Fe in Perovskite: Spin Transitions and Physical Properties

We show the existence of a large complexity of the spin behavior associated with the spin transition in FeSiO3 perovskite at Earth's lower mantle conditions from state-of-the-art first-principles calculations. We analyze the crystallochemical effects induced by iron and we monitor the compressibility as a function of iron spin. We find that at low pressures the high-spin antiferromagnetic structure is the most stable one. At higher pressures, the high-spin to low- spin transition occurs in perovskite in association with a small structural distortion on the Fe site. This distortion can be traced down from phonon analysis and is responsible for the reduction of the coordination volume that favors a smaller volume iron. There is no obvious change in the structural compressibility during the spin transition. We also find a structure with intermediate-spin state energetically favorable and competitive to the anti-ferromagnetic structure. Temperature, structural distortions, larger superstructures and Mg dilution can raise the transition pressure to the low-spin state such as a sequence of spin transitions: antiferromagnetic high spin => ferromagnetic intermediate spin => paramagnetic is likely to occur in natural thermodynamical and thermochemical conditions. Next we derive the seismic properties of FeSiO3 perovskite as a function of Fe2+ spin state and observe that the effects of the spin transition are rather small. For pyrolitic compositions the change in seismic wave velocities due to the spin transition is on the order of 0.2%, invisible in seismology. Nevertheless other properties, like transport and chemical partitioning, might be highly affected by the spin transition and their implications on mantle discontinuities will be discussed.