Pressure-induced phase transitions in NaMgF3 post-perovskite*

Understanding the behavior of MgSiO$_{3}$ postperovskite(PPV) under extreme pressures is fundamental for modeling the interiors of solar giants and extrasolar planets. In 2006, MgSiO$_{3}$ post-perovskite was predicted to dissociate into MgO and SiO$_{2}$ at 1.1 TPa (Umemoto et al., Science 311, 983 (2006)). However, the predicted dissociation pressure is too high to be easily verified experimentally. Instead, a low-pressure analog, NaMgF$_{3}$ neighborite, has been studied to test for structural predictions in MgSiO$_{3}$. NaMgF$_{3}$ was predicted to dissociate at $\sim$40 GPa (Umemoto et al., Geophys. Res. Lett. 33, L15304 (2006)), but this has not been confirmed experimentally (Martin et al., Geophys. Res. Lett. 33, L11305 (2006); Grocholski et al. Geophys. Res. Lett. 37, L14204 (2010)) and the dissociation MgSiO$_{3}$ PPV is now being questioned. Here, we reexamine in detail the pressure dependence of crystal structures and phonon frequencies in NaMgF$_{3}$ and reveal the apparent reason why dissociation was not observed in this material.