Non-d0Mn-driven ferroelectricity in antiferromagneticBaMnO3

Using first-principles density-functional theory we predict a ferroelectric ground state—driven by the off-centering of the magnetic ${\text{Mn}}^{4+}$ ion—in perovskite-structure ${\text{BaMnO}}_3$. 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 $12.8\mu \text{C}/{\text{cm}}^2$. 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.