  The formation of monolayer and multilayer ice with a square lattice structure
has recently been reported on the basis of transmission electron microscopy
experiments, renewing interest in confined two dimensional ice. Here we report
a systematic density functional theory study of double-layer ice in
nano-confinement. A phase diagram as a function of confinement width and
lateral pressure is presented. Included in the phase diagram are honeycomb
hexagonal, square-tube, hexagonal-close-packed and buckled-rhombic structures.
However, contrary to experimental observations, square structures do not
feature: our most stable double-layer square structure is predicted to be
metastable. This study provides general insight into the phase transitions of
double-layer confined ice and a fresh theoretical perspective on the stability
of square ice in graphene nanocapillary experiments.
