Local levels in La1-xSrxScO3-x/2 band-gap under interaction with components of O2, H2, H2O atmospheres

The paper studies the electronic structure of proton-conducting oxides based on the lanthanum scandate La_1-xSr_xScO_3-x/2 for advancing in understanding the mechanisms of hydrogen uptake from dry and humid atmospheres into the lattice of oxides with a perovskite structure. The process of protons incorporation from H₂O containing atmospheres is considered to describe by the reaction ${\text{H}}_2\text{O}+{\text{O}}_{\text{O}}^{\times }+{\text{V}}_{\text{O}}^{??}=2{\text{OH}}_{\text{O}}^{?}$. However, there is no established concept of a mechanism for proton uptake from a dry H₂ atmosphere. At such an uptake, a positively charged proton defect will be formed in the oxide lattice, and a negative charge must appear for compensation of the excess positive charge. Formally, the reaction of this process can be represented as $\frac{1}{2}{\text{H}}_2+{\text{O}}_{\text{O}}^{\times }={\text{OH}}_{\text{O}}^{?}+{\text{e}}^{\text{′}}$. In this case an uncompensated electron appears, and the question arises as to where it is localized. In order to answer this question, it is necessary to study the electronic structure of perovskites.With increasing in dopant concentration x the absorption band at 5.6 eV overlapping with the edge of fundamental absorption increases. Most probably it can be related with oxygen vacancies. When protons are incorporated from the H₂ atmosphere into the La_1-xSr_xScO_3-x/2 lattice, the absorption intensity in this band decreases, which can be due to the transition of the defects causing this band to another charge state. In addition, specific defects that absorb in the red and IR region at hν < 2.2 eV are formed. They are found to be located deep enough in the bang-gap and not to be an electronic traps. It is also shown that in La_1-xSr_xScO_3-x/2 there are electron traps located at a depth of 2–4.5 eV in the band-gap relative to the bottom of the conduction band. On the basis of the obtained data, it can be assumed that these defects are somehow associated with oxygen vacancies, but their charge state is not obvious. It is important that these traps participate in the capture of uncompensated electrons during the proton uptake from the H₂ atmosphere.