The Polar/Antipolar Phase Boundary of BiMnO3–BiFeO3–PbTiO3: Interplay among Crystal Structure, Point Defects, and Multiferroism

The ferromagnetic perovskite oxide BiMnO is a highly topical material, and the solid solutions it forms with antiferromagnetic/ferroelectric BiFeO and with ferroelectric PbTiO result in distinctive polar/nonpolar morphotropic phase boundaries (MPBs). The exploitation of such a type of MPBs could be a novel approach to engineer novel multiferroics with phase-change magnetoelectric responses, in addition to ferroelectrics with enhanced electromechanical performance. Here, the interplay among crystal structure, point defects, and multiferroic properties of the BiMnO–BiFeO–PbTiO ternary system at its line of MPBs between polymorphs of tetragonal P4mm (polar) and orthorhombic Pnma (antipolar) symmetries is reported. A strong dependence of the phase coexistence on thermal history is found: phase percentage significantly changes whether the material is quenched or slowly cooled from high temperature. The origin of this phenomenon is investigated with temperature-dependent structural and physical property characterizations. A major role of the complex defect chemistry, where a Bi/Pb-deficiency allows Mn and Fe ions to have a mixed-valence state, in the delicate balance between polymorphs is proposed, and its influence in the magnetic and electric ferroic orders is defined.