  The ground-state structure of monolayers and nanoplatelets of SnS with a
thickness from two to five monolayers is calculated from first principles. It
is shown that nanoobjects with only odd number of monolayers are ferroelectric.
The ferroelectric, piezoelectric, and elastic properties of these polar
structures are calculated. The appearance of polarization in these nanoobjects
is explained by an uncompensated polarization that exists in an
antiferroelectric structure of bulk SnS. The mechanism of ferroelectricity, in
which the ferroelectric distortion is associated with short-range ordering of
lone pairs, can be regarded as a way of creating ferroelectrics with high Curie
temperature.
