Crystalline solids spinel structure

The spinel ferrites were fabricated by solid state reaction technique. Cobalt and Zinc ferrites CoxZnyFe204,(x=0.7,0.3,0.4,0.2 and y=0.3,0.7,0.6,0.8) were prepared by solid state reaction technique. The crystalline structure of the sample was investigated by X-ray diffraction(XRD). All samples show cubic spinel structure. The lattice parameter decreases with increasing cobalt content. Magnetic properties shows that the prepared sample exhibit ferromagnetic behaviour at room temperature. The saturation magnetization increases with increasing cobalt content. Curie temperature... 

Five aspects of the preparation of solids can be distinguished (i) preparation of a series of compounds in order to investigate a specific property, as exemplified by a series of perovskite oxides to examine their electrical properties or by a series of spinel ferrites to screen their magnetic properties (ii) preparation of unknown members of a structurally related class of solids to extend (or extrapolate) structure-property relations, as exemplified by the synthesis of layered chalcogenides and their intercalates or derivatives of TTF-TCNQ to study their superconductivity (iii) synthesis of a new class of compounds (e.g. sialons, (Si, Al)3(0, N)4, or doped polyacetylenes), with novel structural properties (iv) preparation of known solids of prescribed specifications (crystallinity, shape, purity, etc.) as in the case of crystals of Si, III-V compounds and... 

In this work the performance of Cr-Co spinels as catalysts for total combustion of methane is studied. The spinels were prepared from nitrate precursors. The effect of temperature and time of calcining was studied using x-ray diffraction (XRD) in order to check the crystalline structure and the absence of other phases, N2 physisorption (BET) in order to study the porous structure of these solids and Temperature Programmed Reduction (TPR) in order to determine the presence of different metallic species. 

There are a few catalytic applications in which other mixed oxide structures are cited. Among them are pyrochlore compounds such as La2Zr207, spinels such as AB2O4, and lanthanum beta aluminates, all of them with well-defined crystalline structures. There are also a few examples of oxide solid solutions made up of a rare-earth oxide and a transition-metal oxide. 

The powder X-ray diffraction patterns for the dried precursors correspond to a well crystallized material with a hydrotalcite-like structure. This fact indicates that highly homogenous precursors are obtained. The XRD results for the calcined oxides (Fig. 1) show diffraction peaks corresponding as much to a spinel-like phase as to metallic oxides such as nickel oxide. The relative proportion of these depends to a large extent on the catalyst composition. As these solids are poorly crystallized (except for the Ni-Co-Zn-AI catalyst), it is not possible to identify through this technique which divalent cation is present in the crystalline net, since there is hardly Table 1. Nominal composition and Sbet of the samples. any difference between the diffraction angles of the spinels corresponding to the different metallic cations. The tendency of Co and Zn to form a spinel-like structure, in which cobalt would occupy the octahedral sites, is very... 

Layered and spinel materials have been used successfully as intercalation-type cathode active materials in commercial Li-ion batteries. The physical and chemical properties, electrochemical reactions, structure evolution mechanisms, stability and safety issues have been widely investigated. Based on comprehensive fundamental researches, since 1980s, their electrochemical performances are improved continuous after various modifications, such as doping, surface coating, forming solid solution and composite, controlling morphology, size and crystallinity. Here, basic features of layered and spinel materials are summarized. 

If the heat of mixing of the solid is sufficiently negative, a new phase will form within some range of the composition x. This could be a new crystalline structure in which some substitution of A and B is permitted, either as a solid solution as in Al-Ni (Figure 12.11) or through its defect structure as discussed in Chapter 9. The formation of spinel (MgAl204) shown in Figure 12.12 is an example of the latter. 

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