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

We predict a candidate high-temperature, high-pressure structure of ${\mathrm{FeSiO}}_{3}$ with space-group symmetry Cmmm by applying an evolutionary algorithm within density functional theory (DFT)+U that we call post-perovskite II (PPv-II). An exhaustive search found no other competitive candidate structures with ${AB\text{O}}_{3}$ composition. We compared the x-ray diffraction pattern of ${\mathrm{FeSiO}}_{3}$ PPv-II with experimental results of the recently reported ``H phase'' of (Fe,Mg)${\mathrm{SiO}}_{3}$. The intensities and positions of two main x-ray diffraction peaks of PPv-II ${\mathrm{FeSiO}}_{3}$ 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 the perovskite (Pv) and post-perovskite (PPv) phases of ${\mathrm{FeSiO}}_{3}$. According to the static DFT+U computations, the PPv-II phase of ${\mathrm{FeSiO}}_{3}$ is less stable than the Pv and PPv phases under lower mantle pressure conditions at $T=0$ 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 -${\mathrm{PbO}}_{2}$ (columbite-structure) silica and perovskite, or with magnesiowustite and/or ferropericlase, depending on the bulk composition.