Damage evolution of ion irradiated defected-fluorite La₂Zr₂O₇ epitaxial thin films

Pyrochlore-structure oxides, A₂B₂O₇, may exhibit remarkable radiation tolerance due to the ease with which they can accommodate disorder by transitioning to a defected fluorite structure. The mechanism of defect formation was explored by evaluating the radiation damage behavior of high quality epitaxial La₂Zr₂O₇ thin films with the defected fluorite structure, irradiated with 1 MeV Zr⁺ at doses up to 10 displacements per atom (dpa). The level of film damage was evaluated as a function of dose by Rutherford backscattering spectrometry in the channeling geometry (RBS/c) and scanning transmission electron microscopy (STEM). At lower doses, the surface of the La₂Zr₂O₇ film amorphized, and the amorphous fraction as a function of dose fit well to a stimulated amorphization model. As the dose increased, the surface amorphization slowed, and amorphization appeared at the interface. Even at a dose of 10 dpa, the core of the film remained crystalline, despite the prediction of amorphization from the model. To inform future ab initio simulations of La₂Zr₂O₇, the bandgap of a thick La₂Zr₂O₇ film was measured to be indirect at 4.96 eV, with a direct transition at 5.60 eV.