Transition metals electrical conductivity, spinels

FIGURE 4.12 Electrical conductivity of transition metal oxide spinels. 

Magnetite, Fe30>4, is a unique material. It is a mixed-valency compound (Fe " and Fe " on crystallographic sites of the same symmetry) with a low electrical conductivity below 120K, and a nearly metallic conductivity above this temperature. Between 120 and 770 K, magnetite is an inverse spinel, (Fe )[Fe Fe ]04, and all the transition phenomena occur in the octahedral sites. For T > 770 K, the redistribution of Fe on tetrahedral sites becomes non-negligible (Wu Mason, 1981). The reversible, sharp discontinuity in conductivity was first observed by Okamura (1931). Ferroelectric features have also been observed at low temperatures (Rado Ferrari, 1975 Kato et al, 1983). 

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