Solid-state structures spinels

The solid state structure of magnetite, a spinel(2.), contains iron cations in two different oxidation states (Fe " and Fe ) and in two lattice sites of different coordination (octahedral and tetrahedral) therefore, the catalytic surface of this material may be expected to provide a variety of possible sites capable of acting as adsorption or reaction centers. Also, it has been demonstrated that substitution of other cations for iron can significantly alter the catalytic activity for WGS (4,5). 

Aluminium oxide occurs in two main forms a-alumina corundum) and Y-AI2O3 activated alumina). The solid state structure of 0t-Al2O3 consists of an hep array of ions with cations occupying two-thirds of the octahedral interstitial sites. ot-Alumina is extremely hard and is relatively unreactive (e.g. it is resistant to attack by acids). Its density (4.0gcm ) exceeds that of Y-AI2O3 (3.5gcm ) which has a defect spinel structure (see Box 13.6 and Section 21.10). The a-form is made by dehydrating... 

SPINEL STRUCTURES CONNECTING CRYSTAL FIELD EFFECTS WITH SOLID-STATE STRUCTURES... 

If you have already covered crystal field theory (CFT) (Chapter 4), you are now in a position to bring both CFT and solid-state structures together to better understand a class of compounds called the spinels. MgAl204 is representative of these compounds that are of general formula A"B2 O4. The oxide anions are found to be ccp (fee), and in a normal spinel, the A(II) cations occupy one-eighth of the tetrahedral holes, and the B(III) cations occupy one-half of the octahedral holes. A section of the lattice is shown in Figure 7.25. Be sure you can verify that this figure and the fractions of holes occupied result in the correct stoichiometry (one A for four oxides two B s for... 

The structures of spinels, A B2 4, are determined not only by the radius of the ions involved but also by the crystal field stabilization energies of the cations that occupy octahedral or tetrahedral holes in the cubic close-packed lattice of oxide ions. These structures offer an opportunity to combine a knowledge of crystal field theory obtained in earlier chapters with the knowledge of solid-state structures covered in this chapter. 

Chromates III). Mixed oxides, e.g. FeCr204, having spinel structures and prepared by solid state reactions. 

This section considers a number of extremely important structure types in which A1 combines with one or more other metals to form a mixed oxide phase. The most significant of these from both a theoretical and an industrial viewpoint are spinel (MgAl204) and related compounds, Na- -alumina (NaAlnOi ) and related phases, and tricalcium aluminate (Ca3Al20g) which is a major constituent of Portland cement. Each of these compounds raises points of fundamental importance in solid-state chemistry and each possesses properties of crucial significance to... 

The sensitive dependence of the electrical and magnetic properties of spinel-type compounds on composition, temperature, and detailed cation arrangement has proved a powerful incentive for the extensive study of these compounds in connection with the solid-state electronics industry. Perhaps the best-known examples are the ferrites, including the extraordinary compound magnetite Fc304 (p. 1080) which has an inverse spinel structure (Fe )t[Fe Fe ]o04. 

Tretyakov YuD, Gordeev IV, Kesler YaA (1977) Investigation of some chalcogenides with spinel structure. J Solid State Chem 20 345-358... 

Let us now turn to diffusion in the general case, without worrying about the exact mechanism or the rates of diffusion of the various species. As an example to illustrate how we would analyze a diffusion-limited solid state reaction, we use the general equation describing formation of a compound with spinel (cubic) structure and stoichiometry ... 

The spinel ferrites were fabricated by solid state reaction technique. Cobalt and Zinc ferrites CoxZnyFe204,(x=0.7,0.3,0.4,0.2 and y=0.3,0.7,0.6,0.8) were prepared by solid state reaction technique. The crystalline structure of the sample was investigated by X-ray diffraction(XRD). All samples show cubic spinel structure. The lattice parameter decreases with increasing cobalt content. Magnetic properties shows that the prepared sample exhibit ferromagnetic behaviour at room temperature. The saturation magnetization increases with increasing cobalt content. Curie temperature... 

Figure 1.41 The spinel crystal structure of MgAl204. Reprinted, by permission, from C. Kittel, Introduction to Solid State Physics, p. 447. Copyright 1957 by John Wiley Sons, Inc.

Aluminum forms mixed oxides with many other metals which either replace Al3+ in octahedral positions or occupy tetrahedral sites. The spinel structure of MgAl204, an important prototype, has been investigated by photoelectron spectroscopy.122 Vibrational and solid state NMR methods are also useful for studying aluminate structures.12 125... 

One of the difficulties with the classical solid-state reaction is that mechanical mixing methods are relatively ineffective in bringing the solid reactants in contact with one another. Diffusion lengths, on an atomic scale, are still enormous and the temperatures required may preclude the formation of phases that might be stable at intermediate temperatures. One method, called a precursor method, involves the formation of a mixed-metal salt of a volatile organic oxyanion such as oxalate by wet chemical methods, which result in mixing essentially on the atomic level. The salt is then ignited at relatively low temperatures to form the mixed-metal oxide. The method has been applied successffilly to the preparation of a number of ternary transition metal oxides with the spinel structure. ... 

This sub-set of structures was used to simulate the neutron diffraction pattern for comparison with experimental data. The tetragonal unit cell was also supported by transmission electron microscopy and solid-state NMR [74]. The idea of cations taking up non-spinel sites had also earlier been commented on by Wolverton and Hass for 5-AI2O3 since its ceU dimensions are often quoted as non-integer multiples of the y-phase. However, the debate is still on-going, with a recent comparison of XRD data with periodic DFT results rejecting the new structure in favor of the conventional disordered spinel [78]. 

Several attempts have been made to stabilize the layered LiMn02 phase by substitution of manganese ions with small amounts of chromium and aluminum by conventional solid state reaction and the introduction of cobalf and nickel on the manganese sites via the chimie douce reaction. Although these substituted layered compounds resulted in improvements in capacity retention in comparison to pure LiMn02, they have been found to be thermodynamically unstable with respect to their transformation to the less desired spinel structure. 

The theory of electronic excitation, which occurs during plastic deformation and fragmentation of crystals, is discussed by Molotskii [99]. The mechanochemistry of inorganic solids has been reviewed by Boldyrev et al. [100] with particular reference to the work done in The Institute of Solid State Chemistry at Novosibirsk. The consequences of mechanical activation on the structures and properties of selected spinels have been examined and the rate of the reaction forming barium tungstate investigated. Prospects for the future development of this subject are assessed. 

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