Anomalous vibrational properties and spin phase diagrams of (Mg,Fe)CO3 under extreme conditions

$(\mathrm{Mg},\mathrm{Fe})\mathrm{C}{\mathrm{O}}_{3}$ solid solutions not only play a crucial role in carbon storage and transport within the deep mantle, but also serve as a great case for studying the spin transition behavior of $\mathrm{F}{\mathrm{e}}^{2+}$ in the fairly isometric sixfold coordinated octahedral sites. However, simultaneously high temperature and pressure measurements on the spin crossover of $(\mathrm{Mg},\mathrm{Fe})\mathrm{C}{\mathrm{O}}_{3}$ remain scarce, which greatly limits our knowledge about the pressure-induced spin transition of $(\mathrm{Mg},\mathrm{Fe})\mathrm{C}{\mathrm{O}}_{3}$ under extreme conditions. In this study, we investigated three representative synthesized $(\mathrm{Mg},\mathrm{Fe})\mathrm{C}{\mathrm{O}}_{3}$ samples at high temperatures and pressures up to 65 GPa and 900 K using Raman spectroscopy coupled with diamond-anvil cells using neon (Ne) as a pressure-transmitting medium. Interestingly, $(\mathrm{Mg},\mathrm{Fe})\mathrm{C}{\mathrm{O}}_{3}$ exhibits abnormal vibrational properties and spin phase diagrams at high temperatures and pressures. The onset of spin crossover for the three $(\mathrm{Mg},\mathrm{Fe})\mathrm{C}{\mathrm{O}}_{3}$ samples decreases with increasing temperatures from 300 to 500 K, followed by an inverse increase at higher temperatures. Moreover, the width of spin crossover is $\ensuremath{\sim}7\text{\ensuremath{-}}9$ GPa at 800 K, compared to 3--4 GPa at 300 K. Notably, iron-rich $(\mathrm{Mg},\mathrm{Fe})\mathrm{C}{\mathrm{O}}_{3}$ displays a narrower width of spin crossover than its iron-poor counterpart. These findings provide insights into the effects of temperature and iron concentration on the pressure-induced spin transition of $(\mathrm{Mg},\mathrm{Fe})\mathrm{C}{\mathrm{O}}_{3}$.