Fe2O3-Cr2O3 thin films were formed on Pt substrates at 623 K by low pressure CVD technique using Fe(O2C5H7)3 and Cr(O2C5H7)3 as precursors and O2 as a reaction gas. The dissolution processes of the films under anodic and cathodic polarization in 1 kmol·m-3 H2SO4 were examined in-situ by real-time spectroscopic ellipsometry. Spectra of refractive index nfilm and extinction coefficient kfilm of the optical constant of the films, Nfilm (= nfilm-ikfilm), in a wavelength range 470-780 nm were measured as a function of dissolution time. When the film with a Cr cationic fraction XCr = 0.26 was polarized at a cathodic potential of 0.0 V (vs. Ag/AgCl/3.33 kmol·m-3 KCl), the value of nfilm in a wavelength range less than 700 nm and that of kfilm in the whole wavelength range decreased with time. This suggests that reductive dissolution of Fe2O3 gives rise to an enrichment of Cr2O3 in the film, which is followed by an increase in the void fraction of the film. When the film with XCr = 0.61 was polarized at an anodic potential of 1.1 V, the value of nfilm first increased and then decreased with time, while that of kfilm monotonously decreased with time. The change in nfilm was prominent in a short wavelength range. These results show that the oxidative dissolution of Cr2O3 causes both an enrichment of Fe2O3 and an increase in a void fraction or surface roughness. The influence of the enrichment of Fe2O3 on nfilm is ascendant in the initial stage of dissolution and that of the increased void fraction or surface roughness is predominant in the subsequent stage of dissolution.
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