Modeling defects and plasticity in MgSiO3 post-perovskite

Abstract:

Alexandra M. Goryaeva, Philippe Carrez, Patrick Cordier

UMET, Université de Lille 1, 59655 Villeneuve d'Ascq, France

alexandra.goryaeva@ed.univ-lille1.fr

The discovery of the so-called “last mantle phase transition” [1] immediately became promising to shed light on the puzzling properties of the D′′ layer and to provide new insights into our understanding of the dynamics of the lowermost Earth’s mantle. The MgSiO₃ post-perovskite (Cmcm) exhibits unusual for high-pressure phases layer-like structure which may be responsible for the observed seismic anisotropy of the D'' layer. However, information about mechanical properties, probable slip systems, dislocations and their behavior under stress are not well known still. This work represents a full atomistic study of dislocations in MgSiO₃post-perovskite based on the pairwise potential previously derived by [2].

Lattice friction opposed to the dislocation glide in MgSiO₃ post-perovskite is shown to be highly anisotropic. Thus, remarkably low values of Peierls stress (1 GPa) are found for the glide of [100] screw dislocations in (010), while glide in (001) requires almost 18 times larger stress values. In general, (010) plane is characterized by the lowest lattice friction. Mobility of edge dislocations is found to be much higher than that of screw dislocations, and the latter will control plastic behavior of MgSiO₃ post-perovskite. Comparison of our results with previous study of MgSiO₃ perovskite (bridgmanite) [3], based on similar simulation approach, clearly shows that monotonous increase in Peierls stress of bridgmanite will be followed by a dramatic drop after the phase transition to the post-perovskite phase, which consequently suggests the D′′ located at the CMB to be weaker than the overlying mantle.

References

[1] Murakami M. et al.,Science (2004), 304, 855–858.

[2] Oganov A. et al.,Phys. Earth Planet. Int. (2000), 122, 277-288.

[3] Hirel et al., Acta Mater (2014 ), 79, 117–125.