The electrical conductivity of the lower mantle phase magnesiowüstite at high temperatures and pressures

Experimental measurements of magnesiowüstite electrical conductivity at Fe/Fe + Mg 0.05 to 0.2 and Fe3+/Fe3+ + Fe2+ 0.01 to 0.7 at high pressure and high‐temperature are presented. Below 1000 K, conduction occurs by a small‐polaron process of electron hopping between ferric and ferrous sites, but above 1000 K there is a change in mechanism. This high‐temperature mechanism is postulated to be a large‐polaron process in which holes are promoted in the oxygen valence band via the reactions: 1/2 O2 = OOx + VMg″ + 2h• and FeFe• = FeFex + h•. The hole and its associated polarization field are free to move in the valence band until trapped by a ferrous ion. Activation energies for the low‐temperature, small‐polaron regime are ∼0.3 eV across the range of Fe/Fe + Mg and Fe3+/Fe3+ + Fe2+ studied, in agreement with previous studies. The high‐temperature, large‐polaron activation energy decreases with increasing Fe/Fe+Mg and decreasing Fe3+/Fe3+ + Fe2+, ranging, from 0.4 to 1.1 eV. Both regions show a small, negative activation volume (ΔVlt = −0.33 (19) cm3 mol−1; ΔVht = −0.26(69) cm3 mol−1), consistent with previous high pressure studies of electronic conduction mechanisms. A compilation of the available data shows a discrepancy between measurements at low and high‐temperatures, consistent with the new results presented here. At the temperature of the lower mantle, the dominant conduction mechanism in magnesiowüstite will be the more mobile large‐polaron process. This is less sensitive to iron content than small‐polaron conduction at Fe/Fe+Mg < 0.17 (the likely compositional range of lower mantle magnesiowüstite) and has a different temperature dependence from the low‐temperature process.