The material of the deep earth is inaccessible to direct materials characterization and under such high pressures and temperatures that even laboratory experiments have difficulty reproducing realistic conditions. Simulations can easily explore extreme conditions and provide a new avenue to understand the materials properties of the deep earth. Our work focuses on the spin state of Fe in the lower mantle (600km-2900km), where Fe is the primarily impurity in the MgO and MgSiO3 phases making up the bulk of this region. Fe spin changes with pressure and temperature can impact critical properties from density to elastic response to chemical heterogeneity. We have established properties of Fe spin and its coupling to composition[22-24] and the spin affects on Fe diffusion[25, 26]. We are also establishing how Fe spin couples to the Fe valence, partitioning between phases, and phase stability of the deep earth.
[22] S. H. Shim, A. Bengtson, D. Morgan, W. G. Sturhahn, K. Cataiii, J. Y. Zhao, M. Lerche, and V. Prakapenka, Electronic and magnetic structures of the postperovskite-type Fe2O3 and implications for planetary magnetic records and deep interiors, Proceedings of the National Academy of Sciences of the United States of America 106, p. 5508-5512 (2009).
[23] A. Bengtson, K. Persson, and D. Morgan, Ab initio study of the composition dependence of the pressure-induced spin crossover in perovskite (Mg1-xFex)SiO3, Earth and Planetary Science Letters 265, p. 535-545 (2008).
[24] K. Persson, A. Bengtson, G. Ceder, and D. Morgan, Ab initio study of the composition dependence of the pressure-induced spin transition in the (Mg1-x,Fe-x)O system, Geophys. Res. Lett. 33 (2006).
[25] S. Saha, A. Bengtson, and D. Morgan, Effect of anomalous compressibility on Fe diffusion in ferropericlase throughout the spin crossover in the lower mantle, Earth and Planetary Science Letters 362, p. 1-5 (2013).
[26] D. Morgan, S. Saha, A. Bengtson, K. L. Crispin, and J. A. Van Orman, Effects of spin transition on iron diffusion in ferropericlase, Abstracts of Papers of the American Chemical Society 243 (2012).
Fig.8: First-principles based prediction of (a) migration energy and (b) diffusion of dilute Fe in lower mantle (Mg,Fe)O as a function of spin state (LS = low spin, MS = mixed spin, HS = high spin). From Ref. S. Saha, A. Bengtson, K. L. Crispin, J. A. Van Orman, and D. Morgan, Effects of spin transition on diffusion of Fe(2+) in ferropericlase in Earth’s lower mantle, Physical Review B 84, p. 184102 (2011).