Synchrotron Mössbauer spectroscopic and x-ray diffraction study of ferropericlase in the high-pressure range of the lower mantle region

Systematic change of structural transition and high-spin (HS) to low-spin (LS) transition of ${\mathrm{Fe}}^{2+}$ in synthetic $({\mathrm{Mg}}_{0.6}{\mathrm{Fe}}_{0.4})\mathrm{O}$-ferropericlase for pressures up to that of the lower mantle region were investigated using synchrotron x-ray diffraction (XRD) and synchrotron M\"ossbauer spectroscopic methods. The XRD patterns and the M\"ossbauer spectra were measured up to 160 GPa at room temperature. The results of the synchrotron XRD analysis indicate that the cubic structure of $({\mathrm{Mg}}_{0.6}{\mathrm{Fe}}_{0.4})\mathrm{O}$-ferropericlase is maintained up to 160 GPa. The M\"ossbauer spectra at 19.8 and 24.0 GPa consist of three doublets assigned to HS ${\mathrm{Fe}}^{2+}$ at the octahedral site. At pressures from 61 to 136 GPa, a singlet assigned to LS ${\mathrm{Fe}}^{2+}$ is added to the three HS ${\mathrm{Fe}}^{2+}$ doublets, and its area ratio with respect to the HS ${\mathrm{Fe}}^{2+}$ doublets gradually increase with increasing pressure. At pressures above 136 GPa, the M\"ossbauer spectra consist of only an LS ${\mathrm{Fe}}^{2+}$ singlet, implying that all Fe at these pressures is in a LS state. The resulting spin crossover pressure interval from 61 to 136 GPa indicates the coexistence of both HS and LS ${\mathrm{Fe}}^{2+}$ at pressure conditions from the upper part to the bottom of the lower mantle.