Wiistite defect structure

The defect structure of wiistite can be discussed from the view of a quasi-chemical equilibrium among defects, similar to the case of Nij O. Assuming that the predominant defects are iron ion vacancies, we obtain the following equations ... 

The defect structure of Fei O with the NaCl-type structure had been estimated to be a random distribution of iron vacancies. In 1960, Roth confirmed, by powder X-ray diffraction, that the defect structure of wiistite quenched from high temperatures consists of iron vacancies (Vp ) and interstitial iron (Fcj) (there are about half as many FCj as Vpe). This was a remarkable discovery in the sense that it showed that different types of crystal defects with comparable concentrations are able to exist simultaneously in a substance, Roth also proposed a structure model, named a Roth cluster, shown in Fig. 1.84. Later this model (defect complex = vacancy -F interstitial) was verified by X-ray diffraction on a single crystal and also by in-situ neutron diffraction experiments. Moreover, it has been shown that the defect complex arranges regularly and results in a kind of super-structure, the model structure of which (called a Koch-Cohen model) is shown in Fig. 1.85 together with the basic structures (a) and (b). 

FCj O or wiistite has probably the most studied defect structure. The hypothetical compound where x = 0 would crystallize with a perfect version of the halite structure where iron lies in an octahedral site surrounded by oxygen atoms. Removal of some of the iron should just give iron vacancies with the remaining irons remaining in an octahedral environment. 

Wiistite has a defect structure in which the Fe/0 ratio varies with and T. However in equilibrium with iron, the composition of pure wiistite (often written as FeO to indicate that the composition is not stoichiometric) is nearly constant at the composition Feo,950. Therefore that composition is the standard state for the activities reported by Hahn and Muan. 

Model of defect structure of wiistite. The defect structures of wiistite were well studied by many researchers and it is commonly considered that the basic unit of that is as shown in Fig. 3.6, and the defect cluster structure of that is as shown in Fig. 3.7. It will be seen from Fig. 3.7(a) that one of Fe + cation located in the interspace of tetrahedron is surrounded by four Fe + located in interstitial of octahedron. This structure is called as 4 1 defect clusters, 4 indicates the defect amount of cations and 1 indicates the interstitial amount of tetrahedron. The defect unit possesses five units with negative charges. It was proved for the existence of the mentioned defect structures by theoretical calculation in the works of Catlow, Grimes and Press. 

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. 

In transition metal compounds where the transition metal is stable in more than one oxidation state, by nonstoichiometry. For example, in CaO and FeO (both of which possess the rock salt structure) Schottky defects are thermally formed. Additionally, however, FeO (wiistite) exhibits mixed valence and, accordingly, has Fe2+ and Fe3+ ions and vacancies distributed over the cation sublattice. 

The method can be illustrated by reference to a classical study of the defects present in iron monoxide1. Iron monoxide, often known by its mineral name of wiistite, has the halite (NaCl) structure. In the normal halite structure, there are four metal and four non-metal atoms in the unit cell, and compounds with this structure have an ideal composition MX 0, (see Chapter 1, Section 1.8). Wiistite has an oxygen-rich composition compared to the ideal formula of FeOi.o- Data for an actual sample found an oxygen iron ratio of 1.059, a density of 5728 kg m 3, and a cubic lattice parameter, a, of 0.4301 nm. Because there is more oxygen present than iron, the real composition can be obtained by assuming either that there are extra oxygen atoms in the unit cell, as interstitials, or that there are iron vacancies present. 

It can be considered that the experimental value of diffusion coefficient is a direct proportion with defect concentration in wiistite (Fig. 3.4). There are certain amount of Fe + cations in the structure of wiistite, which not only ensured the electrical neutrality in lattice, but also offered the favorable conditions for Fe + cation to move along with the cavity and for electrons to transfer (Fe + —> O —> Fe +). As a consequence, the activation energy for diffusion of Fe in FeO is the lowest, only as small as 96kJ mol (Table 3.3), compared with that of other metal oxides. This is the structural factor for Fei xO to be reduced easily by H2. 

Catlow et researched on the relative stability of defect cluster in wiistite by HADES (Harwell Automatic Defect Evaluation System) and they proved first by theory that the crystal structure tends to be more stable if it is formed via isolated 4 1 defect clusters. The 6 2 and 8 3 defect clusters that are formed by common faces with those 4 1 defect clusters to be base units are the most stable ones. Catlow and Fender found by calculation that the stability of other types of defect clusters are far lower than that of 6 2 and 8 3 defect clusters. Therefore, they suggested that the 6 2 and 8 3 defect clusters are formed by common faces based on 4 1 defect cluster in wustite, and it is not possible for other types of defect clusters to be formed by common angles or common side. This has a discrepancy with the former results of experiments. 

But this structure is unstable and usually accompanied with the defects of metal ions. As a consequence, there would be certain amounts of Fe + cations to maintain the charge-neutrality. These Fe + cations often enter into the original unoccupied tetrahedral interspaces. For example, when the defect concentration of metal ions is X, the wiistite could be expressed as Fei j 0, where x < 1. And if x = 0.1, according to the principle of charge-neutrality, then ... 

Thereout, it is found that the catalyst has the highest activity among all the fused iron catalysts for ammonia synthesis when its chemical composition and crystal structure of the precmsor are those of wiistite (Fei xO)- It is called Fei xO or wiistite based ammonia sjmthesis catalysts, where the defect concentration x of iron ion is 0.04 experimental results break through the classical conclusion that lasted for more than 80 years, namely the catalyst has the best activity when its chemical composition and crystal structure of the precursor are most close to those of magnetite. It also provides a new approach for a novelcat-alytic system — wiistite Fei xO system for improving the performances of the fused iron catalysts. 

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. 

Nevertheless, in the structure of wiistite, for the reason of Fe + cations being defective, there still exist given amount of Fe + cations besides Fe + cations in order to preserve electric charge neutrality. These Fe + cations enter generally into the original empty interspace of tetrahedron. At the same time, if one of the vacancies of Fe + is formed in the lattice, then in order to maintain the electric charge neutrality, there must be two of Fe + transferring to be two of Fe +, one of which will enter into the tetrahedron interspace and leave behind an empty space in the octahedral interspace, while the another Fe + stays in octahedral interspace. For example, when X = 0.10 in Fei xO, then the structural formula can be written as ... 

In wiistite structure, a large number of defects are iron ions, which allow the diffusion of iron into the lattice, and it is beneficial for the transfer of electrons. This is the structural reason that why Fei xO-based catalyst is easy to be reduced. 

The reduction performance of catalyst is closely related with the composition of its precursor in hydrogen flow. As mentioned earlier, this is due to the different reduction mechanisms for catalysts with different precursors. All precursors of iron oxide such as Fe304, Fei xO and their mixture are possible for fused iron catalysts, while the sequence of the reduction rate as well as the reduction temperature is Fei xO > Fe304 > mixture. Apparently, the catalysts with non-stoichiometric Fei xO with wiistite structure as precursors have the fastest reduction rate and the lowest reduction temperature. As mentioned before, the defects of iron ions in lattice of Fei xO has serious impact on its reduction properties. It can be seen from Fig. 5.13 that the reduction process is faster and more complete when the amount of the defects is larger in wiistite. 

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... 

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... 

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