  For Sn$_2$P$_2$S$_6$ ferroelectrics the second order phase transitions line
is observed until reaching the tricritical point at transition temperature
lowering to 250 K by compression. Observed temperature-pressure phase diagram
agrees with simulated diagram by MC calculations based on early founded by DFT
study local potential for Sn$_2$P$_2$S$_6$ crystals. In addition to the
tricritical point, the possibility of disordered and quadrupolar phases
occurrence was shown. For mixed crystals with tin by lead substitution, the
investigated ultrasound, hypersound and low frequency dielectric properties
also reveal appearance of heterophase peculiarities at decreasing of
ferroelectric transition temperature below so named "temperature waterline"
near 250~K. The tricriticality at similar temperature level also appears in
mixed crystals at sulfur by selenium substitution. Such behavior agree with
Blume-Emery-Griffiths (BEG) model, that is appropriated for investigated
ferroelectric system with three-well local potential for the order parameter
(spontaneous polarization) fluctuations.
