Pressure-induced structural change and nucleation in liquid aluminum

Abstract

The relationship between the atomic structure and dynamics of liquid aluminum (Al) has been studied at 1500 K as a function of pressure via ab initio molecular dynamics simulations. The origin of the structural evolution is unveiled by various techniques. The structure factor and the mean square displacement data indicate the fortuity of a crystalline-like phase at 25 GPa, first evolving into the metastable body-centered cubic-like local order, followed by face-centered cubic, which is different from the crystallization mechanism proposed for the liquid Al by Desgranges and Delhommelle [J. Chem. Phys. 127, 144509 (2007)]. The three-dimensional structural analysis demonstrates the concentration of distorted icosahedron-like clusters, e.g., Voronoi <0, 3, 6, 3> and <0, 2, 8, 2>, which are closely correlated with crystal nucleation and growth. Crystallization in the liquid Al is distinctly correlated with the bond orientational order (Q6) fluctuations, instead of density fluctuations, where the temperature or pressure comparison on the local atomic structure in the liquid Al and degree of crystallization is also elucidated. The electronic structure study reveals that at ambient pressure, some valence electrons are already localized, showing a strong tendency of electron pairing with each other in the interstitial regions.