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 Fe2+ in synthetic (Mg0.6Fe0.4 )O ferropericlase for pressures up to that of the lower mantle region were investigated using synchrotron XRD and synchrotron Mössbauer spectroscopic methods. The XRD patterns and the Mössbauer spectra were measured up to 160 GPa at room temperature. The results of the synchrotron XRD analysis indicate that the cubic structure of (Mg0.6Fe0.4)O ferropericlase is maintained up to 160 GPa. The Mössbauer spectra at 19.8 and 24.0 GPa consist of three doublets assigned to HS Fe2+ at the octahedral site. At pressures from 61 to 136 GPa, a singlet assigned to LS Fe2+ is added to the three HS Fe2+ doublets, and its area ratio with respect to the HS Fe2+ doublets gradually increase with increasing pressure. At pressures above 136 GPa, the Mössbauer spectra consist of only an LS Fe2+ 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 Fe2+ at pressure conditions from the upper part to the bottom of the lower mantle