RheoMan: a five-year, ERC-funded (Advanced Grant), project to model the rheology of the Earth's mantle

Jan 27, 2016 Charged dislocations in bridgmanite Results

New article published in Acta Materialia about the electric charge and climb of dislocations in bridgmanite

In the Earth's lower mantle, bridgmanite is deformed at high temperature and slow strain rates. These conditions are favorable to dislocation climb, a mechanism by which a dislocation moves out of its glide plane by absorbing or emitting vacancies. We used atomic-scale simulations to investigate the interactions between a [100](010) edge dislocation and vacancies in MgSiO3 bridgmanite. The calculations show that the dislocation favors an oxygen-poor configuration, as represented in Fig. 1. Because bridgmanite is an ionic material, this deviation from stoichiometry results in the dislocation to bear a positive electric charge.


Fig. 1 - Atomic structure and electric charge distribution around the [100](010) edge dislocation. The red areas show the zones of positive electric charge that arise because of the deficiency of oxygen ions.

As introduced by Eshelby, the charge of the dislocation is compensated by a cloud of vacancies of opposite sign. Since both the dislocation and vacancies are charged, they interact not only through elastic effects, but also through the Coulomb interaction. Our calculations show that the Coulomb part is very strong, with interaction energies of several eV, superseding elastic effects. As a result the dislocation-vacancy interaction has a radial symmetry, as shown in Fig. 2. Cations vacancies (Mg and Si), being negatively charged, are attracted to the dislocation, while positively-charged oxygen vacancies are repelled by it.


Fig. 2 - Enthalpy of interaction of a magnesium vacancy with the [100](010) edge dislocation. The dashed line is a fit of a model accounting for the Coulomb interaction.

After absorbing cation vacancies the charge of the dislocation changes sign, thus becoming attractive to oxygen vacancies. Finally, we propose that dislocation climb occurs by the sequential absorption of positively and negatively-charged vacancies.

Reference: "The electric charge and climb of edge dislocations in perovskite oxides: The case of high-pressure MgSiO3 bridgmanite" P. Hirel, P. Carrez, E. Clouet, P. Cordier, Acta Mater. 106 (2016) 313-321.