Model for tricritical points in distortive structural phase transitions

We show rigorously ($T=0\mathrm{}$) that tricritical points in systems exhibiting ferro- or antiferrodistortive structural phase transitions are included in a two-dimensional model system covering the order-disorder and displacive regimes. For $T>\mathrm{}0\mathrm{}$, our analysis is based on the mean-field approximation. Antiferroelectric systems turn out to be favorable candidates for investigation of the occurrence of tricritical points among real systems undergoing distortive structural phase transitions.