  The structures of the liquid surface and the liquid-solid interface of sodium
have been characterized with extensive first-principles molecular dynamics
simulations. Friedel oscillations in the electronic charge density at the free
surface were found to persist across the solid-to-liquid melting transition,
with a small but distinctive electronic layering that remains decoupled from
the atomic positions. Strong ionic layering was observed both at the liquid
surface and at the liquid-solid interface, notwithstanding the absence of
Friedel oscillations or under-coordinated atoms in the latter case. Confinement
effects at these soft or hard boundaries drive the atoms into
quasi-close-packed layers; even for this prototypical free-electron metal
Friedel oscillations are not relevant to ordering.

Materials Science