  The macroscopic dielectric permittivity of dielectric crystals is related to
the microscopic atomic polarizability of constituent atoms by the known
Clausius-Mossotti relation obtained in the middle of 19th century. We derive a
similar relation for recently discovered two-dimensional crystals (mono- and
bilayer graphene, boron nitride, etc) and show that, in contrast to
three-dimensional materials, much stronger electron-electron interaction in two
dimensions leads to a spontaneous electric polarization of the ground state of
two-dimensional crystals. The predicted ferroelectric transition may have
interesting applications in electrodynamics and optics.
