Order-disorder versus soft mode behaviour of the ferroelectric phase transition in Sn2P2S6

This paper explains the coexistence of the displacive and the order-disorder features of the ferroelectric phase transition in Sn2P2S6 crystals. Both have been observed in experiments. The height of the potential barrier hindering the order parameter fluctuations, estimated from experimental data, shows that the phase transition in Sn2P2S6 is actually very close to the theoretical case of the order-disorder versus displacive crossover. Moreover, previously performed model calculations can be used For the analysis of the temperature dependence of dielectric susceptibility and other physical properties which do not obey the predictions of standard Landau theory.