Landau–Devonshire derived phase diagram of the BiFeO3­–PbTiO3 solid solution

Since the construction of the first structural phase diagram of the BiFeO3–PbTiO3 solid solution, there has been ambiguity as to the true location of the morphotropic phase boundary (MPB). In this study, a Landau–Devonshire derived phase diagram has been constructed in order to provide a mathematically derived perspective. The compositional dependence on properties has been modeled as a linear dependence between the Landau coefficients of each end member. It is found that the only structural phases found below TC are rhombohedral and tetragonal, which at 25 °C shows an MPB at 0.696BiFeO3–0.304PbTiO3. The MPB also shows temperature dependence similar to other ferroelectric solid solutions in which the tetragonal–rhombohedral phase transition moves toward higher BiFeO3 contents at higher temperatures. Although the replication of the experimentally observed phase diagram can be constructed, this method is found to be insufficient for describing the spontaneous strain behavior.