The influence of oxygen vacancy on the electronic and optical properties of ABO3−δ (A = La, Sr, B = Fe, Co) perovskites

Abstract

ABO_3−δ (A = La, Sr, B = Fe, Co) perovskites are useful in a wide range of applications, including their recent exploration for application in high-temperature optical oxygen sensing for energy conversion devices such as solid oxide fuel cells. To elucidate the dependence of functional properties and oxygen vacancy formation on defect chemistry and composition, first principles calculations are presented. The obtained results show that oxygen vacancy (V_O) formation energies are in the order of LaFeO₃ > LaCoO₃ > SrFeO₃ > SrCoO₃. Furthermore, the influence of V_O on the electronic and optical properties is investigated for the high temperature stable phases (T = 1100 K). For the LaFeO₃ insulator, the V_O donated electrons are all localized on the down-spin d_3z²−r² orbitals of the nearest Fe ions. These defect states located in the band gap induce a drop in the energy onset of absorption as pristine bulk → V²⁺_O → V¹⁺_O → V⁰_O, and especially, an extra absorption peak appears between 0.5 and 1.5 eV due to V⁰_O and V¹⁺_O formation. In the rest of the crystals that expressed a metallic feature, the V_O donated electrons partially localize on the down-spin d_3z²−r² orbital and partially delocalize through the lattice, by which the absorption peaks (0.5–2.0 eV for LaCoO₃, 0.0–0.5 eV for SrFeO₃ and SrCoO₃) from the electronic excitation near the Fermi level are enhanced. A high V_O concentration of oxygen divacancy in SrFeO₃ and SrCoO₃ could enhance charge localization on down-spin d_3z²−r² orbitals, resulting in a remarkable increase of optical absorption at 1.5–3.0 eV.