Polymorphism and Multiferroicity in Bi1–x/3(MnIII 3)(MnIII 4–x MnIV x )O12

We report on structural and magnetic characterizations performed on the quadruple perovskite Bi1–x/3(MnIII 3)(MnIII 4–x MnIV x )O12, showing dramatic changes on the magnetic properties of different polymorphs produced by small compositional variations (0 < x < 0.3). Besides the stoichiometric monoclinic phase, rhombohedral (0.10 < x < 0.27) and cubic (x > 0.27) polymorphs are detected, being related to the loss of orbital ordering originated by the introduction of Mn4+ ions within the structure. For the monoclinic and rhombohedral phases, ferroelectric properties are suggested by single-crystal X-ray diffraction and TEM studies, while their magnetic behavior reveals weak ferromagnetism, induced by Dzyaloshinskii–Moriya (DM) interaction, characterized by high coercive fields, not easily explained by the sole DM effect. The pinning of the magnetic domains to the electric ones, mediated by strong magnetoelectric coupling, could be the mechanism at the origin of such coercive fields. This hypothesis finds further support in the low switching fields detected for the cubic phase, where just local polar character is observed.