Effects of the Spin Transition of Iron in Magnesiow\"{u}stite-(Mg,Fe)O: Applications to the Earth's Lower Mantle*

Magnesiow\"{u}stite [(Mg,Fe)O] is the second most abundant mineral in the Earth's lower mantle. Here I will discuss the spin states of iron in magnesiow\"{u}stite and the isolated effects of the electronic transitions on the elastic, thermodynamic, magnetic, and vibrational properties of magnesiow\"{u}stite under high pressures and high temperatures. Pressure-induced electronic spin transitions of iron from high-spin to low-spin states have been recently observed to occur in magnesiow\"{u}stite under high pressures using high-pressure X-ray emission spectroscopy and synchrotron M\"{o}ssbauer spectroscopy. Based on the synchrotron M\"{o}ssbauer studies of (Mg$_{0.75}$,Fe$_{0.25})$O, the simultaneous disappearance of the quadrupole splitting and the drop of the isomer shift at above 62 GPa are consistent with a high-spin to low-spin electronic transition of iron in the sample between 62 and 70 GPa. Addition of FeO in MgO stabilizes the high-spin state to higher pressures and the high-spin to low-spin transition of iron in magnesiow\"{u}stite results in an abnormal compressional behaviour between the high-spin and the low-spin states$^{1}$. Moreover, there are also significant changes in particular physical properties of magnesiow\"{u}stite such as force constant across the electronic spin transition. Here I have combined results from a variety of high-pressure techniques to understand the effects of the electronic transition on the physical properties of magnesiow\"{u}stite and to explore possible geophysical consequences of the transition in the Earth's lower mantle. $^{1}$J. F. Lin, V. V. Struzhkin, S. D. Jacobsen, M. Hu, P. Chow, J. Kung, H. Liu, H. K. Mao, and R. J. Hemley, Spin transition of iron in magnesiow\"{u}stite in Earth's lower mantle, \textit{Nature,} 436, 377-380, 2005.