In-plane structure and ordering at liquid sodium surfaces and interfaces from ab initio molecular dynamics

TL;DR

This study uses ab initio molecular dynamics to analyze atomic arrangements at liquid sodium surfaces, revealing a mixture of five- and six-fold coordination that shifts towards hexagonal order as temperature decreases.

Contribution

It provides detailed insights into the in-plane atomic structure and ordering at liquid sodium surfaces using first-principles simulations, comparing with classical and solid-liquid interface models.

Findings

Atoms at the surface are mostly 5-fold coordinated.

Decreasing temperature increases hexagonal ordering.

Similar effects observed in classical MD and solid-liquid interface simulations.

Abstract

Atoms at liquid metal surfaces are known to form layers parallel to the surface. We analyze the two-dimensional arrangement of atoms within such layers at the surface of liquid sodium, using ab initio molecular dynamics (MD) simulations based on density functional theory. Nearest neighbor distributions at the surface indicate mostly 5-fold coordination, though there are noticeable fractions of 4-fold and 6-fold coordinated atoms. Bond angle distributions suggest a movement toward the angles corresponding to a six-fold coordinated hexagonal arrangement of the atoms as the temperature is decreased towards the solidification point. We rationalize these results with a distorted hexagonal order at the surface, showing a mixture of regions of five and six-fold coordination. The liquid surface results are compared with classical MD simulations of the liquid surface, with similar effects appearing, and with ab initio MD simulations for a model solid-liquid interface, where a pronounced shift towards hexagonal ordering is observed as the temperature is lowered.