Effect of proton uptake on the structure of energy levels in the band-gap of Sr-doped LaScO3: diffuse reflectance spectroscopy and coherent potential approximation calculations

Abstract

Features of the energy levels in the band-gap of La_1−xSr_xScO_3−x/2 and the effect on those levels of proton uptake from H₂ and H₂O atmospheres were studied by diffuse reflectance spectroscopy and coherent potential approximation (CPA) calculations. It was shown that oxygen vacancies appearing due to acceptor doping with Sr form energy levels near the bottom of the conduction band that are strongly hybridized with the states of the nearest atoms. Excitation of electrons from the valence band to these vacancy levels gives rise to an additional absorption band which overlaps with the fundamental absorption edge. Proton incorporation from both H₂ and H₂O atmospheres leads to formation of proton levels below the valence band. However, during H₂ uptake, electrons from hydrogen atoms occupy oxygen vacancy levels, and as a result additional absorption in the red-IR range appears due to electronic transitions from these levels to the conduction band. On the other hand, H₂O uptake leads to the disappearance of oxygen vacancy levels. Experimental results obtained are similar to literature data on the optical absorption properties of some other proton-conducting perovskites, allowing the conclusion that findings from these CPA calculations on the nature of the energy levels could be extrapolated to some extent to those oxides.