Spinel compound

Electrode materials with a spinel-type structure, A[B2]X4, have become the subject of intensive research over the past 10 years. This interest is partly due to the fact that many spinel compounds exist in nature, and are therefore intrinsically stable materials. Furthermore, the family of spinels encompasses a vast number of compositions because it is possible to vary not only the type and valency of the A and B cations within the A[B2]X4 structure, but also the type and valency of the X ani-... 

The magnirnde and sign of the magnetic hyperfine field in the nickel spinel compounds NrFe204 (Fig. 7.8), NiCr204 (Fig. 7.9), NiFeo.3Cr1.7O4, NiRh204, and... 

The electronic structure of the spinel type compound NiCo204 has been investigated by XANES, EXAFS, and Ni Mdssbauer studies. On the basis of the derived cation valencies, the octahedral and tetrahedral site occupancies as well as the formula in standard notation for spinel compounds could be delineated [25]. 

It also consists with the spinel hypothesis for the structure of active catalytic sites the spinel compound NiCo204 is well known for its catalytic activity in the reaction of oxygen reduction [21],... 

Table 1.19 Lattice energy and CFSE for selected spinel compounds. Values of U, E, and from Ottonello (1986). CFSE values calculated with values proposed by McClure (1957). Data in kJ/mole x = degree of inversion.

The ample diversity of properties that the spinel compounds exhibit is derived from the fact that the possibility of synthesis of multi-component spinels by partial substitution of cations in position A and B giving rise to compounds of formula (AxA i x)(ByB 2 y)04. This accounts for the variety of reactions in which they have been used as catalyst. Moreover, partial substitution of A and B ions giving rise to complex oxides is possible, while maintaining the spinel structure. 

ASOa-perovskites of Table 6 extends from Dza through Can to Ci. Distinct band-splittings are not observed, however, 13) (Fig. 2). As is demonstrated for the YCra Ali-a 03-system, the Oo-transition is situated at 13600 to 13700 cm i independent of the Cr3+ concentration (Fig. 2) leading to a B55 of 705 cm (Table 3), which is more than 5% lower than for the corundum and spinel compounds discussed. This is in agreement with the possibility of relatively strong Ji-bonds in the perovskite structure, which could be inferred from spectroscopic and crystallographic results 3) as well as from NMR-data and MO-calculations 33) for Ni2+,... 

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. 

Similar to the case of the pyrite compounds (PdSb2—PtSb2, etc.) the metallic spinel compounds mix badly with non-metallic ones, as is... 

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 ... 

Kosova N.V., Devyatkina E.T., Kozlova S.G. Mechanochemical way for preparation of disordered lithium-manganese spinel compounds. Proceedings of thelO Int. Meet, on Lithium Batteries Lithium 2000 Como, May 28-June 2, 2000 J. Power Sources 2001 (to be published). 

Many spinel compounds adopt an alternative structure where half the B cations occupy the tetrahedral sites while the remaining B cations and A cations occupy the octahedral sites. These are known as inverse spinels. The occupancy of the latter sites may be random or ordered. The adoption of an inverse spinel versus a spinel structure depends on a variety of factors the relative size of the A and B cations, the relative lattice energies, ligand-field stabilization, energies of octahedral versus tetrahedral coordination, and polarization or covalency effects. For example, in MgABOq one might expect the small... 

This set of schematic equations has been considerably simplified, in particular in that the stoichiometric formulae do not describe the certain variability in the structure as well as in the composition of the products. For example, metakaolinite may contain small amounts of HjO, the spinel compound may show significant differences in the ratio of the two oxides, and the same applies to a certain degree to mullite. 

Figure 2 shows the XRD pattern of the copper ferrite prepared by co-precipitation. The diffraction peaks in the scanning range include 112, 103,211, 220, 224 and 321 and the strongest peak appears at around 35°, these correspond to the structure characteristics of the spinel compounds, the strongest peak corresponds the 211 crystal surface. There is no other peak in the spectrum, indicating that the product is pure compound. The crystalline data were shown in Table 1. 

X-ray diffraction and microscopic studies revealed that calcined stoichiometric mixtures of coprecipitated hydroxides do indeed form spinels and solid solutions. With some mixtures, complete reaction was not always easily attained. For example, in the CuO Fe203 system, excess copper oxide and Fe203 peaks were found in x-ray diffraction patterns in addition to the major spinel phase. Calcined mixtures usually produced pure spinel compounds. 

The analysis shows, as do diffraction methods, that gallium is coordinated tetrahedrally by oxygen. In addition, the Ga-O bond length of 1.83 A is obtained that cannot be determined directly from diffraction methods due to the partial occupation of the Ga sites by Si. As the negative value of Ad indicates, the deviations from the mean bond length are smaller in the sodalite than in the spinel compound. The parameter AEq bears no physical meaning,but o values which are larger than some eV point to false results. 

D Arco, Ph., Silvi, B., Roetti, C. and Orlando, R. (1991) Comparative study of spinel compounds a pseudopotential periodic Hartree-Fock Calculation of Mg2Si04, Mg2Ge04, Al2Mg04 and Ga2Mg04 J. Geophys. Res. 96,(> Ql-6 2. 

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