Investigation on structural development of the n = 2 Ruddlesden-Popper rare-earth ferrites

Abstract

Recently, hybrid improper ferroelectric (HIF) has been experimentally demonstrated in n = 2 Ruddlesden-Popper (RP) (Ca,Sr)3Ti2O7, (Ca,Sr)3Mn2O7, and Sr3Sn2O7. In the mechanism of HIF, higher-order trilinear coupling of octahedral rotation, tilt, and polarization plays a key role in stabilizing the exotic ferroelectric ground state. Thus, HIF is classified in geometric ferroelectric, where constraints of geometrical structure, rather than cation-anion pairings, induce ferroelectric polarization. In order to predict new geometric ferroelectric distortions, the tolerance factor (TF) is frequently referred. The TF of perovskite-related materials is calculated using {R_{AO}-0.37\ln{\frac{s}{N}}}/{\sqrt{2}(R_{BO}-0.37\ln{\frac{s}{N}})}, where R_{AO}, R_{BO}, s and N are valence bond parameter between A and O, B and O, formal valence, and coordination number, respectively. TFs of n = 2 RP phases such as Ca3Ti2O7, Ca3Mn2O7, and Sr3Sn2O7 are 0.927, 0.958, and 0.982, respectively. Interestingly, the famous rare-earth orthoferrites (LnFeO3) display TF ranging from 0.942 (Ln = La) to 0.871 (Ln = Lu), and are crystallized in the space group of Pbnm possessing three modes of octahedral rotations and tilts. In this study, we scrutinize the structural evolution of n = 2 RP rare-earth ferrites as a function of rare-earth ionic radii and discuss possible magnetic hybrid improper ferroelectricity.