Spinel-like structures

The beta-alumina structures show a strong resemblance to the spinel structure. They are layered structures in which densely packed blocks with spinel-like structure alternate with open conduction planes containing the mobile Na ions. The and /S" structures differ in the detailed stacking arrangement of the spinel blocks and conduction planes. Fig. 2.9. 

The coordination of aluminium is changed during the transformation from octahedral to mixed coordination in y - AljOj (the latter has a spinel-like structure containing 16 octahedral aluminium and 8 tetrahedral positions) and in 8-AI2O3 (containing some elements of spinel lattice), then again octahedral in a-Al203 [8]. 

New cubic high-pressure phases of A12S3, Al2Se3, and CuInAl4Seg have been reported all have spinel-like structures.470,471... 

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... 

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