Spinel-type structure oxides

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... 

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. 

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 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... 

Solid electrolytes. These correspond to soHd materials in which the ionic mobility is insured by various intrinsic and extrinsic defects and are called solid ion conductors. Common examples are ion-conducting solids with rock salt or halite-type solids with a Bl structure (e.g., a-AgI), oxygen-conducting solids with a fluorite-type Cl structure (A"02), for instance CaF and yttria-stabilized zirconia (YSZ, ZrO with 8 mol.% Y O,), a pyro-chlore structure (A BjO ), perovskite-type oxides (A"B" 03), La Mo O, or solids with the spinel-type structure such as beta-aluminas (NaAl 0 ) for which the ionic conduction is ensured by Na mobility. 

WF studies of undoped CoO have shown that in the gas/solid equilibrium corresponding with the stability range of the CoO phase, grains of CoO are covered with a bidimensional layer of a spinel-type structure, CO3O4. These interface stractures have been observed in other oxide systems, such as Cr-doped NiO and Cr-doped CoO. ... 

X-ray diffractograms of the samples (Fig. 13.23) presented only a NiAl204 cubic spinel-type structure (JCPDS 10-0339). All samples showed mean crystallite nanosizes. The crystallite size is strongly dependent on the fuel/oxidant ratio, which increases with increasing urea loading. Nanosized crystallites of NiAl204... 

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. 

Moreover, as especially mentioned in the paragraph dedicated to the MgAl204 (spinel) type, in these compounds examples of interesting and intriguing interrelations are often observed between the structures of intermetallics and of complex oxides. 

---