Fig. 3

H diffusion for the Si ⇔ Mg + 2H defects at 2500K. They are computed in a 2 × 1× 1 convention supercell with one defect (a) and in a 2× 2× 2 primitive supercell containing two defects (b). a Only the trajectory of H is shown, with all the other atoms omitted for clarity. H atoms describe crosses-like patterns on the outside parts of the SiO₄ tetrahedra. H atoms undergo lengthy excursions before falling back into the vacancy site. b The trajectory described by all the atoms during 35 ps is represented here. Pink, yellow, red and white represent respectively the Mg, Si, O, and H atoms. Not all excursions of H result in net diffusion, as seen from the defect in the lower left part of the image: H falls back into the initial site after escaping into the adjacent empty octahedron for a short time. The unit cell is outlined with dashed line