Phase transitions beyond post-perovskite in NaMgF₃ to 160 GPa

Neighborite, NaMgF₃, is used as a model system for understanding phase transitions in ABX₃ systems (e.g., MgSiO₃) at high pressures. Here we report diamond anvil cell experiments that identify the following phases in NaMgF₃ with compression to 162 GPa: NaMgF₃ (perovskite) → NaMgF₃ (post-perovskite) → NaMgF₃ (Sb2S₃-type) → NaF (B2-type) + NaMg₂F₅ (P21/c) → NaF (B2) + MgF₂ (cotunnitetype). Our results demonstrate the existence of an Sb2S₃-type postpost- perovskite ABX₃ phase. We also experimentally demonstrate the formation of the P2₁/c AB₂X₅ phase which has been proposed theoretically to be a common high-pressure phase in ABX₃ systems. Our study provides an experimental observation of the full sequence of phase transitions from perovskite to post-perovskite to post-post-perovskite followed by 2-stage breakdown to binary compounds. Notably, a similar sequence of transitions is predicted to occur in MgSiO₃ at ultrahigh pressures, where it has implications for the mineralogy and dynamics in the deep interior of large, rocky extrasolar planets.