| The recent discovery of spin transitions in iron under lower mantle conditions has raised many questions about the nature of the spin changes and their impact on lower mantle properties. A comprehensive understanding of the properties of the lower mantle is important in order to create more accurate models of heat convection, density, rheology, and phase stability in the Earth. The three phases known to exist in the lower mantle are perovskite, ferropericlase, and, just below the mantle (D"), post-perovskite. Results from ab initio calculations are presented on the zero-temperature iron high- to low-spin transition pressure in (Mg1-xFex)O ferropericlase and (Mg1-xFex)SiO3 perovskite. In perovskite, a dramatic decrease in transition pressure as iron concentration increases is predicted. These results indicate that perovskite with higher iron content will be low-spin well within the lower mantle, contrary to previous calculations. The spin crossover trend in pervoskite is opposite to that seen in ferropericlase, demonstrating that the energetics for spin crossover are highly dependent on the structural environment of iron. Additional calculations on the stable spin state and magnetic arrangement in Fe2O3 post-perovskite, the high-pressure end-member of (Mg1-xFex)SiO3 perovskite, predict a strong coupling between spin transition and both magnetic order and iron site occupation.;There is still significant uncertainty about the spin state of iron within perovskite and post-perovskite phases. Mossbauer spectroscopy, a powerful tool for measuring iron's spin, reveals that the stable spin state may be neither high-spin nor low-spin, but rather an intermediate-spin state. Ab initio methods are used to predict Mossbauer quadrupole splitting values of iron as a function of spin, pressure, valence, structural environment, and cation short-range order. These results are used to help interpret the implications for iron's spin state based on the experimentally observed quadrupole splittings. |
