Giant red shift of the absorption spectra due to nonstoichiometry in GdCoO3–δ

Abstract

The GdCoO3–δ perovskite is a semiconductor with the energy gap Eg ≈ 0.5 eV from electrical transport measurements. It reveals unusual optical absorption spectra without transparency window expected for semiconductors. Instead we have measured the narrow transmittance peak at the photon energy e0 = 0.087 eV. To reconcile the transport and optical data we have studied the effect of oxygen vacancies on the electronic structure of the GdCoO3–δ. We have found that oxygen vacancies result in the in-gap states inside the charge-transfer energy gap of the GdCoO3. It is a multielectron effect due to strong electron correlations forming the electronic structure of the GdCoO3–δ. These in-gap states decrease the transparency window and result in a narrow absorption minimum. The predicted temperature dependence of the absorption spectra has been confirmed by our measurements.