Atomistic Modeling of Dislocations in MgSiO3 Post-Perovskite

Abstract:

The recently discovered MgSiO₃ post-perovskite phase (Cmcm) is only stable at high pressure and temperature conditions corresponding to the lowermost ~150 km of the mantle (the D'' layer) [1, 2]. The unusual, for a high-pressure phase, layer-like structure of the post-perovskite may be responsible for the observed seismic anisotropy of the D'' layer. However, information about mechanical properties, easier slip systems, dislocations and their behaviour under stress are not well known still. This work represents a theoretical study of the post-perovskite within the semi-empirical approach using the Buckingham interatomic potential parameters previously derived by [3].

To describe the energy cost incurred as a result of a shear and to deduce the most favorable slip systems, the GSF excess energies are calculated at 120 GPa. The lowest energy barrier as well as the smallest values of the ideal shear stress (ISS), are related to the slip systems with the smallest [100] Burgers vector (b=2.521 Å) and to the slip system [001](010) with the glide plane cutting only Mg-O bonds. Good agreement with the ab-initio results [4] verifies the accuracy of the chosen inteatomic potential model [3].

The C-lattice of the post-perovskite results in four potential Burgers vectors: [100], [010], [001] and ½[011]. Taking into account the estimated GSF energies for different slip systems, we focus on mobility of screw and edge dislocations with Burgers vectors [100], [010] and ½[011]. The evaluated values of lattice friction suggest (010) slip plane parallel to the Si- and Mg-layers in the post-perovskite strucure to be the most probable .

References

[1] Murakami, M. et al., Geophys. Res. Lett. (2005), 32, L03304.

[2] Oganov, A. & Ono S., Nature (2004), 430, 44– 448.

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

[4] Carrez Ph. et al., Philosoph. Mag. (2007), 87, 3229-3247.