Nonstoichiometric wiistite

The density of wiistite has been determined fairly precisely, and for Feo9470 is equal to 5.747 g/cm (Robie et al., 1967), whence F= 12.04 cm /mol. 

Kurepin (1975) calculated that the values of AG - are positive, although very small (from +75 cal at 700°C to +105 cal at 1300°C), and at high temperatures can be expressed by the equation  

Combining this equation with the equation AG = f(T) of the reaction of formation of FeO, we obtain 

For lower temperatures a nonlinear extrapolation from 1000°K (wiistite of the composition Fco 9470) to 298°K likewise is made. 

Subsequent calculation of the high-temperature equilibrium of magnetite with wiistite, taking into account the values of the activity of the components of the solid solutions and the nonstoichiometry of the phases (Kurepin, 1975), made it possible to obtain the following equilibrium  

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A now classic example of defect clustering is the proposed defect stmcture of wiistite Fei xO. Fully stoichiometric FeO would be predicted to adopt the rock salt stmcture with iron fully occupying the octahedral sites. Nonstoichiometric wiistite contains a significant amount of Fe +, some of which has been shown by neutron and X-ray diffraction studies to occupy tetrahedral sites. Since occupation of the tetrahedral site would generate short intercation contacts between Fe + in the tetrahedral sites and iron in the octahedral sites, it is unlikely that they would be simultaneously occupied in any one part of the stmcture. The so-called Koch clusteT results when alternate tetrahedral sites are occupied in the fee cell and all the surrounding octahedral sites are vacant (Figure 3.37), i.e., 4 Fe + interstitials and... 

Actually, the defect structure in some wiistite compositions is even more complicated. In Feo 9O the vacancies and Fe ions cluster together in a specific arrangement, to be described later, giving a volumetric defect. The clusters themselves at low temperatures are spaced in a periodic array to form a supercell. In addition, within the vacancy clusters in highly nonstoichiometric wiistite some cations occupy tetrahedral sites rather than octahedrally coordinated sites as in the perfect rocksalt structure. It is probably mostly Fe ions which occupy tetrahedral sites the Fe ions are smaller than the Fe ions and the tetrahedral sites are smaller than the octahedral sites in wiistite. 

The mechanism of the scaling of iron is so complex as to require special mention. Above 570 °C, wiistite (Fei xO) is thermodynamically stable and forms the relatively thick basal layer in the oxide film. This is followed by a magnetite (FesCU) layer which is followed by a final layer of Fe2C>3. Magnetite itself tends to become nonstoichiometric under oxidizing conditions, with excess Fe3+, so that its composition and color can vary from Fe3.oooC>4 (black) toward cubic Fe2.667 04 (i.e., 7-Fe203, chocolate brown). Thus, as outlined in Section 4.6, the oxidation of iron above 570 °C involves mainly... 

In some cases the crystal defects involve impurities that lead to nonstoi-chiometric compounds. For example, wiistite, which has the idealized formula FeO, actually varies from FeL0Oi.0 to Feo.yjCVo. These variations occur when some of the Fe2+ ions in the compound are replaced by FeJ+ ions in a way that preserves the electrical neutrality of the solid. There are many other examples of nonstoichiometric compounds. One extreme case involves TiO, which can vary from Tit 0Oo.7 to Tio.sOi.o-... 

The law of definite proportions was a crucial step in the development of modern chemistry, and by 1808, Proust s conclusions had become widely accepted. We now recognize that this law is not strictly true in all cases. Although all gaseous compounds obey Proust s law, certain solids exist with a small range of compositions and are called nonstoichiometric compounds. An example is wiistite, which has the nominal chemical formula FeO (with 77.73% iron by mass), but the composition of which, in fact, ranges continuously from Feo.950 (with 76.8% iron) down to Fco.siO (74.8% iron), depending on the method of preparation. Such compounds are called berthollides, in honor of Berthollet. We now know, on the atomic level, why they are nonstoichiometric (see the discussion in Section 21.6). 

Nowadays, six different crystalline forms of iron oxide are recognized (see Fig. 18.1) [2,10]. In iron oxides, iron can be in only trivalent (Fe ) state, only divalent (Fe ) state, or in both divalent and trivalent state. FeO (a black material mineralogically known as wiistite) contains only divalent iron and is very frequently nonstoichiometric with oxygen... 

Among all catalysts with the iron oxides and their mixtures as precursor studied, Fei xO based catalyst with nonstoichiometric and wiistite structure has the fastest reduction rate and lowest reduction temperature. In a wiistite structure, large amounts of defects are iron ions, which enable the diffusion of Fe in oxide lattices, and will be preferable to electron transferences. This is the structural factor for the easy reduction of Fei xO based catalysts. 

In the case of large deviations from stoichiometry, simple associates or more extended defect clusters can be formed. One example is the Koch-Cohen defect cluster in nonstoichi-ometric wiistite (FeO). This defect cluster bears a strong resemblance to the structure of Fc304. One can think of nonstoichiometric FeO as fragments of FCjOj intergrown in the rock salt stmcture of FeO. Another well-known cluster in the oxide-interstitial defect cluster is... 

In this nonstoichiometric Fe(i t)0 material, conduction is electronic and, in fact, it behaves as a p-type semiconductor—that is, the Fe ions act as electron acceptors, and it is relatively easy to excite an electron from the valence band into an Fe " acceptor state with the formation of a hole. Determine the electrical conductivity of a specimen of wiistite with a hole mobility of 1.0 X 10 m A -s, and for which the value of x is 0.040. Assume that the acceptor states are saturated (i.e., one hole exists for every Fe ion). Wiistite has the sodium chloride crystal structure with a unit cell edge length of 0.437 nm. 

One of the first nonstoichiometric compounds where the defects associated with the nonstoichiometry were identified is wiistite. The nominal composition of wiistite is FeO and the structure is that of rocksalt. Wiistite exists in equilibrium only above 570°C. It exists at 1350°C over the composition range Fe 95O to Feo.840 The stoichiometric composition FeO, which would contain only divalent cations, Fe, is not stable at any temperature at atmospheric pressure. Quenching from above 570° retains wiistite at low temperatures as a... 

While the structure of wiistite will be fully described and discussed later in this chapter, it has been briefly mentioned in the introduction to indicate the variety of imperfections which may be expected in a highly nonstoichiometric compound. 

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