  Phase-Field Crystal (PFC) models are able to resolve atomic length scale
features of materials during temporal evolution over diffusive time scales.
Traditional PFC models contain solid and liquid phases, however many important
materials processing phenomena involve a vapor phase as well. In this work, we
add a vapor phase to an existing PFC model and show realistic interfacial
phenomena near the triple point temperature. For example, the PFC model
exhibits density oscillations at liquid-vapor interfaces that compare favorably
to data available for interfaces in metallic systems from both experiment and
molecular dynamics simulations. We also quantify the anisotropic solid-vapor
surface energy for a 2D PFC hexagonal crystal and find well defined step
energies from measurements on the faceted interfaces. Additionally, the strain
field beneath a stepped interface is characterized and shown to qualitatively
reproduce predictions from continuum models, simulations, and experimental
data. Finally, we examine the dynamic case of step-flow growth of a crystal
into a supersaturated vapor phase. The ability to model such a wide range of
surface and bulk defects makes this PFC model a useful tool to study processing
techniques such as Chemical Vapor Deposition or Vapor-Liquid-Solid growth of
nanowires.
