Spinel

The main drawback with the use of perovskites is the poor thermal stability of the materials compared with hexa-aluminate based catalysts. Zwinkels et al. have compared the thermal stability of two different hexa-aluminates with a SrZr03-perovskite, a pyrochlore, see Section 3.2.4, and two spinels, see Section 3.2.3. The perovskite, as well as the pyrochlore and one of the spinels decreased in their surface area significantly. One of the explanations of the much lower stability of the perovskites compared with the hexa-aluminates is that the crystal growth will take place in three dimensions and thereby yield a material with a low surface area. Lowe et al. have studied several different perovskites and their thermal stability and conclude that the surface area of the perovskites is not sufficient for use in high temperature catalytic combustion. Similar results have been shown by Cristiani et al.  

Packings of spheres having occupied tetrahedral and octahedral interstices usually occur if atoms of two different elements are present, one of which prefers tetrahedral coordination, and the other octahedral coordination. This is a common feature among silicates (cf. Section 16.7). Another important structure type of this kind is the spinel type. Spinel is the mineral MgAl204, and generally spinels have the composition AM2X4. Most of them are oxides in addition, there exist sulfides, selenides, halides and pseudohalides. 

In the following, we start by assuming purely ionic structures. In spinel the oxide ions form a cubic closest-packing. Two-thirds of the metal ions occupy octahedral interstices, the rest tetrahedral ones. In a normal spinel the A ions are found in the tetrahedral interstices and the M ions in the octahedral interstices we express this by the subscripts T and O, for example Mgr[Al2](904. Since tetrahedral holes are smaller than octahedral holes, the A ions should be smaller than the M ions. Remarkably, this condition is not fulfilled in many spinels, and just as remarkable is the occurrence of inverse spinels which have half of the M ions occupying tetrahedral sites and the other half occupying octahedral sites while the A ions occupy the remaining octahedral sites. Table 17.3 summarizes these facts and also includes a classification according to the oxidation states of the metal ions. 

Arbitrary intermediate states also exist between normal and inverse spinels they can be characterized by the degree of inversion X  

X = 0 normal spinel X = 0.5 inverse spinel The distribution of the cations among the tetrahedral and octahedral sites is then expressed 

MnJ Mnf] 004. If it were to be converted to an inverse spinel, half of the Mnm atoms  

5 0 was taken for tetrahedral ligand fields. Mn3O4 is a normal spinel. 

Consequently, natural chromite is more realistically formulated as (Mg,Fe )(Al,Fe i,Cr)204 FeCr204 is its idealized or end-member composition. The substituent ions replace Fe + and Cr + in a random way, much as solute molecules displace solvent molecules in a liquid solution. Consequently, chromite samples could be described as solid solutions of MgAl204 and MgFe O4 in Fe Cr204. 

A closely related group of AB2O4 oxides has the inverse spinel structure. Here, again, there is a ccp array of, but the B atoms are equally divided between T- and 0-sites, and all of the A ions appear in 0-, not T-, holes. Thus, we have B(AB)04 with B in one-eighth of the T-holes, A in one-quarter of the 0-holes, and B in one-quarter of the 0-holes. 

When sintered at 1073 K for several hours in vacuo, the microcrystals of MgCr204 assume a very regular habit, as demonstrated clearly by HRTEM (Fig. 30). 

The IR spectrum of adsorbed CO should be dominated by the spectroscopic manifestations of CO adsorbed on the (111) face. Indeed, the IR spectrum of adsorbed CO (Fig. 31) is dominated by a major peak centered 

Therefore, CO adsorbed on Cr3+ centers located on the (111) faces is expected to be characterized by somewhat higher frequencies. However, it has been shown for the (0001) face of a-Cr203 (where Cr3+ is in a very similar environment) that Cr3+ moves inward to a more shielded position upon relaxation, which leads to a reduced electric field strength at the chromium centers. Because of the stability of the surface complexes even at room temperature, it is not excluded that n backdonation may also play a role in the bond between the CO and the Cr3+ centers (which usually shift the CO frequency downwards). As discussed previously (see Section IV.A.4) the shift A v induced by increasing CO coverage is caused by lateral 

In addition to the main peak in Fig. 31, two weak bands in the 2170- to 2185-cm 1 range are detected, and these also slightly shift with coverage. These bands are tentatively attributed to CO adsorbed on Cr3+ centers located on less extended faces or on edges or steps. No other bands are observed in the spectrum of adsorbed CO. Thus, Cr3+-CO adducts are formed exclusively on the MgCr203 surface. 

