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 $\left({\mathrm{Mg}}_{0.6}{\mathrm{Fe}}_{0.4}\right)\mathrm{O}$-ferropericlase for pressures up to that of the lower mantle region were investigated using synchrotron x-ray diffraction (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 $\left({\mathrm{Mg}}_{0.6}{\mathrm{Fe}}_{0.4}\right)\mathrm{O}$-ferropericlase is maintained up to 160 GPa. The Mössbauer spectra at 19.8 and 24.0 GPa consist of three doublets assigned to HS ${\mathrm{Fe}}^{2+}$ 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 Mössbauer 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.

Figure 1

(a) X-ray diffraction pattern measured using a Bruker D8 ADVANCE and (b) Mössbauer spectrum measured at SPring-8 BL11XU of $\left({\mathrm{Mg}}_{0.6}{{\mathrm{Fe}}^{2+}}_{0.4}\right)$-ferropericlase at 1 atm. The residual is shown below the spectrum. The doublets are least-square fits to the empirical data assuming quadrupole split components with different intensities.

Figure 2

X-ray diffraction patterns of $\left({\mathrm{Mg}}_{0.6}{{\mathrm{Fe}}^{2+}}_{0.4}\right)$-ferropericlase measured at room temperature. Diffraction peaks with Miller indices belonging to $\left({\mathrm{Mg}}_{0.6}{{\mathrm{Fe}}^{2+}}_{0.4}\right)$-ferropericlase. KCl: pressure medium. The diffraction pattern at 0 GPa is measured using a Bruker D8 ADVANCE.

Figure 3

Pressure-volume relationship of $\left({\mathrm{Mg}}_{0.6}{{\mathrm{Fe}}^{2+}}_{0.4}\right)$-ferropericlase at room temperature.

Figure 4

Mössbauer spectra of $\left({\mathrm{Mg}}_{0.6}{{\mathrm{Fe}}^{2+}}_{0.4}\right)$-ferropericlase measured at room temperature. The residual is shown below for each spectrum. The doublets and singlet are least-square fits to the experimental data assuming quadrupole split components with different intensities. (a) and (b) Spectra fitted with three doublets. (c)–(g) Spectra fitted with three doublets and one singlet. (h) and (i) Spectra fitted with one singlet.

Figure 5

Pressure dependence of (a) the isomer shift of the doublets and the singlet, (b) the quadrupole splitting of doublet, and (c) the area ratio of the doublets (closed circles) and the singlet (open circles). The isomer shift and quadrupole splitting were calculated from the doublets.

Figure 6

The relationship between the wüstite component and the spin crossover region. The results of this paper are indicated by large diamond symbols. Symbols are as follows: open circle: experiments with gas pressure medium and no annealing before measurements [2, 12, 57], open square: experiments with liquid or solid pressure medium and no annealing before measurements, open triangle: experiments with no pressure medium and no annealing before measurements, closed square: experiments with liquid or solid pressure medium and annealing before measurements, closed triangle: experiments with no pressure medium and annealing before measurements. NaCl [2, 9, 47, 58], KCl (this paper) [15, 31, 58], KBr (this paper), heavy mineral oil [62], and boron epoxy [60] are used as solid or liquid pressure media. Ne [9, 35, 54, 55, 59, 61] and Ar [55, 56] are also used as gas pressure media.