Crystal field anisotropy correlated charge disproportionation in Ruddlesden-Popper Sr3-xCaxFe2O7-δ perovskites

Abstract

A series of Ca2+-doped Ruddlesden-Popper phase perovskites Sr3-xCaxFe2O7-δ (x = 0∼0.4) were synthesized. Strong asymmetric FeO6 octahedral distortion was confirmed at x = 0.2 owing to the anisotropic chemical pressure applied from preferable occupation of Ca2+ ion at Sr2 site. Unusual high-charged oxidation state of iron ion with enhanced p→d charge transfer was collectively evidenced by X-ray photoelectron spectroscopy and Mössbauer spectroscopy. Combined electron emission spectroscopy further revealed a narrowing of the surface band gap accompanied by an apparent lowering of the conduction band edge at x = 0.2. These observations can be explained by a nontrivial partial charge disproportionated (CD) regime: d5L + d5L → d5L1−n + d5L1+n, which reflects a significant correlation between anisotropic chemical pressure and CD ground state. The d5L1−n ground state, which dominates the conduction band, is largely stabilized by enhancing the d(Fe)-p(O) hybridization along the c-axis. In addition, abrupt changes in both magnetism and dc electrical properties were also observed at x = 0.2. It is believed that the anisotropic crystal field correlated electronic state fluctuation of iron may facilitate magnetic phase competition and carrier transport. This work provides a comprehensive understanding of the crystallographic anisotropy and correlated valence electron behavior in Fe-base Ruddlesden-Popper perovskites, as well as quasi two-dimensional perovskite materials.