Spinel type structure

Both compounds Mn304 (i.e., hausman-nite) and /-Mn203 crystallize with a tetragonally distorted spinel-type structure (see Fig. 17). 

Figure 17. Schematic drawing of the spinel-type structure of Mn304 and y - Mn20,. The structure is built up of Mn06 octahedra (white) and Mn04 tetrahedra (shaded).

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-... 

Electrodes that are prepared from acid-leached LT-LiCo, xNix02 compounds (0< x<0.2) show significantly enhanced electrochemical behavior over the parent LT-LiCo1 xNix02 structure. The improved performance has been attributed to the formation of compounds with a composition and cation arrangement close to the ideal Li[B2]04 spinel structure (B = Co, Ni) [62]. These spinel-type structures have cubic symmetry, which is maintained on lithiation the unit cells expand and contract by only 0.2 percent during lithium insertion and extraction. 

The cubic y-modification has been recently observed under a pressure of 15 GPa and temperatures above 2000 K by the laser heating technique in a diamond cell [23] and in shock-wave compression experiments with pressures >33 GPa at 1800 K and >50 GPa at 2400 K [29]. This modification is often designated as the c-modification in the literature in analogy to the cubic boron nitride (c-BN). It has a spinel-type structure in which two silicon atoms are octahedrally coordinated by six nitrogen atoms, one silicon atom is coordinated tetrahedrally by four nitrogen atoms (Fig. 3c). The atomic coordinates for the cubic modification are given in Table 2. From calculations it is shown that this structure should have a high hardness similar to that of diamond and c-BN [23]. 

Magnetite (FC3O4) is a component of the water-gas shift reaction that crystallizes to the inverse spinel structure. The general formula of the oxides known as spinel is AB204. In the normal spinel-type structure, A is an A2+ metal, and B is a B3+ metal. O2- forms a CCP anionic framework, where the A atoms occupy 1/8 of the tetrahedral sites and the B atoms occupy 1/2 of the octahedral sites (see Figures 1.6 and 1.7). An example of a normal spinel is Mg A1204. 

Oxides of Trivalent Elements and Spinel-Type Structures... 

Although it is clear that surface Lewis acid sites on alumina are due to coordi-natively unsaturated AP ions, it is not fully clear what is the coordination of such surface ions. Most authors agree that at least three different types of Lewis acid sites (weak, medium, strong) exist on transitional aluminas, arising in some way from the two or three coordinations of the ions in the bulk spinel-type structure. 

Most HTSC oxides have the structure of perovskite (though some of them have the spinel-type structure), which pertains to more than 35 structural classes [14], and includes more than a hundred typical unit cells [15]. Along with cuprates, certain bismuthates also exhibit HTSC properties. For fundamental studies of superconductivity in oxides, both the absolute Tc values and the variety of properties and structures are essential. Therefore, the titanium compounds with relatively low Tc values are also actively studied. 

The XRD profile and the TEM of CAT A were shown in Figure 2 and Figure 4, respectively. The structure of CAT A before reaction was the same as that of FejO which has a spinel type structure. The small peaks assigned to CuO and ZnO were observed. The existences of metallic Cu and Zn could not be ascertained by XRD analysis. On the TEM observation, the CAT A before reaction consisted of the particles of uniform size, 10-20 nm diameters. The compositions of particles were uniform also. These results suggest that Cu and Zn components are dissolved into the spinel type structure. 

We make clear the following facts in this study. The K/Cu-Zn-Fe oxides catalyst has a spinel type structure. The K/Cu-Zn-Fe oxides catalyst is deactivated by the segregation of catalyst components to FeCOj, ZnO, and Cu during the reaction. The segregation is prevented by the addition of Cr component to the catalyst. The long life of K/Cu-Zn-Fe-Cr oxides catalyst can be explained by its slow segregation rate. It is considered that the Cr component stabilizes thermally the spinel type structure of the catalyst. 

The first explanation can probably be ruled out despite thermodynamic data indicating that certain spinel-type structures such as ZnFc204 and Zn2Si04 are less soluble (more stable) than any of the pure solid phases represented in Figure 4.14. There is no evidence that these minerals can crystallize in the pressure-temperature regime typical for soils (Tiller and Pickering, 1974). They are sometimes found in soils as... 

The second modification of alumina is the less compact cubic y-Ai203 (d 3.4gcm ) it is formed by the low-temperature dehydration (<450°> of gibbsitc, y-Al(OH)3, or boehmite, y-AIO(OH). It has a defect spinel-type structure (p. 248) comprising a face-centred cubic (fee) arrangement of 32 oxide ions and a random occupation of 21 of the 24 available cation... 

Tin(IV) nitride, Sn3N4, was first isolated in 1999 from the reaction of Snl4 with KNH2 in liquid NH3 at 243 K followed by annealing the solid product at 573 K. Su3N4 adopts a spinel-type structure, related to that of y-Si3N4 described above. Tin(IV) nitride is the first nitride spinel that is stable under ambient conditions. 

Evidence of spinel structure within some of the alumina and titania catalysts was established by x-ray powder techniques. Samples were removed from the reaction bed of some of the runs where high yields of sodium sulfate had been obtained using either alumina or titanium dioxide as catalyst. The presence of compounds having spinel type structure was established by x-ray powder diffraction. The amount of spinel was variable and of small quantity because neither the proportions of necessary ions nor the reaction conditions were ideal for spinel formation. 

It seems possible, from the results obtained with magnesium aluminate and from x-ray evidence, that a satisfactory catalyst for the Hargreaves reaction will be any compound possessing a spinel-type structure. 

Hydrothermal treatment of precipitated FeS yields ferromagnetic Fc2S3, which has a spinel type structure. The structure of CuFeSj has also been reported. ... 

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