Covalent effects in magnetic ferroelectrics MnMO3 (M = Ti, Sn)

By means of first-principles calculations based on density functional theory (DFT), DFT+U and hybrid functional methods, we report a comparative study of the magnetic, electronic, and ferroelectric properties of high-pressure-induced compounds MnMO3 (M=Ti, Sn). The results correctly describe the insulating character and G-type antiferromagnetic ground state for both compounds, which is in good agreement with the experimental observations. We predicted large spontaneous ferroelectric polarizations of MnTiO3 and MnSnO3 by using the Berry-phase method. In particular, the proper covalent interaction mechanism driving the ferroelectric transition is discussed and explained in term of the analysis of potential-energy surfaces, Born effective charges, and electric localization function. Our results indicate that MnTiO3 and MnSnO3 represent unique examples of ferroelectric perovskites in which the ferroelectric instabilities originate from the combined action of geometric effects and chemical activity of the B-site atom, thus extending the concept of d(0)-ness (MnTiO3) and lone-pair mechanism (MnSnO3) to magnetic ferroelectrics.