UCL Discovery

Abstract

Experimental measurements of magnesiowustite 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 O-2 = O-O(X)+ V-M(g)t(t) 2h(.) and Fe-F(e). =Fe-Fe(X)+ 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 similar to 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 (Delta V-h1 = -0.33 (19) cm(3) moi(-1); Delta V-ht = -0.26(69) cm(3) 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 magnesiowustite 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 magnesiowustite) and has a different temperature dependence from the low-temperature process.