Spin transition and equations of state of (Mg, Fe)O solid solutions

We have performed a series of experiments to investigate the compositional effect on the compression behavior of (Mg, Fe)O solid solutions at high pressure. The in-situ synchrotron X-ray diffraction data revealed abnormal volume contractions at about 40, 60, and 80 GPa for (Mg-0.80, Fe-0.20)O, (Mg-0.61, Fe-0.39)O, and (Mg-0.42, Fe-0.58)O, respectively. The volume contractions are associated with the reported electronic transition of high-spin to low-spin in Fe2+, and caused by the reduction of Fe2+ ionic radius across the transition. A least-squares fit of the compression data to the Birch-Murnaghan equation of state yielded bulk modulus K-0 (GPa) = 160 - 10X(FeO) for the high-spin (Mg, Fe)O and K-0 = 170( 3) GPa for the low-spin (Mg, Fe)O. The equations of state of (Mg, Fe)O established in this study are directly applicable to the Earth's lower mantle in composition and pressure ranges and provide essential data for modeling the density profile of the lower mantle.