Anisotropy of Earth's D" layer and stacking faults in the MgSiO3 post-perovskite phase

Abstract: The post-perovskite phase of (Mg,Fe)SiO3 is believed to be the main mineral phase of the Earth's lowermost mantle (the D" layer). Its properties explain numerous geophysical observations associated with this layer - for example, the D'' discontinuity, its topography and seismic anisotropy within the layer. Here we use a novel simulation technique, first-principles metadynamics, to identify a family of low-energy polytypic stacking-fault structures intermediate between the perovskite and post-perovskite phases. Metadynamics trajectories identify plane sliding involving the formation of stacking faults as the most favourable pathway for the phase transition, and as a likely mechanism for plastic deformation of perovskite and postperovskite. In particular, the predicted slip planes are (010) for perovskite (consistent with experiment) and (110) for postperovskite (in contrast to the previously expected (010) slip planes). Dominant slip planes define the lattice preferred orientation and elastic anisotropy of the texture. The (110) slip planes in post-perovskite require a much smaller degree of lattice preferred orientation to explain geophysical observations of shear-wave anisotropy in the D" layer.

• The paper demonstrates that intermediate stacking faults during the perovskite to post-perovskite transition explain seismic anisotropy in the D″ layer.
• It employs first-principles metadynamics combined with classical and ab initio simulations to identify {110} slip planes as the key mechanism in PPv.
• Results indicate low-energy differences (~20-30 meV/atom) between polytypes, allowing their stabilization under high pressures consistent with seismic data.

Anisotropy of Earth’s D'' Layer and Stacking Faults in MgSiO Post-perovskite

The investigation into the structural properties of the Earth's D'' layer through MgSiO$_3$ post-perovskite (PPv) phases presents significant advancements in understanding the geophysical processes at extreme depths of the Earth. Utilizing an innovative simulation approach, namely first-principles metadynamics, this study identifies key mechanisms pertinent to phase transitions and plastic deformations between perovskite (Pv) and PPv states.

The research focuses on elucidating the dynamic nature of the Earth's D'' layer, particularly attributing seismic anomalies to transformations within MgSiO$_3$ phases. Through metadynamics, the authors reveal intermediate polytypic stacking faults that interpolate between Pv and PPv structures. This suggests plane sliding and stacking fault formation as the dominant route for the Pv-PPv phase transition, thereby positing a plausible mechanism for plastic deformation in both phases. Notably, for PPv, the anticipated slip planes are determined to be {110}, challenging the previously assumed {010} planes, and aligning closely with observed seismic anisotropy data.

Quantitative results demonstrate that alternative polytypes are only slightly higher in enthalpy (~20-30 meV/atom) around transition pressures, allowing these phases to potentially stabilize through thermal or chemical factors. This low-energy scenario engenders implications that minor polytypes may subsist in the D'' region, influencing seismic wave behavior.

The elasticity and mechanical properties derived from NVT-molecular dynamics simulations indicate a revised understanding of shear-wave anisotropy. Predicated on {110} slip planes, the work shows alignment with observed vSH/vSV values, requiring significantly smaller lattice preferred orientations than earlier models proposed. This observation mitigates prior discrepancies, attributing a greater fitting to geophysical observations with a modest alignment degree, offering refined explanations for convective patterns and anisotropic properties, especially within subduction zones.

From a computational perspective, the study employs both classical and ab initio methods, utilizing the DL_POLY and VASP codes respectively, and leveraging the generalized gradient approximation alongside the all-electron PAW method. Structural explorations were conducted on supercells containing 160 atoms, providing substantive insights into the transition mechanisms at pressures exceeding 100 GPa.

The identification of {110} slip planes also reshapes interpretations of experimental data, particularly diamond-anvil cell outcomes. The implications of these mechanistic insights could extend to analogous silicate materials, proposing broader relevance across mineral physics under high-pressure environments.

Overall, this research contributes considerably to the theoretical and computational mineral physics framework, refining models of the Earth's mantle dynamics. Future developments may see this methodology applied to other post-perovskite analogs, potentially yielding novel insights into planetary interior evolution and properties. Continued studies may further enhance our grasp of the interplay between lattice dynamics, structural transitions, and their macroscopic seismic manifestations.