Distorted spinels, magnetite and maghemite

Tetrahedra can also be distorted by the presence of d and d cations to gain additional stabilisation energy. A c/a 1 distortion. Fig. 2.13, produces an additional splitting of the triplet and the doublet in such a way that the d, 2, 2 orbital becomes more stable, as well as the d orbital in the triplet. Conversely, a c/a I deformation leads to an additional stabilisation for d 2 in the doublet, and d and dy in the triplet. Fig. 2.14. 

A model for the transition from the low-temperature, distorted structure to the high-temperature, cubic structure has been proposed by Wojtowicz 

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). 

The Verwey transition as well as the order of the phase change, is strongly dependent on the O/metal ratio. An increase in O content shifted the transition temperature from 120 to 81 K, with a variation in peak width from 2 to 11 K (Kakol, 1990). For 0 x 0.0117 in the formula Fe3 04 (the equivalence between this notation and the d term used in the original papers is x = 3 5), a first-order transition was observed for 0.0117 X 0.1053, the transition showed a second- or higher-order character (Shepherd et ai, 1986). The value x = 0.0117 corresponded to the critical value at which one out of 32 ferrous cations (one in every two unit cells) was oxidised to a ferric ion. A similar trend and composition limits have been observed for Fej- Zn O.  

A simple model for the Verwey transition has been proposed (Honig, Spalek Gopalan, 1990) octahedral sites in magnetite were represented by a site pair, with a ground energy state (an electron trapped), a first excited state (the electron resonating between the two components of the site pair) and a second excited state (two electrons in the site pair). An important characteristic of this model was that the Verwey transition was driven by the coulomb repulsive interaction between electrons in the site pair. 

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