Electrical conductivity of the lower‐mantle ferropericlase across the electronic spin transition

Electrical conductivity of the lower‐mantle ferropericlase‐(Mg0.75,Fe0.25)O has been studied using designer diamond anvils to pressures over one megabar and temperatures up to 500 K. The electrical conductivity of (Mg0.75,Fe0.25)O gradually rises by an order of magnitude up to 50 GPa but decreases by a factor of approximately three between 50 to 70 GPa. This decrease in the electrical conductivity is attributed to the isosymmetric high‐spin to low‐spin transition of iron in ferropericlase. That is, the electronic spin transition of iron results in a decrease in the mobility and/or density of the charge transfer carriers in the low‐spin ferropericlase. The activation energy of the low‐spin ferropericlase is 0.27 eV at 101 GPa, consistent with the small polaron conduction (electronic hopping, charge transfer). Our results indicate that low‐spin ferropericlase exhibits lower electrical conductivity than high‐spin ferropericlase, which needs to be considered in future geomagnetic models for the lower mantle.