Ferroelectricity in oxygen-deficient ferrite perovskites

The family of oxides with composition R_xA_1-xFeO_3-δ (R: rare earth, A: alkali-earth) exhibits oxygen-deficient phases that can be realized without disrupting the underlying perovskite lattice structure. For example, in ferrite, the presence of oxygen vacancies leads to the transformation of octahedral units into tetrahedral chains, thus stabilizing Brownmillerite and Grenier phases, depending on the relative number of alternating octahedral and tetrahedral layers. Interestingly, tetrahedral chains can be twisted with two different handedness leading to a local dipole. Using DFT+U calculations and the Quantum Espresso Code, we investigated the ferroelectric behavior arising from the tetrahedral chain twist in the R_0.33A_0.67FeO_2.67 Grenier structures. We find that tetrahedral twisting involves rotation of neighboring octahedral layers, leading to a parallel/anti-parallel arrangement of dipole moments in the tetrahedral chains. We also find that the relative stability of the polar Grenier phase and the barriers required to reverse the polarity of the chains depend on the size of the rare-earth and A-cations, thus providing a guideline for the optimal design of functional materials for resistive switching devices.