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

摘要

Systematic change of structural transition and high-spin (HS) to low-spin (LS) transition of Fe~(2+) in syn-thetic (Mg_(0.6)Fe_(0.4))O-ferropericlase for pressures up to that of the lower mantle region were investigated using synchrotron x-ray diffraction (XRD) and synchrotron Moessbauer spectroscopic methods. The XRD patterns and the Moessbauer spectra were measured up to 160 GPa at room temperature. The results of the synchrotron XRD analysis indicate that the cubic structure of (Mg_(0.6)Fe_(0.4))0-ferropericlase is maintained up to 160 GPa. The Moessbauer spectra at 19.8 and 24.0 GPa consist of three doublets assigned to HS Fe~(2+) at the octahedral site. At pressures from 61 to 136 GPa, a singlet assigned to LS Fe~(2+) is added to the three HS Fe~(2+) doublets, and its area ratio with respect to the HS Fe~(2+) doublets gradually increase with increasing pressure. At pressures above 136 GPa, the Moessbauer spectra consist of only an LS 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 Fe~(2+) at pressure conditions from the upper part to the bottom of the lower mantle.