement of nickel atom is much slower than that of liquid Ni at lower pressure.