Interactions between bare and protonated Mg vacancies and dislocation cores in MgO

Abstract

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, influence strain rates by affecting dislocation climb, pinning the dislocation, and, potentially, by altering 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 $V_{{{\text{Mg}}}}^{\prime \prime }$ protonated Mg vacancies occupying atomic sites close to ½ 〈110〉 screw dislocations, and ½ 〈110〉 {100} and ½ 〈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 ½ 〈110〉 {100} edge dislocations, with calculated minimum segregation energies of − 3.54 eV for and − 4.56 eV for ${\text{2H}}_{{{\text{Mg}}}}^{{\text{x}}}$. The magnitudes of the minimum segregation energies calculated for defects binding to ½ 〈110〉 {110} edge or ½ 〈110〉 screw dislocations are considerably lower. Interactions with the dislocation strain field lift the threefold energy degeneracy of the ${\text{2H}}_{{{\text{Mg}}}}^{{\text{x}}}$ defect in MgO. These calculations show that Mg vacancies interact strongly with dislocations in MgO, and may be present in sufficiently high concentrations to affect dislocation mobility in both the glide- and climb-controlled creep regimes.