The diffuse phase transitions (DPT) in the ST‐BT system are theoretically and experimentally justified by the coexistence of two phases (one tetragonal, the other cubic) in the temperature range of the transition. This fact is emphasized by X‐ray identification of both phases at room temperature in (Ba0.76Sr0.24)TiO3 and (Ba0.715Sr0.285)TiO3 solid solutions. The ferroelectric‐para‐electric (F‐P) and the lower ferroelectric transitions are discussed in terms of thermal phase equilibria in the F‐P diagram proposed.

If the ferroelectric solid solutions are regarded as homogeneous phases between two or more components, then, according to the ferroelectric–paraelectric diagrams, the diffused phase transitions can be explained by the occurrence of at least two coexisting phases in thermal equilibrium: one ferroelectric the other paraelectric within a certain temperature range. The representative F–P diagrams and transition mechanisms for the binary and ternary solid solutions are presented.

The F–P diagram of the BaTiO3PbTiO3 system is computer calculated starting with the thermodynamical data of the components. Using an ideal solubility for the cubic phase and a regular solubility for the tetragonal phase, the F–P diagram obtained after taking the interaction parameter αF as 2000 J/mol, meets best the experimental data. Estimation of the ferroelectric interaction energy relying on the spontaneous polarization of BaTiO3 and PbTiO3 lead to a value close to αF thermodynamically calculated.