Spinels and related compounds

Grtenwood. Umic Cr stoh, Lattice Defects and Nonstoichiometr, Butterworths. London, 1968. 194 pp. See also J. K. Burdett, G. D. Price and S. L. Price, J. Am. Chem. Soc. 104, 92-5 (1982). 

The anion can be O, S, Se or Te. Most of the a cations have radii (6-coordinate) in the range 65-95 pm and larger cations such as Ca (100 pm) and Hg (102 pm) do not form oxide spinels. The radii of B fall predominantly in the range 60-70 pm though Al (53 pm) is smaller, and In (80 pm) normally forms sulfide spinels only. 

Examples of spinels with other combinations of oxidation state are  

A ByX4 TiMg204, PbFe204, SnCu2S4 A bU4 MoAg204, MoNa204, WNa204 

Many of the spinel-type compounds mentioned above do not have the normal structure in which A are in tetrahedral sites (t) and B are in octahedral sites (o) instead they adopt the inverse spinel structure in which half the B cations occupy the tetrahedral sites whilst the other half of the B cations and all the A cations are distributed on the octahedral sites, i.e. (B)t[AB]o04. The occupancy of the octahedral sites may be random or ordered. Several factors influence whether a given spinel will adopt the normal or inverse structure, including (a) the relative sizes of A and B, (b) the Madelung constants for the normal and inverse structures, (c) ligand-field stabilization energies (p. 1131) of cations on tetrahedral and octahedral sites, and (d) polarization or covalency effects.  

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 Fe304 (p. 1080) which has an inverse spinel structure (Fe )t[Fe Fe ]o04. 

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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 long-known K2Hg(CN)4 and related compounds are still the object of structural studies. Thus, a neutron-diffraction study confirmed the spinel structure of K2Hg(CN)4 at room temperature (298 K) with exactly tetrahedral anions, Hg(CN)42 (r(Hg—C) 215.2, r(C—N) 114.9 pm).112... 

Realizing the importance and effectiveness of the magnesium species in the sorption of SO, in the late 1980s, Akzo Nobel patented the use of hydrotalcite and related compounds for nse in an FCC to reduce SO emissions [8,9,10]. Hydrotalcite contains more active Mg species than spinel. Hydrotalcite-based compounds typically contain 3-4 moles of Mg per mole of Al, while spinels contain only 1 mole of Mg per 2 moles of Al (Figure 16.4). 

The magnetic properties of oxides and related compounds have been tabulated in comprehensive data collections [3.4]. A review of the basic magnetic properties of garnets (A3 [B2](Si3)Oi2 and spinel ferrites Me0Fc203 or Me Fe 04 is given by Guillot in [3.3]. 

The spinel family of oxides with composition AB2O4 has the A and B cations distributed in octahedral and tetrahedral sites in a close-packed oxygen structure (Supplementary Material SI). Impurity doping can take place by the addition of a dopant to octahedral or tetrahedral sites. In this, the spinel family of compounds is quite different from the A2B04 perovskite-related phases of the previous section in that both cation sites are similar in size and can take the same cations. 

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. 

The difficulties in synthesizing the mixed spinel seem related to both thermodynamic and kinetic issues. Hydroxides (and/or basic salts) of divalent elements are more stable than the ferrite at low temperatures. The large reactivity difference between iron and chromium explains the rapid crystallization of iron oxides or oxyhydroxides compared with the chromium compounds, as well as the segregation of both elements. 

Titanium IV) oxide, T1O2. See titanium dioxide. Dissolves in concentrated alkali hydroxides to give titanates. Mixed metal oxides, many of commercial importance, are formed by TiOj. CaTiOj is perovskite. BaTiOa, per-ovskite related structure, is piezoelectric and is used in transducers in ultrasonic apparatus and gramophone pickups and also as a polishing compound. Other mixed oxides have the il-menite structure (e.g. FeTiOj) and the spinel structure (e.g. MgjTiO ). 

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