Fundamental aspects about the first steps of irradiation-induced phase transformations in fluorite-related oxides

International audienceFundamental aspects of ion-irradiation-induced phase transformations in fluorite-related oxides, from cubic to monoclinic structures, are investigated through xenon implantation at medium energy in yttrium sesquioxide (Y2O3). For that purpose, an original approach considering this material fabricated as a thin film is proposed to control the pristine structure before irradiation and to allow fine measurements of internal stresses. It is evidenced that this oxide can be strongly disordered up to an unexpected amorphization, depending on the initial damage state. We emphasize that point defects created during the irradiation process generate strong internal stresses which turn out to be a major parameter in the mechanism involved during such phase transition. When the oxide structure is initially weakly disordered, the evolutions of the biaxial and triaxial residual stresses, together with the lattice parameter, presents two different steps. Those steps are interpreted at the atomic scale by the nucleation of oxygen vacancy aggregates which collapse in extended dislocation loops that are considered as monoclinic nuclei. The geometry of these extended defects has been considered to explain why the monoclinic phase appears with clearly defined crystallographic orientation relationship (4 0 )B//(2 2 2)C relative to the initial cubic phase. The link between the energy deposited by irradiation and the critical size of these defects, analyzed from a thermodynamical approach by the evaluation of the Gibbs free energy, also elucidates the presence of sharp thresholds observed between the different phases (amorphous/cubic and monoclinic/amorphous) along the depth of the film