Verwey transition

Subsequently the transition has been called the Verwey transition and the transition temperature the Verwey temperature Ty. Verwey also guessed that, below Ty, the mobile electrons order as Fe on [110] rows and Fe on [llO] rows of B-site cations to produce a distortion to orthorhombic symmetry with lattice parameters approximately y/2 + 6)ao X (V2 - 6)ao x ao, where 5 is a small fraction and ao is the cubic lattice parameter. Although Bickford was able to confirm that Fc304 is magnetically orthorhombic at temperatures T < Ty, it is now known that the low-temperature structure is in fact monoclinic with lattice parameters V2ao x y/lz x 2ao and that the electronic ordering is more complex than originally proposed by Verwey. 

In contrast to Fe3, 04 and Fe304 jF the Verwey transition disappears even for small concentrations of zinc in the system ZniFe3 x04 l. This observation indicates that... 

FbjO has the inverse spinel structure, with all the Fe " ions and half of the Fe " ions located in octahedral sites (B sites) in the oxygen network and the remaining half of the Fe ions located on tetrahedral sites (A sites). It undergoes a ferrimagnetic-paramagnetic transition around 850 K and another transition around Ty = 123 K (Verwey transition). The material is a semiconductor both above and below the Verwey... 

A recent review of the experimental situation has been given by Honig(1985). It is pointed out that the electrical properties, particularly near to the transition, are very sensitive to purity and specimen preparation, and that much of the extensive experimental work is therefore open to doubt. None the less, the broad features of the behaviour of this material are clear. The history of the so-called Verwey transition in this material goes back to 1926, when Parks and Kelly (1926) detected an anomalous peak near 120 K in the heat capacity of a natural crystal of magnetite. The first detailed investigations were those of Verwey and co-workers (Verwey 1939, Verwey and Haayman 1941, Verwey et al. 1947), who showed that there was a near discontinuity in the conductivity at about 160K. The conductivity as measured by Miles et al. (1957) is shown in Fig. 8.1. 

The assumption that the carriers are small or intermediate polarons in no way militates against discussions of ths band structure of the ground state (see e.g. Camphausen et al. 1972, Cullen and Callen 1971,1973). The absence of Jahn-Teller distortion (Goodenough 1971) also, in our view, indicates not the absence of a polaron mass-enhancement but rather a value.of V0jB not too far from the critical value. These conclusions seem to be in agreement with the considerations of Sokoloff (1972), who used a description in terms of a degenerate band of small polarons. Samara (1968) showed that pressure lowers the temperature of the Verwey transition. If this depended only on e2/ ca then the opposite should be the case. But pressure will increase B, and push the substance nearer to the critical value for the metal-insulator transition. 

Fig. 8.2 Conductivity of Fe304 xFx for various values of x as a function of reciprocal temperature (Whall et al 1978) (1) x=0.025 (2) 0.05 (3) 0.1 (4) 0.15. It will be seen that the Verwey transition disappears somewhere between x = 0.025 and 0.05.

The disappearance of the sharp Verwey transition was discussed by Mott (1979), who suggested that at low temperatures the material is a Wigner glass , the electrons (Fe2 + ions) being frozen into random sites and the whole system stabilized by the fluorine. Discussion of the thermopower measurements show, according to Mott (1979), that a hopping mechanism is operative at low T. Ihle and Lorenz (1985), however, consider that the electrons in the wrong sites move by a small polaron band mechanism. 

In this material also the sharp Verwey transition disappears with traces of impurity—in (Ti4 XVX)404 when x > 0.01 (Schlenker et al 1976, Gourmala et al 1978, Ahmed et al 1978). Below 90 K the conductivity then behaves like exP [—(T0/T)1/4]. The above authors believe that the conduction process giving these low activation energies is the hopping of an electron from one V-Ti3 + pair to a V-Ti4+ pair, there being some compensation to make this possible. 

Mott (1979, p. 357) discusses the amount of impurity needed to make the sharp Verwey transition disappear. The small value, x 0.01, for Ti407 is difficult to explain. 

Verble, J. L. (1974) Temperature-dependent lightscattering studies of the Verwey transition and electron disorder in magnetite. Phys. Rev. (B), 9,5236—48. 

The Verwey transition in Fe304 is associated with a marked jump in conductivity, but the material remains a semiconductor both above and below the transition temperature (123 K) below 123 K, there is... 

Figure I- Verwey transition in Fe304 caused by charge ordering (from Honig,b).

Figure 39 Change in the magnetic structure for TbBaFe205 at the charge ordering or Verwey transition, (a) Mixed valence state, (b) Charge ordered state. (Reprinted with permission from P. Karen, P.M. Woodward, J. Linden, T. Vogt, A. Studer, P. Fischer. Phys. Rev., 2001, B64, 214405. 2001 by the American Physical Society)...

The application of Mossbauer studies to magnetic materials is well illustrated by the spectra of magnetite (Fe304) shown in Fig. 2.48. Although this ferrimagnetically ordered material is an inverse spinel with nominally tetrahedral Fe + and both Fe and Fe on octahedral sites, at temperatures above 119 K (the Verwey transition temperature) only... 

Fig. 2.48. The Fe Mossbauer spectra of magnetite (Fe304) at temperatures (a) above and (b) below the Verwey transition (after Sawatzky et al., 1969).

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