  The films of order-disorder type ferroelectrics were considered in the mean
field approximation taking into account depolarization field. It was shown that
size effects in this system could be described on the base of bulk system
equation of state with Curie temperature dependent on the film thickness. The
critical size hc and critical temperature Tc of phase transition from
ferroelectric to paraelectric phase was calculated allowing for the
depolarization field contribution. The comparison of the polarization
dependence on the film thickness, temperature and electric field for the films
of order-disorder and displacement type ferroelectrics is performed. In
particular it was shown that all the dipoles become ordered at T=0
independently on the film thickness for h>hc contrary to the displacement type
ferroelectrics. Critical thickness appeared larger and polarization
distribution sharper for the displacement type ferroelectrics than for
order-disorder type ferroelectrics.
