Electronic structure, orbital symmetry transformation, charge transfer, and valence state studies on Fe³⁺ - substituted CaCu₃Ti₄O₁₂ quadruple perovskites using X-ray photoelectron spectroscopy

Here, we report a detailed study on electronic structure of quadruple perovskite system, CaCu_3-xTi_4-xFe_2xO₁₂ (x = 0.0, 0.1, 0.3, 0.5 and 0.7), using the powerful X-ray photoelectron spectroscopy (XPS). The Ca2p and Ti2p XPS spectra indicate that Ca and Ti ions exist in the divalent and tetravalent state, respectively, and no visible change is observed in their valence states on Fe³⁺-substitution (x). The asymmetrically shaped peaks of Ti2p_3/2 infer the significant orbital hybridization of Ti atoms with other metallic cations like Cu and Fe, i.e. the Ti2p-Cu3d-Fe3d mixed states, which shows an increment with the Fe³⁺-content. The spectra also indicate that the Fe-substitution causes an increase in the strength of CuFe3d-O2p hybridization while weakening the strength of TiFe3d-O2p orbital hybridization. The Cu2p spectra suggest that Cu ions exist in two valence states i.e. the Cu²⁺ and Cu³⁺ out of which Cu²⁺ is dominant. The XPS results show clear evidence of charge transfer amongst Ti–Cu ions and evolution of n-type charge carriers. The O1s XPS spectra indicate anti-bonding states of t_2g and e_g type molecular orbitals separated by an energy separation of ~1.6 eV. A systematic relative change in the intensity of t_2g and e_g peaks with Fe-substitution point towards a clear transformation of symmetry of the system from TiO₆ octahedral to the square-planar.