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

A series of Ca 2+ -doped Ruddlesden-Popper phase perovskites Sr 3-x Ca x Fe 2 O 7-δ (x = 0∼0.4) were synthesized. Strong asymmetric FeO 6 octahedral distortion was confirmed at x  = 0.2 owing to the anisotropic chemical pressure applied from preferable occupation of Ca 2+ 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: d 5 L  +  d 5 L → d 5 L 1− n  +  d 5 L 1+ n , which reflects a significant correlation between anisotropic chemical pressure and CD ground state . The d 5 L 1− 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.