Spinels, classes

Compounds that have the empirical formulas MCr02 and DCr204 where M is a monovalent and D a divalent cation, are known as chromites. These are actually mixed oxides and probably are better written as M20-Cr203 and D0-Cr203, respectively. The oxides of D are largely spinels, ie, the oxygen atoms define a close-packed cubic array having the octahedral holes occupied by the Cr(III) cation and the tetrahedral holes occupied by D (54). Chromite ore is an important member of this class of oxides. 

Soluble pigments The most important pigments in this class are the metallic chromates, which range in solubilities from 17 0 to 0-00005 g/1 CrO . An examination has recently been carried out of the mechanism of inhibition by chromate ions and it has been shown by chemical analysis of the stripped film, Mdssbauer spectroscopy and electron microprobe analysis that the air-formed film is reinforced with a more protective material in the form of a chromium-containing spinel (Chapter 17). The situation is, however, complicated by the possibility that some chromates, particularly the basic ones, may inhibit through the formation of soaps. There is evidence that lead chromate can function in this way. 

The number of oxide type minerals is quite large. Rostov (1956) has identified 160 specific minerals, grouped them into classes (chrysoberyl, spinel, corundum, periclase, etc.), and proposed a classification system. Only a few examples will be discussed here. 

Accompanying the commercialization of lithium ion technology, the emergence of 4.0 V class cathode materials based on spinel, LiCo02, and LiNi02 presents a more stringent requirement for the selection... 

Aromatic N-containing heterocycles (pyridine, imidazole etc.,) and their alkyl derivatives represent an important group of products and they have received considerable attention because of their various applications. For instance, methyl pyridines (picolines) and dimethyl pyridines (lutidines) are a class of industrially valuable compounds for the production of dyes and fine chemicals [108]. Sreekumar et al [109] reported pyridine methylation to 3-picolines over Zni xCoxFe204 spinel systems at reaction temperature between 325 and 425°C. The... 

The second group consists of those structures that have the same number of parameters as independent bond lengths. In this case the structural parameters are determined by requiring the bond lengths to have their normal values. For oxides this can be done by using appropriate tables of bond lengths, such as those of Shannon and Prewitt. Conspicuous and important examples of this second class are the spinel and garnet structures. 

Few oxide superconductors were known prior to 1985 and we shall now return to these so that we can discuss these materials in reference to their crystal structure classes. There are only three broad structural categories in which most of the oxide superconductors occur. The important structural types include sodium chloride (rocksalt, or Bl-type), perovskite (E2X), and spinel (Hlx). 

The name ferrite was originally given to a class of mixed oxides having an inverse spinel structure and the formula AFe204 where A is a divalent metal ion. The term has been extended to include other oxides, not necessarily containing iron, which have similar... 

Five aspects of the preparation of solids can be distinguished (i) preparation of a series of compounds in order to investigate a specific property, as exemplified by a series of perovskite oxides to examine their electrical properties or by a series of spinel ferrites to screen their magnetic properties (ii) preparation of unknown members of a structurally related class of solids to extend (or extrapolate) structure-property relations, as exemplified by the synthesis of layered chalcogenides and their intercalates or derivatives of TTF-TCNQ to study their superconductivity (iii) synthesis of a new class of compounds (e.g. sialons, (Si, Al)3(0, N)4, or doped polyacetylenes), with novel structural properties (iv) preparation of known solids of prescribed specifications (crystallinity, shape, purity, etc.) as in the case of crystals of Si, III-V compounds and... 

Fig. 37. Cubic system. (See also Figs. 16-22.) a. Unit cell type, b and c. Two habits of pyrites, FeSs. Class m3, d. Tetrahedrite, Cu3SbSa. Class 43m. e. Spinel, MgAls04. Class m3m. /. Almandine (Garnet), Fe8Al2(Si04)3. Class mZm.

Spinel oxides with a general formula AB2O4 (i.e. the so-called normal spinels) are important materials in industrial catalysis. They are thermally stable and maintain enhanced and sustained activities for a variety of industrially important reactions including decomposition of nitrous oxide [1], oxidation and dehydrogenation of hydrocarbons [2], low temperature methanol synthesis [3], oxidation of carbon monoxide and hydrocarbon [4], and oxidative dehydrogenation of butanes [5]. A major problem in the applications of this class of compound as catalyst, however, lies in their usually low specific surface area [6]. 

Spinel. Colorless (pure), blue (Co), and olher colored synthelic spinels made by the Vemeuil process are widely seen in class rings and in other jewelry uses, where the blue is often mislabeled as synthetic sapphire. 

The spinels are a class of double oxide of general formula AB204 industrially important members of this class include aluminates (e.g. MgAl204), ferrites (e.g. MgFe204) and chromites (e.g. MgCr204). 

Some cubic crystals with faces indexed by Miller indices, (a) Cubic unit cell and labels of axes and angles (b) and (c) two habits of pyrites FeS2, class m3 (d) tetrahedrite Cu3SbS3, class 43m (e) spinel MgAl204, class m3m and (f) almandine (garnet) Fe3Al2(Si04)3, class m3m. 

According to the cation distribution over the 8(a) and 16(d) sites, the spinels of formula AB2O4 can be divided into normal and inverse classes. Examples of normal spinels are MgA CU and MgTi204, in which the oxidation states of A1 and Ti are both +3 their respective structural formulas are Mg2+ t A]2+ 004 and [Mg2+]t[Ti2+]o04, in which the subscripts t and 0 indicate tetrahedral and octahedral sites, respectively. 

Spinels form a large class of compounds that are important in catalysis. The structure of normal spinels is related to that of MgAl204, the unit cell of which contains 32 oxygen atoms in a nearly perfect cubic array (30, 31). The Mg and A1 cations occupy tetrahedral and octahedral sites, respectively. 

Both Y-AI2O3 and TI-AI2O3 are members of a class of binary oxides in which the oxygen packing is of the cubic close packing type, with the cations situated partly in tetrahedral and partly in octahedral positions. These compounds are called spinels after the mineral spinel which has the composition MgAhC. In spinel Mg2+ occupies tetrahedral and Al3+ occupies octahedral positions. The unit cell of a cubic close packed structure is shown in Fig. 8.5. 

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