Aluminum oxide crystal structure

C.19 Aluminum oxide, alumina, exists in a variety of crystal structures, some of which are beautiful and rare. Write the formula for aluminum oxide, which is a binary compound of aluminum and oxygen. The mass of a rectangular slab of aluminum oxide of dimensions 2.5 cm X 3.0 cm X 4.0 cm is 102 g. What is the density of aluminum oxide ... 

Aluminum oxide, A1203, is known almost universally as alumina. It exists with a variety of crystal structures, many of which form important ceramic materials (see Section 14.22). As a-alumina, it is the very hard, stable, crystalline substance corundum impure microcrystalline corundum is the purple-black abrasive known as emery. Some impure forms of alumina are beautiful, rare, and highly prized (Fig. 14.25). A less dense and more reactive form of the oxide is y-alumina. This form absorbs water and is used as the stationary phase in chromatography. 

A colorless mineral known as corundum (composed of aluminum oxide) is colorless. A red variety of corundum known as ruby, a precious stone, owes its color to impurities of chromium within the crystal structure of corundum. Blue and violet varieties of corundum are classified as sapphires, the blue being the result of iron and titanium impurities, and the violet of vanadium impurities within the corundum crystal structure. Another colorless mineral is beryl (composed of beryllium aluminum silicate) but blue aquamarine, green emerald, and pink morganite, are precious varieties of beryl including different impurities aquamarine includes iron, emerald chromium and vanadium, and morganite manganese. 

We begin with the structure of a noble metal catalyst. The emphasis is on the preparation of rhodium on aluminum oxide and the nature of the metal-support interaction. Next we focus on a promoted surface in a review of potassium on noble metals. This section illustrates how single crystal techniques have been applied to investigate to what extent promoters perturb the surface of a catalyst. The third study deals with the sulfidic cobalt-molybdenum catalysts used in hydrotreating reactions. Here we are concerned with the composition and structure of the catalytically active... 

Spinel Mixed Metal Oxide Pigments. The crystal structure of spinel, MgAl204, is made up of an approximately cubic closely packed array of oxygen ions (Fig. 26). The unit cell contains 32 oxygen ions. Of the 64 possible tetrahedral sites, 8 are occupied by magnesium ions of the 32 possible octahedral sites, 16 are occupied by aluminum ions. 

An ionic compound typically contains a multitude of ions grouped together in a highly ordered three-dimensional array. In sodium chloride, for example, each sodium ion is surrounded by six chloride ions and each chloride ion is surrounded by six sodium ions (Figure 6.11). Overall there is one sodium ion for each chloride ion, but there are no identifiable sodium-chloride pairs. Such an orderly array of ions is known as an ionic crystal. On the atomic level, the crystalline structure of sodium chloride is cubic, which is why macroscopic crystals of table salt are also cubic. Smash a large cubic sodium chloride crystal with a hammer, and what do you get Smaller cubic sodium chloride crystals Similarly, the crystalline structures of other ionic compounds, such as calcium fluoride and aluminum oxide, are a consequence of how the ions pack together. 

Corundum is an aluminum oxide that possesses a hexagonal crystal structure, The compound is extremely hard (2000 on the Knoop scale), sp gr 3.95, and is widely used in abrasives and refractories. Corundum is manufactured by fusing alumina or bauxite in an electric arc furnace operated at about SIOO C. 

The density of beryllium is 1.847 g/cm3 based upon average values of lattice parameters at 255C (a — 22.856 nm and c — 35.832 nm). Beryllium products generally have a density around 1.850 g/cm3 or higher because of impurities, such as aluminum and other metals, and beryllium oxide. The crystal structure is close-packed hexagonal. The alpha-form of beryllium transforms to a body-centered cubic structure at a temperature very close to the melung point. 

The first crystal structure evidence of a molecular cage entity within an alumo-silicon oxide was reported in 1987 with tetrakis(tertamethylammonium)alumino-silicate.53 Although there is no organic ligand on the silicon atom, the compound contains a separated silsesquioxane-like Si4Al4012 unit (half of the silicon atoms of the silsesquioxane being substituted by aluminum atoms) with all silicon and... 

Consider the crystal structure of LiA102 in which each A1 is surrounded by six oxide ions, each of which is bound to two aluminum atoms. If each Li is surrounded by four oxide ions, how many Li ions surround each oxide ion ... 

Chromia Chromite Cr2Fe04 is the most commonly used chromium-containing mineral for ceramic formulations. This mineral has a spinel crystal structure, where the iron may be replaced by magnesium and aluminum. Chromite is used in ceramics largely as a refractory in the form of burned and chemically bonded bricks. For this purpose, a low-silica material is desired. When low silica is desired, chromic oxide is extracted from chromite by dissolution in add, removal of the iron impiu-ity by liquid—liquid extraction, and precipitation of the hydroxide, which is subsequently calcined to the oxide. Chromic oxide is used as a color additive to azes and enamels and in ferrite production to give magnetic materials. 

Mixed y-Ga203-Al203 oxides of different stoichiometry were prepared by the solvothermal method from Ga(acac)3 and Al(OPr-i)3 as starting materials and were used as catalysts for selective reduction of NO with methane. The initial formation of gallium oxide nuclei controls the crystal structure of the mixed gallium-aluminum oxides. It is found that the acid density per surface area is independent of the Al Ga feed ratio but depends on the reaction medium (diethylenetriamine, 2-methylaminoethanol, toluene, 1,5-pentanediol etc.), whereby in diethylenetriamine the catalyst had lower densities of acid sites and showed a higher methane efficiency. 

Particularly attractive method for preparation of synthetic zeolite is recrystallization of natural aluminosilicates, such as kaolinite (halloysite), previously formed for elimination of plastic flow of highly thixotropic, pulverized zeolite. Some additional components of initial mixtures, such as texture modifiers (hard coal, lignite, cellulose, silica, aluminum oxide) are also introduced. They enrich the structure of zeolite adsorbent in transport pores and prevent an excessive compression of the clay material during the formation process. This results in an increase in product efficiency during the crystallization of zeolite phase. 

Metal oxide nanocomposites were synthesized by electrical discharge method using a combination of aluminum and copper electrodes submerged into water. The crystal structure, lattice parameters and grain size of the nanopowders were determined by XRD using Cu K radiation (Fig. 3b). The XRD pattern exhibited the presence of cubic copper with a lattice constant of 0.3615 nm, as well as aluminum and copper oxide and hydroxide phases. The positions of all peaks were in agreement with the JCPDS standards. 

Matsui has found for an I.G. catalyst that the aluminum oxide at its surface is present in the form of ferrous aluminate which covers parts of the metal surface, consisting of a-iron. The same author prepared a small single crystal of iron coated by an oxide film and studied its structure by the transmission method of electron diffraction. The pattern shows, Fig. 14, that the (111) plane of magnetite is produced parallel to the (111) plane and also to the (110) plane of iron, suggesting that with the... 

---