Rheology of Earth Materials :
Closing the gap between timescales in the laboratory and in the mantle

Pipe Diffusion mechanism in MgO

Using a numerical approach based on atomistic simulations (empirical potential and kinetic monte carlo) coupled with elasticity theory, we demonstrate the efficiency of the vacancy diffusion along edge dislocation, especially for the anionic vacancy for which a decrease of 22 % on the migration barrier is observed ! 

This is due to the fact that the vacancies strongly interact with the edge dislocation core which modify the local diffusion path. Favored diffusion path along the line has been identified (see Figure below) :


We show that edge dislocation in MgO behaves as a strong sink for vacancies which, combined with a lower migration energy at dislocation core region, strongly enhances the diffusion of point defect in the vicinity of the dislocation with respect to the bulk material (see the short movies below).


At low and intermediate temperatures, pipe diffusion results in an increase of the diffusivity of several order of magnitude. Accounting more precisely for the effect of pipe diffusion may therefore be a key to reconcile the experimentally measured scattering of diffusivity in MgO. Indeed we show that the diffusivity strongly depends on the dislocation density and hence the pipe diffusion process and a bend on the diffusivity curve is observed which is also present on experimental data (see Figure below) :


To learn more:

M. Landeiro Dos Reis, Y. Giret, P. Carrez, P. Cordier (2022) Efficiency of the vacancy pipe diffusion along an edge dislocation in MgO. Computational Materials Science, 211, 111490. https://doi.org/10.1016/j.commatsci.2022.111490