Applications of multigrain crystallography for the study of post-perovskite microstructures

Abstract:

Silicate post-perovskite is found at the Earth's core-mantle boundary, 2900 km below the Earth's surface. Understanding the mechanical properties of this phase is important for constraining the dynamics of this region. It is, however, a multiscale problem with post-perovskite defects driving the behavior of single-crystals, themselves parts of polycrystals, polycrystals plastically deforming due to mantle convection. Here, we wish to address one aspect of this multiscale problem by analyzing the behavior of individual grains of post-perovskite inside a polycrystalline aggregate. Post-perovskite is not stable under ambient pressures and should be studied under the pressures of the Earth's core-mantle boundary: 135 GPa, We use diamond anvil cells to generate such pressure conditions and study grains of post-perovskite using multigrain crystallography, in-situ, at pressures above 1 megabar. Experiment was performed at the ID11 beamline of the ESRF on a 80 μm diameter and 20 μm thickness post-perovskite sample to identify grains of post-perovskite and follow their behavior inside a polycrystalline aggregate. Diamond anvil cells allow experiments under extreme conditions but access to sample diffraction and sample rotation are limited. Such limitation in sample access can be a problem for multigrain crystallography. Hence, in this presentation, we will investigate the fine details of such analysis. In particular, we will focus on evaluating the proportion of grains that can be indexed successfully and how grains can be localized within the sample. Eventually, these results will allow us to follow the orientation and position of hundreds of grains inside a polycrystalline aggregate undergoing physical processes such as plastic deformation or phase transformations.