  Basing on the temperature dependences of optical birefringence for Sn2P2S6
and Sn2P2(Se0.28S0.72)6 crystals subjected to hydrostatic pressures, we prove
unambiguously that Sn2P2S6 reveals a tricritical point on its (p,T)-phase
diagram with the coordinates (p,T) = (4.3 kbar, 259 K), so that the
second-order phase transition transforms into the first-order one whenever the
pressure increases above 4.3 kbar. We also find that increasing hydrostatic
pressure applied to Sn2P2(Se0.28S0.72)6 leads to the change in the phase
transition character from tricritical to first-order. Further increase in the
pressure up to ~ 2.5 kbar imposes splitting of the first-order
paraelectric-to-feroelectric phase transition into two phase transitions, a
second-order paraelectric-to-incommensurate one and a first-order
incommensurate-to-ferroelectric transition.
