  For Sn$_2$P$_2$S$_6$ ferroelectrics, the appearance of spontaneous
polarization is related to stereoactivity of tin cations and valence
fluctuations of phosphorous cations. Here, the continuous phase transition and
its behavior under pressure is determined by thee-well local potential and can
be described in an anharmonic quantum oscillator model. For such a model, the
spectrum of pseudospin fluctuations at different temperatures and pressures has
been calculated and compared with the data of Raman spectroscopy. It was
revealed that the ferroelectric lattice instability is related to several low
energy optic modes.
