Native vacancy migrations in zircon

Energy minimization methods were used to simulate the migration of Zr, Si, and O vacancies in zircon (ZrSiO4). Two sets of interatomic potentials were employed for comparison: one with O-Si-O three-body terms for the SiO4, and one without. Results for Si were inconclusive, but consistent with maintaining the integrity of the SiO4 molecular units. Both Zr and O vacancies can migrate on three-dimensional sublattice networks, thus supporting the experimentally observed diffusional isotropy. The predicted Zr vacancy migration energy (1.16-1.38 eV) was in good agreement with experiment if supplemented by Zr vacancy formation via Schottky or Frenkel defects (6.21-12.28 eV/defect). Oxygen vacancy migration energies were predicted to be 0.99-1.16 eV, somewhat lower than the experimental value of 4.64 eV measured in natural zircons, which thus may include significant contributions from vacancy formation mechanisms at 3.31-6.52 eV/defect. (orig.)