B1−B2 phase transition of ferropericlase at planetary interior conditions

Abstract

Using ab initio simulations based on density functional theory, we have analyzed the crystal structure and thermodynamic stability of ${\mathrm{Mg}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}x}\mathrm{O}$ ferropericlase, showing how the $P\text{\ensuremath{-}}T$ phase diagram associated with the $B1\text{\ensuremath{-}}B2$ phase transition of pure MgO is influenced by the presence of iron substitutional alloys. We find that a small concentration of Fe atoms contribute to an increase of the transition pressure at fixed temperature, extending the stability of $B1$ crystalline structure. Moreover, we find a significant nonhomogeneous distribution of the iron atoms between the two phases at low temperatures, with strong partitioning in the $B1$ phase, an interesting phenomena that could lead to important dynamic consequences. Finally, we analyze the effect of the iron impurities on the volume thermal expansion.