The following are the major conclusions (i) The MgCr204 surface properties are dominated by the reactivity of Cr3+ centers, (ii) the Mg2+ ions do not play a detectable role in the CO/spinel chemistry within the investigated pressure range, and (iii) the surface oxygen ions do not have a basicity comparable to that of oxygen ions on MgO and La203. 

At temperatures just below Tc, the molecular field energy 

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

Tricoball teiroxide, C03O4. Black solid (ignition of CoO in air). Has spinel structure and other spinels MC02O4 are also known. 

Manganates III), again mixed-metal oxides present in the spinel Mn304, Mn Mn 204. 

Rinmann s green, ZnCojO. A spinel formed when cobalt nitrate solution is placed on zinc oxide and the mixture heated to redness. The green colour forms a delicate test for Zn. 

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 ). 

The structure of these solid compounds is not known with certainty but an approximate formula might be NaAlOj.xHjO. Many aluminates occur in minerals, for example the spinels of general formula M (A102)2 where M may be Mg, Zn or Fe these have a mixed oxide structure, i.e. consist essentially of M AF and O ions. 

Sillimanite, see Aluminum silicon oxide (1/1) Smithsonite, see Zinc carbonate Soda ash, see Sodium carbonate Spelter, see Zinc metal Sphalerite, see Zinc sulflde Spherocobaltite, see Cobalt(II) carbonate Spinel, see Magnesium aluminate(2—)... 

Spinel ferrites, isostmctural with the mineral spinel [1302-67-6] MgAl204, combine interesting soft magnetic properties with a relatively high electrical resistivity. The latter permits low eddy current losses in a-c appHcations, and based on this feature spinel ferrites have largely replaced the iron-based core materials in the r-f range. The main representatives are MnZn-ferrites (frequencies up to about 1 MH2) and NiZn-ferrites (frequencies 1 MHz). 

The soft magnetic spinel ferrites, by far the most important cubic ferrites, were first introduced by Philips under the trade name Ferroxcube (14) and are now widely commercially available under various trade names. The world market for soft magnetic ferrites amounts to about one biUion dollars (1991), about 350 million dollars of which is in the United States. 

Common Properties of Spinel Ferrites and M-Type Ferrites... 

The commercial sintered spinel and M-type ferrites have a porosity of 2—15 vol % and a grain size in the range of 1—10 ]lni. In addition, these materials usually contain up to about 1 wt % of a second phase, eg, CaO + Si02 on grain boundaries, originating from impurities or sinter aids. 

Spinel Ferrites. In spinel ferrites having the composition where A and B are metals, cubic close-packed oxygen ions leave two kinds of... 

Eig. 2. Spinel stmcture where Q is oxygen, Q > A-sites, O, B-sites. A unit cell consists of eight fee subceUs with different cation occupants. Adapted from... 

The site preference of several transition-metal ions is discussed in References 4 and 24. The occupation of the sites is usually denoted by placing the cations on B-sites in stmcture formulas between brackets. There are three types of spinels normal spinels where the A-sites have all divalent cations and the B-sites all trivalent cations, eg, Zn-ferrite, [Fe ]04j inverse spinels where all the divalent cations are in B-sites and trivalent ions are distributed over A- and B-sites, eg, Ni-ferrite, Fe Fe ]04 and mixed spinels where both divalent and trivalent cations are distributed over both types of sites,... 

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