Non- ferroelectricity from semicovalent superexchange in bismuth ferrite

Empirical rule, i.e., magnetic unpairing occupation tends to be chemically incompatible for ferroelectrics, has for long inhibited the availability of room-temperature single-phase multiferroics. Here, we readdress the ferroelectricity of type-I multiferroic , and conclusively clarify hybrid origins of lone-pair electrons' instability. Surprisingly, the sole lone-pair mechanism cannot account for overall polarization. Thus, beyond the popular belief, we propose that, with half-filling configuration and stereochemical activity of Bi's lone pair preparticipating, renormalized nonempty Fe orbitals render a strong p-d charge-transfer insulator, and further induce non- ferroelectricity through antiferromagnetic semicovalent superexchange. The magnetoelectric coupling scenario is numerically elaborated by mean-field p-d model, and first-principles generalized gradient approximation calculations. The magnetically induced ferroelectricity also reconciles the competitivity of octahedral rotations and ferroelectricity, which turns into cooperative behavior as octahedral rotations straighten the bonds and strengthen superexchange interactions. Such mechanism is in principle ubiquitous in magnets with superexchange interaction and manipulatable to hybridization engineering.