Stability of the MgSiO₃ analog NaMgF₃ and its implication for mantle structure in super-Earths

First-principles calculations on MgSiO₃ suggested a breakdown into MgO + SiO₂ at pressure above 1000 GPa with an extremely large negative Clapeyron slope, isolating the lowermost mantles of larger super-Earths (∼10M_⊕) from convection. Similar calculations predicted the same type of breakdown in NaMgF₃ to NaF + MgF₂ at 40 GPa, allowing for experimental examination. We found that NaMgF₃ is stable to at least 70 GPa and 2500 K. In our measurements on MgF₂ (an SiO₂ analog), we found a previously unidentified phase ("phase X") between the stability fields of pyrite-type and cotunnite-type (49-53 GPa and 1500-2500 K). A very small density increase (1%) at the pyrite-type → phase X transition would extend the stability of NaMgF₃ relative to the breakdown products. Furthermore, because phase X appears to have a cation coordination number intermediate between pyrite-type (6) and cotunnite-type (9), entropy change (δS) would be smaller at the breakdown boundary, making the Clapeyron slope (dP/dT = δS/δV) much smaller than the prediction. If similar trend occurs in MgSiO₃ and SiO ₂, the breakdown of MgSiO₃ may occur at higher pressure and have much smaller negative Clapeyron slope than the prediction, allowing for large-scale convection in the mantles of super-Earth exoplanets.