Segregation of bare and protonated Mg vacancies to dislocation cores in MgO

Richard Skelton, Andrew Walker 

Water can be incorporated into the lattice of mantle minerals in the form of protons charge-balanced by the creation of cation vacancies. These protonated vacancies, when they interact with dislocations, increase strain rates by enhancing dislocation climb and, potentially, by reducing the Peierls barrier to glide. We use atomic scale simulations to investigate segregation of Mg vacancies to atomic sites within the core regions of dislocations in MgO. Energies are computed for bare and protonated Mg vacancies occupying atomic sites close to 1/2<110> screw dislocations, and 1/2<110>{100} and 1/2<110>{110} edge dislocations. These are compared with energies for equivalent defects in the bulk lattice to determine segregation energies for each defect. Mg vacancies preferentially bind to 1/2<110>{100} edge dislocations, with calculated minimum segregation energies of -3.54 eV for {VMg}′′ and -4.56 eV for {2HMg}X. The magnitudes of the minimum segregation energies calculated for defects binding to 1/2<110>{110} edge or 1/2<110> screw dislocations are considerably lower. Interactions with the dislocation strain field lift the 3-fold energy degeneracy of the {2HMg}X defect in MgO. For edge dislocations, defect configurations in which the O-H bond vector is perpendicular to the glide plane normal are preferred, which may have implications for the ability of protonated vacancies to influence dislocation glide.