LOCAL LEVELS IN THE BAND-GAP OF La_1-xSr_xScO_3-x/2 OXIDES UNDER INTERACTION WITH COMPONENTS O₂, H₂, H₂O GAS PHASES

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 watercontaining atmospheres is considered to describe by the reaction H₂O+ O_O^× +V_O^** = 2OH_O^*. However, there is no established concept of a mechanism for proton uptake from a dry hydrogen 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 such process can be represented as 1/2 H₂O+ O_O^×=OH_O^*+e'. 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 concentration of dopant x, the absorption band at 5.6 eV overlapping with the edge of fundamental absorption is found to increase. A similar band has been observed in other proton-conducting perovskites, and it can be related either with oxygen vacancies or with acceptor levels, since the amount of both the ones increases with the concentration of dopant x. When protons are incorporated from the dry hydrogen atmosphere into the La_1-xSr_xScO_3-x/2 lattice, the absorption intensity in this band decreases, that can be due to the transition of the defects causing this band to another charge state. In addition, specific defects that absorb light in the red and infrared region at hν < 2.2 eV are formed. They are found to be located deep enough in the bang-gap and not to be the 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 eV to 4.5 eV in the band-gap relative to the bottom of the conduction band. On the basis of the data obtained, 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 dry hydrogen atmosphere.

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