Structures of filled ice nanotubes inside carbon nanotubes

We report ordered structures of water in carbon nanotubes that are different from previously found n-gonal ice nanotubes and may be called filled ice nanotubes. Spontaneous formation of the filled ice nanotubes is observed in molecular dynamics (MD) simulations of water at fixed densities and a fixed temperature. Outer layers of filled ice nanotubes are characterized by a roll-up vector (n, m) while inner files of molecules do not have definite ordered structures. With this notation the filled ice nanotubes are of the (6, 0), (7, 0), (8, 0) and (8, 1) types, the last of which has a helical structure in its outer layer whereas the outer layers of the first three have the same achiral structures as the n-gonal ice nanotubes. Structure analysis is done for their hydrogen-bond networks and average dipole moments.