  Although crystallized neutron star crust is responsible for many fascinating
observational phenomena, its actual microscopic structure in tremendous
gravitational and magnetic fields is not understood. Here we show that in a
non-uniform magnetic field, three-dimensional ionic Coulomb crystals comprising
the crust may stretch or shrink while their electrostatic pressure becomes
anisotropic. The pressure depends non-linearly on the magnitude of the stretch,
so that a continuous magnetic field evolution may result in an abrupt crystal
elongation or contraction. This may provide a trigger for magnetar activity. A
phonon mode instability is revealed, which sets the limits of magnetic field
variation beyond which the crystal is destroyed. These limits sometimes
correspond to surprisingly large deformations. It is not known what happens to
crust matter subject to a pressure anisotropy exceeding these limits. We
hypothesize that the ion system then possesses a long-range order only in one
or two dimensions, that is becomes a liquid crystal.
