Low-temperature $P{-}T$ phase diagram of the (Mg,Fe)SiO$_3$ perovskite

The electron spin states of iron in minerals of the Earth’s mantle at high pressures mostly determine the physicochemical properties of deep layers of the Earth and are of great interest not only for geophysics but also for fundamental physics of strongly correlated electron systems. In this work, using Raman and synchrotron Mössbauer nuclear forward scattering (NFS) spectroscopies, iron-containing magnesium-silicate perovskite (Mg,Fe)SiO$_3$ ($10\%$ Fe) has been studied in the cryogenic temperature range of $35$–$300$ K and at high pressures up to 48 GPa, which are created in diamond anvil cells. The analysis of NFS spectra has indicated that iron ions are in a nonmagnetic (para- or diamagnetic) state in the entire region of temperatures and pressures and the electronic properties can be controlled by means of the quadrupole splitting parameter. It has been found that an increase in the pressure and a decrease in the temperature are accompanied by a significant increase in the parameter $\Delta$ from $2$ mm/s to $\sim4$ mm/s, which indicates that the electronic state of Fe$2^+$ ions changes. The maximum $\Delta$ value has been observed at $P> 20$ GPa, but the pressure behavior of a transition strongly depends on the temperature. Possible mechanisms of the transition have been discussed.