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

ABO(3-delta) (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 d3z(2)-r(2) 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 -> V0O, and especially, an extra absorption peak appears between 0.5 and 1.5 eV due to V0O 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 d3z(2)-r(2) 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 d3z(2)-r(2) orbitals, resulting in a remarkable increase of optical absorption at 1.5-3.0 eV.