Spinel, chemical formula

Spinel compounds have a common chemical formula, AB2X4. Structurally they have a cubic symmetry and are derived from magnesium alurmnate, MgAl O., a naturally occurring mineral. 

The spinel structure consists of a f.c.c. anion sublattice that has half its octahedral interstices and one-eighth of its tetrahedral interstices (there are twice as many tetrahedral as octahedral interstices) occupied by cations. For oxygen spinels, the chemical formula is usually written M2+[N2+]04 for a normal 2-3 spinel, N3+[M2+N3+]04 for an inverse 2-3 spinel, where the cations in brackets are in octahedral interstices, those outside of the brackets are in tetrahedral interstices. The 2-4 spinels M2+[M2+N4+]04 and N4+[M2+]04 are also common. In most ferrospinels, N3+ = Fe3+ or (1 — )Fe3+ + N3+. (Fe2+[Fe2+N4+]04 is also a ferrospinel.) Most ferrospinels... 

Spinel compounds have a common chemical formula AB2X4 and a cubic structure. Of the more than 100 known spinel compounds, only those with oxygen anions are used as inorganic pigments. The following are among those most commonly used in plastics ... 

In the intermediate case, the ions of divalent metal do not form their own structural layer but are localized in the vicinity of vacant octahedrons [ Al2(OH)6] 2oo using OH" groups of this layer, as well as OH" groups and H2O molecules of the freshly formed third network of layered packing. In this case, interplanar distance is equal to 0.76 nm. Hence, the crystal chemical formula for this compound is [Me(0H)2-H20][Al2(0H) ]. It was obtained in Mg(OH)2 - A1(0H)3 system. The decomposition of such compounds at moderate temperatures (about 800°C) can result in the formation of stoichiometric spinels, which was demonstrated in [15]. 

Assignment of A or B sites to cations in spinel type oxides based upon their chemical formulae can offen be misleading. Thus, for the mixed-valence oxide NiFe204, we might conclude that the Ni+ ions are on the tetrahedral A... 

The mineral spinel is MgAl204. Spinels have the general formula AB2O4 although later we will also write it as AO B203, where n describes the nonequimolarity. Bragg and Nishikawa actually solved the spinel structure independently in 1915. The spinel structure is so important because the magnetic ferrites are spinels. For the ferrites we express the chemical formula as MO FciOs, where M is a divalent metal ion like Mn, Ni, Fe, or Co (or a mixture of such ions). 

Vacancies If an atom is not present on the site that it should occupy in a perfect crystal then a vacancy is located at that site. A Schottky defect is a set of vacancies created by removing one atom for each atom in the chemical formula. Thus, in a stoichiometric crystal such as MgO, we get a pair of vacancies, one on the Mg sublattice and one on the O sublattice, as shown in Figure 11.1. In spinel, a Schottky defect consists of seven vacancies. 

Complex oxides, such as YAG, spinel, mullite and ferroelectrics, contain more than one type of metallic element in their chemical formula [140, 160-175]. In this... 

In a fluorite-type oxide, whose chemical formula is expressed as MO2, the structure type is named afler Cap2, in which the cation/anion radius ratio is such that the anions achieve a simple cubic packing with the cations occupying half the available sites with 8-fold coordination. The packing of the anions is not as compact as those found in other oxides such as spinel or corundum-type structures, which have closest packing of oxide ions. 

Great attention has been devoted to nanoparticles of ferrites these compounds are spinels with the chemical formula A +B2 04, where A + and B + correspond to metallic 3d cations, respectively. The ferrite spinel structure has a face-centered cubic (fee) structure in the close packed cubic arrangement of oxide ions. The structure contains two interstitial sites, occupied by oxygen coordinated metallic cations, with tetrahedral (A)-site, and octahedral [B]-site, resulting in a different... 

Chromite is a member of the spinel group. Spinels have the general formula, R"0 R2 "03. R" represents a divalent metal ion, such as Fe++, Mg++, Mn++, and so forth. R " is a trivalent ion. Some examples are AF++, and Ci ++. The chemical formula for chromite is FeO Cr203. The characteristics of this group of minerals are as follows ... 

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