  Ferropericlase, (Mg,Fe)O, is the second-most abundant mineral of the Earth's
lower mantle. With increasing pressure, the Fe ions in the material begin to
collapse from a magnetic to non-magnetic spin state. We present a
finite-temperature first-principles phase diagram of this spin crossover,
finding a broad pressure range with coexisting magnetic and non-magnetic ions
due to favorable enthalpy of mixing of the two. Furthermore, we find the
electrical conductivity of the mineral to reach semi-metallic values inside the
Earth.
