Phase Transitions Beyond Post-Perovskite in Neighborite (NaMgF₃) to 1.6 Mbars

Neighborite, NaMgF_3, is used as a model system for understanding ultra-high pressure phase transitions in ABX₃ systems such as MgSiO₃. In this work, we have studied the high-pressure behavior of NaMgF₃ to 1.6 Mbars using the laser-heated diamond anvil cell technique coupled with synchrotron x-ray diffraction. Our work reveals a complex sequence of high-pressure phases beyond post-perovskite in this ABX₃ system. The phase transition sequence in NaMgF₃ is: NaMgF₃ (perovskite) → NaMgF₃ (post perovskite) → NaMgF₃ (Sb₂S₃-type) → NaF (B2-type) + NaMg₂F₅ (P2₁/c) → NaF (B2) + MgF₂ (cotunnite-type). Our results demonstrate the existence of a post-post-perovskite phase, followed a two-stage dissociation into binary fluorides. This work is the first experimental report of a dissociation of a post-perovskite in any known ABX₃ system. A similar sequence of transition is predicted to occur in MgSiO₃ at ultrahigh pressures where it has implications for the mineralogy and dynamics large, rocky extra-solar planets.