Cluster compounds silicates

A very large number of theoretical studies have been performed on MgO and AI2O3. Only some of the early studies and some of the most recent will be described here, in order to give some idea of the extent of progress over the past two decades. Important advances have recently been made in the application of ionic models to such materials as well as in band-theory studies and embedded-cluster studies. After reviewing the early work, contemporary studies of structure, stability, phase relations, and dynamic properties will be described, followed by recent studies of spectral properties and characteristics of the electron-density distribution for each of these materials. Attention is then turned to Si02, the silica polymorphs, and various compounds and clusters that may be used to model tetrahedrally coordinated Si in silica and the silicates. 

Such clusters of FeOe octahedra also exist in other inorganic compounds. For instance they exist in ilvaite (CaFe TFe 2Si20700H) where silicate layers replace phosphate anions and FeOs octahedra form ribbons. It is interesting to note that such a material is active and selective for the reaction although to a lesser extent than the previous hydroxyphosphates [24]. The lower catalytic behaviour may be due to the presence of Ca " cations in the structure and/or to the silicate counter anion whose basicity in the sense of Pearson is different from that of the phosphate anion and/or the infinite size of Fe06 octahedra clusters. 

Boron has a great affinity for oxygen and occurs in nature only in boric acid or borates. Borates are composed from clusters of flat trigonal BO3 and tetrahedral BO4 groups. The structural chemistry of borates is as rich and complicated as those of silicates, borides, or boranes. Boron oxide is an essential part of borosilicate glasses such as Pyrex. Boron halides are volatile molecular compounds. They are Lewis acids and react violently with water. The subhalides consist of boron chains or clusters that have terminally bound halogen atoms. They are substitution derivatives of the lower boranes. 

In conclusion, in this study the sol-gel synthesis of ZnO nanoclusters embedded in silica has been faced by the gel-derived binary system ZnO-Si02. As compared to oxide mixtures prepared by conventional solid-state reactions, in gel-derived systems the formation of ceramic silicate compounds is observed at lower temperatures than expected for traditional melting processing [173]. Through XPS it was possible to evidenee the importance of the choice of proper precursor compounds and proper annealing conditions to produce stable ZnO guest clusters dispersed into the host silica glassy matrix. 

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