  For the first time in a bulk proper uniaxial ferroelectrics, double
antiferroelectric-like hysteresis loops have been observed in the case of
Sn$_2$P$_2$S$_6$ crystal. The quantum anharmonic oscillator model was proposed
for description of such polarization switching process. This phenomenon is
related to three-well local potential of spontaneous polarization fluctuations
at peculiar negative ratio of coupling constants which correspond to inter-site
interaction in given sublattice and interaction between two sublattices of
Sn$_2$P$_2$S$_6$ modeled crystal structure. Obtained data can be used for
development of triple-level cell type memory technology.
