Aluminates, spinel structure

Aluminum forms mixed oxides with many other metals which either replace Al3+ in octahedral positions or occupy tetrahedral sites. The spinel structure of MgAl204, an important prototype, has been investigated by photoelectron spectroscopy.122 Vibrational and solid state NMR methods are also useful for studying aluminate structures.12 125... 

The magnesium aluminate spinels that we have tested are MgAl204 and Mg Al Og. The latter is a solid solution (2,3) of pure spinel (MgAl204) and MgO. The spinel structure (4) is. based on a cubic close packed array of oxide ions. Typically, the crystallographic unit cell contains 32 oxygen atoms one eighth of the tetrahedral holes are occupied by the divalent metal ions (Mg2+), and one half of the octahedral holes are occupied by the trivalent metal ions (Al3+). 

Lithium aluminate, LiAlsOg with the inverse spinel structure, is a material with possible applications in ceramic blankets for thermal control of fusion reactors. Li and Li NMR has been used to measure the spin-lattice relaxation of lithium in this compound (Stewart et al. 1995). The results indicate that Li relaxes most significantly through interactions with paramagnetic impurities, whereas Li relaxes much more strongly through dipole-dipole interactions. 

M. Ishimaru, et al., Atomistic structures of metastable and amorphous phases in ion-irradiated magnesium aluminate spinel. J. Phys.-Condens. Matter 14(6), 1237-1247 (2002). 

Both of these materials can be "activated" by Mn which also forms a spinel, i.e.- Mn2Sn04. The latter as a phosphor, i.e.- Mg2GKi04 Mn , is used as the source of green light in most copsdng machines today. It is interesting to note that the cubic spinel structure is formed by a majority of dibasic cations like Mg and that many trivalent or quadrivalent cations can form oxide-based anions like stannate. aluminate, titanates, vanadates and the like. 

If without additives, a carrier such as A3 consists of a mixture of 8-, 0-, and a-aluminas between 900° and 1000°C. The presence of the metal oxides, introduced by impregnation, effectively maintains a cubic type structure at a calcining temperature of 900°-1000°C. With alumina, these oxides form spinel-structured compounds which are more or less well crystallized. Because of the insertion of alumina, the lattice parameter of these compounds is expanded with respect to that of the stoichiometric spinel. The properties of the carrier are thus maintained up to a temperature which depends on the considered mixed oxide. At this temperature—about 1000 °C with magnesium aluminate and 900°C with zinc and copper aluminates—the stoichiometric spinel recrystallizes while a-alumina is rejected (6). The carrier then suddenly loses its mechanical and structural properties. None of the mentioned additives could improve the stability of the Cl carrier above 1000°C. 

Evidence of spinel structure was determined in the reaction bed by x-ray powder diffraction. The mineral spinel, magnesium aluminate, possessed unusual catalytic activity. Artificial spinels, prepared from other elements but still having the basic spinel structure, also were found to be active catalysts. 

One of the keys to the properties of the hexa-aluminates is the crystal structure. The hexa-aluminate structure consists of blocks of spinel-structure divided by mirror planes in which the large A ions are situated. This results in a layered structure. There are two different structure types, the magnetoplumbite and P-alumina structure where, for example. La and Sr in the A-position will yield magnetoplumbite while Ba yields the fl-alumina structure. ... 

Reijnen, P. J. L. (1967). Sintering behaviour and microstructure of aluminates and ferrites with spinel structure with regard to deviations from stoichiometry. Science of Ceramics, 4, 169-88. 

Structural relaxation in tetrahedrally coordinated Co along the gahnite-Co-aluminate spinel solid solution. Am. Mineral, 97,... 

Nickel aluminate (NiAl204) is a good example of a mixed cation oxide with normal spinel structure, where Al occupies the octahedral sites and Ni occupies the tetrahedral sites [98,113]. It can be used as support for catalysts due to its resistance to high temperatures and acidic or basic environments, providing chemical and physical stability for the catalyst. 

Both structure and composition affect phonon conductivity in singlephase crystalline ceramics. Complex structures scatter lattice waves to a greater extent. Hence, thermal conductivity is lower in those structures. For example, magnesium aluminate spinel, consisting of two oxides, has a lower thermal conductivity than the single oxides alumina or magnesia. 

Other, but not all, ceramic crystal structures may be treated in a similar manner included are the zinc blende and perovskite structures. The spinel structure is one of the A B Xp types, which is foimd for magnesium aluminate or spinel (MgAl204). With this structure, the ions form an FCC lattice, whereas Mg ions fill tetrahedral sites and ions reside in octahedral positions. Magnetic ceramics, or ferrites, have a crystal structure that is a slight variant of this spinel structure, and the magnetic characteristics are affected by the occupancy of tetrahedral and octahedral positions (see Section 20.5). 

Alumina is incorporated as a sohd solution of the iron aluminate spinel, hercynite, in the crystal lattice. The alumina concentration should be less than the solubility of alumina in magnetite. This corresponds to a maximiun content of about 3% alumina. Any excess of alumina does not go into solid solution, and leads to a reduction in catalytic activity, particitlarly when using catalysts promoted with alumina. The presence of alumina as a structural promoter also leads to the formation of wustite and stabihzes the reduced catalyst. Small amounts of magnesia can also dissolve into magnetite and act as a promoter. The calcium component exists in the form of ferrites or alirminates by neutrahzing acidic components—such as silica—and protects the potash that activates the catalyst. 

XRD difiBactograms, shownin Fig. 1 for selected satrqrles, confirmed spinel structure and low crystalhnity of zinc aluminate as-prepared and after catalysts preparation. Average grain size calculated using Scherrer equation amount to 3 run for the as-prepared ZnAl204 support, and 5-6 run for the catalyst samples. 

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