抄録
Phase diagrams in the applied electric field vs the temperature plane were studied for several cases of improper ferroelectric and ferroelastic transitions. An interesting phase diagram including a re-entrant phase and a tricritical point is obtained when the system undergoes the ferroelastic-ferroelectric transition in the absence of the electric field. The anomalies in the temperature dependence of transition parameters, the electric susceptibility and the elastic stiffness constant are discussed. The nature of transitions is examined from the viewpoint of the Clausius-Clapeyron and the Ehrenfest relations.
