PROBE THE ELECTRONIC SPIN STATE OF IRON IN LOWER MANTLE PEROVSKITE THROUGH A COMBINATION OF X-RAY EMISSION SPECTROSCOPY AND SYNCHROTRON MÖSSBAUER SPECTROSCOPY TO MEGABAR PRESSURES

Recent experimental studies discovered pressure-induced changes in the spin state of iron in aluminum (Al)-bearing (Li et al. 2004) magnesium silicate perovskite (hereafter called “perovskite”) and in its Al-free counterpart (Badro et al. 2003; Jackson et al. 2005). Two different techniques, x-ray emission spectroscopy (XES) and synchrotron Mössbauer spectroscopy (SMS), were used by different groups to detect spin state changes under pressure. In this study, we apply both techniques to the same sample in order to derive additional information on the pressure induced spin state changes in Al-bearing perovskite.

XES spectra of an Al-bearing perovskite (Mg_0.87Fe_0.09)(Si_0.94Al_0.10)O₃ from 21 to 100 GPa suggest that iron experiences a gradual loss of magnetic moment with increasing pressure (Li et al. 2004). At 100 GPa, the residual magnetic moment is about 60% of the initial value at 1 bar. According to conventional Mössbauer data, our sample contains 53% Fe³⁺ and 47% Fe²⁺, each taking one crystallographic site. Assuming that valence states and sites do not change with pressure, a number of scenarios can account for the XES observations, including 1) Fe²⁺ changes into the low-spin state while Fe³⁺ stays in the high-spin state, 2) both Fe²⁺ and Fe³⁺ change into intermediate-spin states, and 3) Fe²⁺ stays in the high-spin state while Fe³⁺ changes into the low-spin state. From XES data alone, these alternative scenarios cannot be readily resolved.

Our new SMS data are in excellent agreement with the results of conventional Mössbauer measurements at ambient conditions. The high pressure SMS results show that the difference in the isomer shifts of Fe²⁺ and Fe³⁺ decreases with increasing pressure. Considering the expected changes in IS associated with spin changes, scenario 3) can be excluded. The likely scenarios that can explain both the XES and SMS data are that Fe²⁺ changes from the high-spin state to the low-spin state, or that both Fe²⁺ and Fe³⁺ change into intermediate-spin states by 100 GPa. Thus, the combination of these two complimentary techniques provide new insights into the spin state of iron in lower mantle perovskite.