Iono-covalent materials

Models of electronic structure in iono-covalent materials application to polar surfaces... 

In highly ionic compounds, the surface relaxation may be assigned to a competition between short-range repulsive forces and electrostatic attractions, produced by the bond breaking at the surface. In covalent materials, on the other hand, covalent rather than electrostatic forces are responsible for the surface relaxation. We will first develop these two limiting cases, before considering the more intricate problem of mixed iono-covalent materials. 

Under the proper circumstances, most soft chemical processes can allow ready manipulation of the ionic component of many nonmolecular materials. Indeed, the solid-state literature has seen an enormous growth in the number of reports, wherein the utility of these synthetic strategies are exploited. However, these methods typically leave the covalent framework of iono-covalent strucmres intact. It would be extremely desirable to exercise kinetic control over the entire strucmre of a solid, thereby maximizing the ability to tune its properties. 

The word ceramics is derived from the Greek keramos, meaning solid materials obtained from the firing of clays. According to a broader modern definition, ceramics are either crystalline or amorphous solid materials involving only ionic, covalent, or iono-covalent chemical bonds between metallic and nonmetallic elements. Well-known examples are silica and silicates, alumina, magnesia, calcia, titania, and zirconia. Despite the fact that, historically, oxides and silicates have been of prominent importance among ceramic materials, modern ceramics also include borides, carbides, silicides, nitrides, phosphides, and sulfides. 

The GeSe2 system has been the subject of extensive first-principles molecular dynamics simulations in which the electronic structure is taken into explicit account, as befits a material in which the electronegativities of the different chemical species are similar and the bonding takes an iono-covalent character [24-40]. 

In Class-II materials components are chemically linked by strong covalent or iono-covalent bonds. The molecules used as starting building blocks possess two distinct functionalities alkoxy groups (R-O-M bonds) and metal-carbon (M-C) links. The alkoxy groups can be formed into an oxo-polymer network by hydrolysis-polycondensation reactions in a sol-gel. Hybrids can be obtained from organically modified silicon alkoxides such as polyfunctional or polymer functionalised alkoxysilanes. The network-forming functionalities can be covalently connected in a sol-gel in several ways ... 

The first chapter of the book summarizes classical approaches, introduces the concept of ionicity, and describes the mixed iono-covalent character of the oxygen cation bond in bulk materials. The next three chapters focus on the characteristics of the atomic structure (relaxation, rumpling and reconstruction effects), the electronic structure (band width, gap width, etc.) and the excitations of clean surfaces. Metal-oxide interfaces are considered in the fifth chapter with special emphasis on the microscopic interfacial interactions responsible for adhesion. The last chapter develops the concepts underlying acid-base reactions on oxide surfaces, which are used in catalysis, in adhesion science, and in colloid physics, and discusses their applicability to the adsorption of hydroxyl groups. A comprehensive list of references is included. 

The essentially iono-covalent, non-metallic compounds that constitute ceramics are compounds formed between metals and non-metals. The opposition - the word is not too strong - between a metallic material and a metallic oxide can be illustrated by the comparison between a metal, aluminum, and its oxide, AI2O3 (alumina in English, where oxides are named by adding the ending a to the name of the metal aluminum/alumina, silicon/silica, magnesium/magnesia, uranium/urania, etc.) ... 

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