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. 2005 Dec 5;102(50):17918–17922. doi: 10.1073/pnas.0508919102

Fig. 4.

Fig. 4.

Structural effect of spin transition. (a) Difference in d-spacing for the 200 diffraction lines (Δd200) of Mg0.8Fe0.2O – Mg0.1Fe0.9O magnesiowüstite compositions, measured as a function of pressure at 300 K. Circles indicate experiments with an alcohol mixture as a pressure medium, and squares refer to experiments for which Ar was the pressure medium; filled symbols are for data collected on compression, and open symbols are for data collected on decompression. The blue curve (with estimated 1σ uncertainty shown as a dashed line) is obtained by fitting Δd200 measured at pressures below 35 GPa to a linear dependence of normalized pressure on the Eulerian strain (12, 14). The red line (with 1σ envelope) is for the finite-strain fit of the observed Δd200 at pressures above 35 GPa. For comparison, the Δd200 calculated from the measurements in ref. 8 on (Mg0.83Fe0.17)O are shown in gray (these were not used to constrain the blue and red curves and error envelopes, but they show good agreement with our results within mutual uncertainties). (Inset) The absolute d-spacings for the 200 lines of the two compositions. The d-spacings approach each other with increasing pressure up to the spin transition at 35 GPa and then diverge (i.e., appearance of the low-spin state softens the equation of state of the x = 0.80 composition). (b) Abundance of Fe in the low-spin state, as determined from high-pressure Mössbauer spectra collected from Mg0.8Fe0.2O at 6 K. (Inset) The measured isomer shift (IS) for both low- and high-spin Fe components. Note that the x-ray emission measurements (8) exhibit spectra intermediate between those of high-spin (HS) and low-spin (LS) states at pressures of 54–67 GPa for an x = 0.83 sample, in good agreement (within mutual uncertainties of abundances and pressures) with our results. The isomer shift is proportional to the s-electron density at the nucleus (ρS), and the large difference observed between the two spin states agrees with the expectation that the radius of the Fe2+ ion decreases significantly across the spin transition.