Optical properties and electronic structure of cobalt-deficit GdBaCo$_{1.86}$O$_{5+\delta}$ single crystals

The optical properties of cobalt-deficit GdBaCo$_{1.86}$O$_{5+\delta}$ single crystals with different oxygen contents have been studied using the ellipsometry method in the spectral range of 0.16–4.8 eV. The optical conductivity spectrum has been characterized by a low-energy structure with the center at $\sim$ 1.3 eV and an intense broad absorption band at 1.8–4.0 eV. It has been found that the electronic transitions in the cobalt-deficit samples are partially blocked in a narrow spectral range of 0.5–0.8 eV due to weakening of the contribution from the interstitial Co$(3d)$–Co$(3d)$ transitions. The ordered GdBaCo$_2$O$_{5.5}$ crystal exhibits a strong absorption in the energy band gap ($E_g<$ 0.26 eV) due to the structural disorder, defects, and vacancies. A significant decrease in the interband absorption intensity in the tetragonal phase of the GdBaCo$_{1.86}$O$_{5.07}$ compound as compared to that in the case of the orthorhombic GdBaCo$_{1.86}$O$_{5.32}$ phase has been explained by the existence of only the pyramidal environment of a cobalt ion.