Abstract

We use density functional theory (DFT) calculations to show that oxygen vacancies ((Formula presented.)) and mobility induce noncentrosymmetric polar structures in SrTi (Formula presented.) Fe (Formula presented.) Co (Formula presented.) O (Formula presented.) (STFC, (Formula presented.)) with (Formula presented.), enhance the saturation magnetization, and give rise to large changes in the electric polarization (Formula presented.). We present an intuitive set of rules to describe the properties of STFC, which are based on the interplay between (Co/Fe)- (Formula presented.) defects, magnetic cation coordination, and topological vacancy disorder. STFC structures consist of layered crystals with sheets of linearly organized O (Formula presented.) -coordinated Fe–Co pairs, sandwiched with layers of O (Formula presented.) -coordinated Ti. (Co/Fe)- (Formula presented.) defects are the source of crystal distortions, cation off-centering and bending of the oxygen octahedra which, considering the charge redistribution mediated by (Formula presented.) and the cations’ electronegativity and valence states, triggers an effective electric polarization. Oxygen migration for (Formula presented.) leads to (Formula presented.) >∼10 µC/cm (Formula presented.) due to quantum-of-polarization differences between (Formula presented.) structures. Increasing the oxygen deficiency to (Formula presented.) yields (Formula presented.), the O migration of which resolved polarization for (Formula presented.) is (Formula presented.) 3 µC/cm (Formula presented.). Magnetism is dominated by the Fe,Co spin states for (Formula presented.), and there is a contribution from Ti magnetic moments (∼1 (Formula presented.)) for (Formula presented.). Magnetic and electric order parameters change for variations of (Formula presented.) or oxygen migration for a given oxygen deficiency. Our results capture characteristics observed in the end members of the series SrTi(Co,Fe)O (Formula presented.), and suggest the existence of a broader set of rules for oxygen-deficient multiferroic oxides.