  We show that a simple two dimensional model of spin crossover materials gives
rise to spin-state smectic phases where the pattern of high-spin (HS) and
low-spin (LS) metal centers spontaneously breaks rotational symmetry and
translational symmetry in one direction only. The spin-state smectics are
distinct thermodynamic phases and give rise to plateaus in the fraction of HS
metal centers. Smectic order leads to lines of Bragg peaks in the x-ray and
neutron scattering structure factors. We identify two smectic phases and show
that both are ordered in one direction, but disordered in the other, and hence
that their residual entropy scales with the linear dimension of the system.
This is intermediate to spin-state ices (examples of `spin-state liquids')
where the residual entropy scales with the system volume, and antiferroeleastic
ordered phases (examples of `spin-state crystals') where the residual entropy
is independent of the size of the system.
