The Spinel Lattice

Ni-Co ferrites with the general formula Nii cCoxFe204 were tested for the methylation of pyridine [110]. It was observed that the systems possessing x values >0.5 are selective for 3-picoline formation, whereas the ones with x values 0 and 0.2 give a mixture of 2- and 3-picolines. Pyridine conversion increased with the progressive substitution of Ni ions by Co ions. The cation distribution in the spinel lattice influences their acidic and basic properties, and these factors have been considered as helpful to evaluate the activity of the systems. 

The inverse spinel structure differs in that one type of cation occupies the tetrahedral and half of the octahedral sites of the spinel lattice, and the other cations occupy the remaining octahedral sites. This is indicated by writing the formula of, for example, zinc titanium spinel as Zn(TiZn)04. Spinels containing di- and tetravalent cations are mostly of the inverse type. Normal and inverse spinels should be regarded as idealized limiting structures, intermediate forms are often observed in practice. A... 

With these energies, it is possible to calculate not only the preferred cation distribution over two types of available anion interstice, as in the spinel lattice, but also the valency distribution given two atoms on the same site, each with multiple-valence alternatives. This point... 

MgAl204 has been reported by Petermann et al. (50). Several types of excited state absorption transitions have been observed and can be understood in terms of a configurational coordinate mode. As a consequence the Mn ion cannot lase in the spinel lattice. 

In the proposed two-step process, it is important to attain high efficiencies for conversion of coal to COx (CO -I- CO2) in the first-step reaction and then for conversion of CO2 to CO in the second-step reaction by an external heat input. From the thermodynamic conditions and the low cost, the redox pair of Fe304/a-Fe was one of the promising redox systems for the two-step process, but it still required the operating temperature above 1200°C[2]. It is well known that many kinds of metal ions can be incorporated into the spinel lattice structure of magnetite by replacing ferrous or ferric ions. There is the possibility that metal-substitution for Fe or Fe " in magnetite causes a phase transition to the metallic phase, which proceeds readily even at low temperatures and improves the conversion efficiencies of coal and CO2 to CO in the two-step process. 

Binding energies of the different metals contained in the catalyst after calcination at 773 or 873 K results are presented in Table 1. No fundamental differences were found at either temperature. The values of B.E. obtained for Ni and Co are those typically reported for Ni0-t-NiAl204 and well-defined C0AI2O4 structmes, respectively [7,8]. Nevertheless, the results shown in Table 2 (first and fourth rows) clearly indicate that an increase in the calcination temperature preferentially favors the incorporation of Co cations into the spinel lattice, increasing the relative amount of nickel in the catalyst surface. This can be related to the greater tendency of cobalt to form a well-defined bulk spinel phase [5]. 

The spinel lattice is not geometrically simple but can be considered in terms of a cubic close-packed array of ions with one-eighth of the tetrahedral holes occupied by A ions and half of the octahedral holes occupied by B ions. The unit cell contains eight formula units, i.e. [AB2X4]g. 

Some mixed metal oxides AB2X4 in which at least one of the metals is a block element (e.g. CoFe204) possess an inverse spinel structure which is derived from the spinel lattice by exchanging the sites of the A ions with half of the B ions. 

Despite the linear relation between the composition of the feedstock and that of the deposited flhns, it can be noted that the concentration of nickel doping in the deposited films was lower than that of the liquid feedstock. This effect might be justified either by a selective depletion of the nickel precursor during the transport or by its lower sticking coefficient on the surface of the substrate at the deposition temperature. The IR analyses of the deposited NixCo3.x04 (0 < x < 1) films, not shown, revealed their spinel structure, and a shift of the characteristic vibration bands indicated the incorporation of nickel into the spinel lattice. Further evidence about the controlled incorporation of nickel into the lattice of cobalt oxide can be drawn from conductivity measurements. Fig. 5b shows a substantial increase in conductivity as a function of the concentration of nickel incorporated into the film. The formation of a secondary phase should be expressed by a discontinuity, which is clearly absent in the investigated range of conditions. 

Blasse (1964) listed close to 200 spinels having either a "normal" or "inverted spinel structure. What this means is that the cations normally occupying the "A site would occupy the "B" site would be exchanged, depending upon the ionic radius of the two cations. Thus, if we could make Mg smaller in radius, and Al were made larger, we would have Al MgO as an inverted spinel. In normal spinels, the divalent cations occupy tetrahedral sites while the trivalent cations occupy the octahedral sites. The Inverted state depends upon which cations are involved and their relative ionic size. Thus, we have two cation "sub-lattices" in the spinel lattice, the tetrahedrcd or A-sublattlce and the octahedral B-sublattice. 

Thus, we write the solid state diffusion reaction for spinel as follows in 3.1.86. on the next page. In this case, Frenkel pair diffusion predominates and is faster than any other possible mechanism in the spinel lattice. 

Of all solid state reactions, the formation of oxide spinels is at present the most thoroughly investigated [4, 5, 33]. The first reason for this is the relatively simple crystallographic structure of the spinel lattice. Essentially, this consists of a nearly close-packed face-centred-cubic sublattice of oxygen ions. The tetrahedral and octrahedral interstices of this sublattice are filled in a certain way by the cations. The second reason is that spinels are technically very interesting substances, and one would like to be able to find optimal methods for their preparation. For instance, ferrites are used as control or circuit elements in the electronics industry, and chromite brick is used as cladding in ovens which are used for the production of steel. Therefore, the formation of spinels will now be discussed in detail as a model of a classical solid state reaction. 

Fe, Co, Ni. This provides a range of colours in which the excellent durability characteristics of the host crystal structures are retained. An important commercial example of a mixed oxide pigment based on the spinel lattice is cobalt aluminate blue (Cl Pigment Blue 28), which is usually represented as C0AI2O4,... 

Sometimes, an introduction of excess lithium is done in Uthium manganese spinels (typically, less or equal to 6 atomic % excess hthium). The excess lithium improves the cathode s stabihty in the charged state and the cycling stability of the material. The presence of extra lithium in the spinel lattice also pins lithium ions into their sites in the crystal lattice and prevents the potential migration of mobile lithium ions into manganese lattice sites. 

Figure 7.4 shows a simplified scheme of the distribution of cations between the two types of interstices of the spinel lattice. A1 ions occupy both tetrahedral (squares) and octahedral (circles) interstices. However, for reason of charge balance of the 16 available octahedral sites, only 13 are filled, leaving 2 vacancies per 32 oxygen ions in the spinel unit ceU. 

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