Electronic structure and stability field of iron-bearing pppv Mg₂SiO₄ at ultra-high pressure

At pressures ~500 GPa, which corresponds to the deep interior of super-Earths, we can expect the existence of MgSiO₃ post-perovskite (PPv) and MgO. Previous calculations predicted that NaCl-type MgO and MgSiO₃ PPv might combine to form I4^–2d-type Mg₂SiO₄ (pppv).(Umemoto et al., 2017). This phase might be the primary mantle silicate in those massive exoplanets. Therefore, it is essential to understand the properties of this pppv silicate phase, especially in solid-solution with Fe₂SiO₄. Here we present an ab initio study on the properties of Fe-bearing pppv from 400GPa to 1TPa in a range of iron concentrations, x_Fe, varying from ~3% to 12%. Given the strongly correlated nature of iron, LDA + U_sc and conventional DFT methods were used to study the electronic structures. The dependence of U on pressure and spin state is carefully considered in the (Mg_1−xFe_x )₂SiO₄ system. The influence of pressure, temperature, and structure on the spin state has also been discu#@ssed. These results will help model the mantle of super-Earth-type exoplanets.





K. Umemoto et al., (2017). DOI: 10.1016/j.epsl.2017.08.032