Cubic structure alkaline earth oxides

B. Other Alkaline Earth Oxides with Cubic Structure... 

No literature melting data are available. BeO and BeS have a different crystal structure (cubic, zincblende type) than the remaining alkaline-earth oxides and sulfides (cubic, NaCl type). 

The simplest of structures is the rock salt structure, depicted in Figure 2.2a. Magnesium oxide is considered to be the simplest oxide for a number of reasons. It is an ionic oxide with a 6 6 octahedral coordination and it has a very simple structure — the cubic NaCl structure. The structure is generally described as a cubic close packing (ABC-type packing) of oxygen atoms in the (111) direction forming octahedral cavities. This structure is exhibited by other alkaline earth metal oxides such as BaO, CaO, and monoxides of 3d transition metals as well as lanthanides and actinides such as TiO, NiO, EuO, and NpO. 

We have recently suggested a new approach to the preparation of active sites in sulfated zirconia catalysts [5, 6]. In this case, the catalysts are prepared by deposition of sulfate ions on crystalline zirconium dioxide samples with highly defective structure. According to numerous reports, the monoclinic phase typical for ZrOa is not suitable for this purpose. We have shown that active materials could be obtained by impregnation of zirconia-based oxides with cubic crystalline structure. It should be noted that the cubic structure is not thermodynamically stable for pure zirconia at low temperatures. It can be stabilized by introducing different additives, in particular, alkaline-earth metal cations [7]. Recently, similar results have been obtained for ZrOa stabilized by Y2O3 [8]. 

During the next 15 years there were a number of additional innovations in the application of HPHT technology to the synthesis of industrial superabrasives, virtually all of which were pioneered by GE—the Superabrasives business working in close collaboration with the Corporate Research and Development laboratory. In 1956, Wentorf synthesized cubic boron nitride (cBN). Boron nitride does not exist in nature, but the hexagonal form of BN, which has a structure very similar to that of graphite, was a well known synthetic material. The second-hardest material known, cBN has much better abrasion resistance with ferrous alloys and oxidation resistance than diamond. The best catalyst-solvents for cBN synthesis are alkali- and alkaline-earth nitrides and related inorganic salts. GE introduced cBN commercially in 1969 under the tradename Borazon cBN. 

It can be seen from Fig. 14 that this tetragonal oxide, bears a relation to the cubic perovskite (BaTiOj, [11]) structure-type which lies in the cubic space group Im-3m, not shown here, in which, by removal of an axial pair of oxygen vertices, one ean generate the Kentuckia structure-type from the perovskite stmcture-t5 e. However, it should be pointed out that the perovskite structure-t5 e is a 6-,12-connected network, in which the transition metal titanium and chalcogenide oxygen centers, attain octahedral 6-coordination, while the alkaline earth barium cation bears a closest packed coordination sphere of 12. Whereas in the Kentuckia structure-t5 e, the lattice, by great contrast as can be seen in Fig. 14, bears an... 

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