Modeling defects and plasticity in MgSiO3 post-perovskite: Part 2—screw and edge [100] dislocations

Abstract

In this study, we propose a full atomistic study of [100] dislocations in MgSiO3 post-perovskite based on the pairwise potential parameterized by Oganov et al. (Phys Earth Planet Inter 122:277–288, 2000) for MgSiO3 perovskite. We model screw dislocations to identify planes where they glide easier. We show that despite a small tendency to core spreading in {011}, [100] screw dislocations glide very easily (Peierls stress of 1 GPa) in (010) where only Mg–O bonds are to be sheared. Crossing the Si-layers results in a higher lattice friction as shown by the Peierls stress of [100](001): 17.5 GPa. Glide of [100] screw dislocations in {011} appears also to be highly unfavorable. Whatever the planes, (010), (001) or {011}, edge dislocations are characterized by a wider core (of the order of 2b). Contrary to screw character, they bear negligible lattice friction (0.1 GPa) for each slip system. The layered structure of post-perovskite results in a drastic reduction in lattice friction opposed to the easiest slip systems compared to perovskite.

Modeling defects and plasticity in MgSiO3 post-perovskite: Part 3—Screw and edge [001] dislocations

“…Full atomic modeling of [100] (Goryaeva et al 2015b) and [001] dislocation glide (this study) in MgSiO 3 postperovskite under lower mantle conditions demonstrates easy slip in (010) plane parallel to the structural layering. Being characterized by significantly different crystal chemistry, Mg-O and Si-O layers in the high-pressure MgSiO 3 phase lead to a very strong plastic anisotropy with (010) concentrating most of plastic shear.…”

“…Being characterized by a larger Peierls stress of 3 GPa, the mobility of screw dislocations is expected to govern plastic strain for this slip system. However, it is worth noticing that, in contrast to [100] dislocations in post-perovskite (Goryaeva et al 2015b), where the Peierls stress σ p is found to be one order of magnitude lower for edge dislocations than for screw, σ p of edge dislocations is only 33% lower in case of the [001](010) system (Table 1).…”

“…The atomic systems used throughout this study to explore the dislocation core properties and dislocation motion are designed according to the methodology described by Hirel et al (2014) for MgSiO 3 perovskite (bridgmanite) which was also shown to be appropriate for modeling both edge and screw [100] dislocations in post-perovskite (see discussion in Goryaeva et al 2015b). In this approach, a single dislocation is embedded into a simulation cell (Fig.…”

“…Simultaneous motion of both partials allows assuming a simple sinusoidal shape of the Peierls potential. Therefore, the maximum energy barrier for each partial can be defined as (Koizumi et al 1993): (Goryaeva et al 2015b(Goryaeva et al , 2016 are 15 and 2 meV/Å for screw and edge dislocations, respectively. Although higher than for the [100] dislocations, lattice friction opposed to the [001] dislocation glide in (010) is still very low.…”

“…All these ambiguities, associated with the analog approach in plasticity, call for direct investigation of MgSiO 3 at relevant P-T conditions. Recent atomic-scale studies of [100] dislocations in MgSiO 3 post-perovskite (Goryaeva et al 2015b(Goryaeva et al , 2016) emphasize the importance of [100] slip in (010) Mg-layers in this material. Alone, this single slip system cannot account for the plasticity of a crystalline aggregate and further mechanisms should be investigated.…”