Non-d0 Mn-driven ferroelectricity in antiferromagnetic BaMnO3

Using first-principles density-functional theory we predict a ferroelectric ground state-driven by the off-centering of the magnetic Mn4+ ion-in perovskite-structure BaMnO3. Our finding is surprising since the competition between energy-lowering covalent bond formation and energy-raising Coulombic repulsions usually only favors off-centering on the perovskite B site for nonmagnetic d(0) ions. We explain this tendency for ferroelectric off-centering by analyzing the changes in electronic structure between the centrosymmetric and polar states and by calculating the Born effective charges; we find anomalously large values for Mn and O consistent with our calculated polarization of 12.8 mu C/cm(2). Finally, we suggest possible routes by which the perovskite phase may be stabilized over the usual hexagonal phase to enable a practical realization of a single-phase multiferroic.