Iron in the major lower mantle (LM) minerals undergoes a high spin (HS) to low spin (LS) transition at relevant pressures (23–135 GPa). Previous failures of standard first principles approaches to describe this phenomenon have hindered its investigation and the clarification of important consequences. Using a rotationally invariant formulation of $\mathrm{LDA}+U$ we report a successful study of this transition in low solute concentration magnesiowüstite, $({\mathrm{Mg}}_{1-x}{\mathrm{Fe}}_x)\mathrm{O}$, ($x<0.2$), the second most abundant LM phase. We show that the HS-LS transition goes through an insulating (semiconducting) intermediate mixed spins (MS) state without discontinuous changes in properties, as seen experimentally. We show that the HS state crosses over smoothly to the LS state passing through an insulating MS state where properties change continuously, as seen experimentally.

The postperovskite transition in MgSiO ₃ at conditions similar to those expected at the D″ discontinuity of Earth's lower mantle offers a paradigm for interpreting the properties of this region. Despite consistent experimental and theoretical predictions of this phase transformation, the complexity of the D″ region raises questions about its geophysical significance. Here we report the thermoelastic properties of Cmcm postperovskite at appropriate conditions and evidences of its presence in the lowermost mantle. These are ( i ) the jumps in shear and longitudinal velocities similar to those observed in certain places of the D″ discontinuity and ( ii ) the anticorrelation between shear and bulk velocity anomalies as detected within the D″ region. In addition, the increase in shear modulus across the phase transition provides a possible explanation for the reported discrepancy between the calculated shear modulus of postperovskite free aggregates and the seismological counterpart in the lowermost mantle.