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 (cotunnite-type). Our results demonstrate the existence of an Sb2S3-type post-post-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.
Summary of constraints on the high-pressure and -temperature phase diagram of NaMgF3. The solid symbols (triangles: Pv, star: pPv, circles: Sb2S3, inverted triangle: pPv + Sb2S3, vertical diamonds: NaMg2F5 + NaF, horizontal diamonds: NaF + MgF2) represent the present experimental data. Open symbols (red, ref. 24; blue, ref. 25; and green, ref. 32) show experimental literature data for the respective phases. The brown open circle indicates the high-temperature cubic phase (33, 34). The dashed black lines are schematic phase boundaries.