  Equilibrium phase diagrams for neutron star matter at subnuclear densities
are obtained at zero temperature. Spherical, rod-like and slab-like nuclei as
well as spherical and rod-like nuclear bubbles are taken into account by using
a compressible liquid-drop model. This model is designed to incorporate
uncertainties in the nuclear surface tension and in the proton chemical
potential in a gas of dripped neutrons. The resultant phase diagrams show that
for typical values of these quantities, the phases with rod-like nuclei and
with slab-like nuclei occur in the form of Coulomb lattice at densities below a
point where the system becomes uniform. Thermal fluctuations leading to
displacements of such nuclei from their equilibrium positions are considered
through explicit evaluations of their elastic constants; these fluctuations can
be effective at destroying the layered lattice of slab-like nuclei in the
temperature region typical of matter in the neutron star crust.
