Ca²⁺ - substitution effect on the electronic structure of CaCu₃Ti₄O₁₂ studied by electron spectroscopy for chemical analysis

A detailed electronic structure study of quadruple perovskite series, Ca_1+xCu_3-xTi₄O₁₂ (for x = 0.0, 0.1, 0.2, 0.5, 1.0), using electron spectroscopy for chemical analysis (ESCA) has been carriedout. The Ti2p ESCA spectra imply that Ti ions remain in the tetravalent state and show the presence of Ti clusters, TiO₆ and TiO₅. The Cu2p spectra indicate the dominance of the divalent state of Cu ions. The Ca2p spectra show that Ca atoms are divalent but spectral features are suggestive of the presence of Ca superficial atoms in the system which is possibly due to the precipitation of Ca atoms in the grain boundary regions. The Ca2p in conjunction with O1s spectra shows that there is a strong possibility of hybridization of Ca2p states with the O1s and/or the Cu3d orbitals. The remarkable shift in O1s spectra towards the lower energy side for the samples with x ≥ 0.5 is mainly due to the substitution of large cation Ca²⁺ (1.0 Å), for small cation Cu²⁺ (0.73 Å) in the series. That results in bond length enhancement and weakening of the bond strength. It is proposed that the shift of O1s peaks can also be partially caused by oxygen vacancies; nevertheless, more experiments are required to substantiate this.