Prediction of a new potential high-pressure structure of FeSiO$_3$

TL;DR

This paper predicts a new high-pressure FeSiO3 structure using evolutionary algorithms and DFT+U, compares its XRD pattern with experimental data, and discusses its potential stability in Earth's lower mantle.

Contribution

The study introduces a novel high-pressure FeSiO3 phase, PPv-II, identified through computational methods and compares its properties with known phases.

Findings

PPv-II matches experimental XRD peaks of the H-phase.

PPv-II is less stable than Pv and PPv phases at 0 K.

PPv-II could be stabilized by entropy in Earth's lower mantle.

Abstract

We predict a new candidate high-temperature high-pressure structure of FeSiO$_3$ with space-group symmetry Cmmm by applying an evolutionary algorithm within DFT+U that we call post-perovskite II (PPv-II). An exhaustive search found no other competitive candidate structures with ABO$_3$ composition. We compared the X-ray diffraction (XRD) pattern of FeSiO$_3$ PPv-II with experimental results of the recently reported H-phase of (Fe,Mg)SiO$_3$. The intensities and positions of two main X-ray diffraction peaks of PPv-II FeSiO$_3$ compare well with those of the H-phase. We also calculated the static equation of state, the enthalpy and the bulk modulus of the PPv-II phase and compared it with those of perovskite (Pv) and post-perovskite (PPv) phases of FeSiO$_3$. According to the static DFT+U computations the PPv-II phase of FeSiO$_3$ is less stable than Pv and PPv phases under lower mantle pressure conditions at 0 K and has a higher volume. PPv-II may be entropically stabilized, and may be a stable phase in Earth$'$s lower mantle, coexisting with $\alpha$-PbO$_2$ (Columbite-structured) silica and perovskite, or with magnesiowustite or ferropericlase, depending on bulk composition.