Redox-mediated transformation of a Tb2O3(111) thin film from the cubic fluorite to bixbyite structure

Abstract

We used temperature programmed desorption (TPD) and low energy electron diffraction (LEED) to investigate the isomeric structural transformation of a Tb₂O₃ thin film grown on Pt(111). We find that repeated oxidation and thermal reduction to 1000 K transforms an oxygen-deficient, cubic fluorite (CF) Tb₂O₃(111) thin film to the well-defined bixbyite, or c-Tb₂O₃(111) structure, whereas annealing the CF-Tb₂O₃(111) film in UHV is ineffective in causing this structural transformation. We estimate that the final stabilized film consists of about ten layers of c-Tb₂O₃(111) in the surface region plus about eight layers of CF-Tb₂O₃(111) located between the c-Tb₂O₃(111) and the Pt(111) substrate. Our measurements reveal the development of two distinct O₂ TPD peaks during the CF to bixbyite transformation that arise from oxidation of c-Tb₂O₃ domains to the stoichiometrically-invariant ι-Tb₇O₁₂ and δ-Tb₁₁O₂₀ phases and demonstrate that the c-Tb₂O₃ phase oxidizes more facilely than CF-Tb₂O₃. We present evidence that nucleation and growth of c-Tb₂O₃ domains occurs at the buried TbO_x/CF-Tb₂O₃ interface, and that conversion of the interfacial CF-Tb₂O₃ to bixbyite takes place mainly during thermal reduction of TbO_x above ∼900 K and causes newly-formed c-Tb₂O₃ to advance deeper into the film. The avoidance of low Tb oxidation states may facilitate the CF to bixbyite transformation via this redox mechanism.