High Pressure Formation of the Eight‐Fold Coordinated Post‐Post Spinel MgFe ₂ O ₄

MgFe 2 O 4 was probed to 74(1) GPa and 2,840(130) K as a low‐pressure analog to post‐post spinel Mg 2 SiO 4 predicted in super‐Earths using synchrotron multigrain X‐ray diffraction techniques in the laser‐heated diamond anvil cell. With high‐temperatures above 65 GPa the eight‐fold coordinated δ ‐MgFe 2 O 4 (Th 3 P 4 ‐type) is stable with Mg and Fe disordered into one site, analogous to that predicted in Mg 2 SiO 4 . While the phase‐boundary slope between the h ‐ and δ‐phases of MgFe 2 O 4 and Fe 3 O 4 differ, the substitution of Fe 2+ and Mg 2+ has little overall pressure effect on the stability of the post‐post spinel phase. This eight‐fold coordinated δ ‐MgFe 2 O 4 is stable at ∼400 GPa lower pressures compared to the predicted δ ‐Mg 2 SiO 4 , suggesting the need for further explorations into the conditions needed to substitute Fe 3+ into δ ‐Mg 2 SiO 4 and the effect of ferric iron on the onset of eight‐fold coordination in super‐Earth mantles.