Are CaIrO3 and MgGeO3 isomechanical to MgSiO3-post-perovskite ?

The recent discovery of MgSiO3 post-perovskite (pPv) and prediction of its elastic properties using atomistic modelling has major implications for the interpretation of seismic anisotropy of the D" layer. Because they don't take into account lattice preferred orientations induced by convective flow, the elastic properties are not sufficient to interpret seismic anisotropy and it is necessary to investigate the plasticity of this mineral. However, it is well known that pressure and temperature near the core-mantle boundary make experimental studies extremely difficult. To circumvent this difficulty, experimental studies are often carried out on analogous phases (stable at lower pressures) which are supposed to exhibit the same mechanical properties as the high-pressure phase. In this work, we calculate the dislocation properties of MgGeO3 pPv at 120GPa and CaIrO3 pPv at ambient pressure using the Peierls-Nabarro (PN) model. The so-called PN model is a fundamental concept of the dislocation theory which describes the resistance of the lattice to dislocation motion, a very important factor for the plasticity of silicates. The PN model also provides an analytical description of the dislocation core and of its potential spreading in the glide plane. Known for several decades, the PN model has triggered a renewed interest when Christian and Vitek (1970) showed that realistic models of dislocations could be built by incorporating generalized stacking faults (GSF) into the PN model. Here, we use the ab initio total-energy package VASP to calculate GSF, which are incorporated in the PN model. In that way, we obtain a model of the dislocation core profile and the value of the stress required to move a dislocation (the so-called Peierls stress) for ten slip systems of each compound. These results are compared to those recently published on MgSiO3 post- perovskite to assess the potential relevance of the analogue approach in studying the rheology of the D" layer. We show that, besides the crystal structure, atomic bonding is an important feature in constraining plastic strain anisotropy. The greatest contrast between Ca-O and Ir-O bond strengths compared to Mg-O and Ge-O or Si-O makes CaIrO3 behave very distinctly from MgSiO3. Although still present, differences are smaller between MgGeO3 and MgSiO3.