Electronegativity Scales In Crystals And Their Applications

The concept of electronegativity (EN) was proposed by Pauling in 1932, which describes "the power of an atom in a molecule to attract electrons to itself". With the development of new materials and the increase of interdisciplinary cooperation, EN has been a basic atomic parameter which is widely used in the fields of chemistry, physics and materials science. EN is closely related to many properties of materials such as superconductive, magnetic and optical properties, and it can be used to simplify the complicated molecular phenomena and properties of materials to some quantitative correlations, which provides a theoretical basis for the design of new materials, In this thesis, we develope the concept of EN by including the specific bonding environment of atoms in crystals, and then apply EN to the study of structure-property relationship of materials and the design of superhard materials. The detailed contents are listed as following:Based on the effective electrostatic potential of ions, an ionic EN model is proposed. According to this model, the EN values of 82 elements with different valence states, coordination numbers and spin states are quantitatively determined, which is the most comprehensive EN scale so far. Due to the detailed consideration of actual chemical environment of ions, these EN values well reflect the electron-attracting power of ions. This EN scale can not only be well used to estimate some useful physical and chemical parameters such as the Lewis acid strength and the hydration free energy of cations, it can also be used to predict the charge transfer energies of trivalent lanthanides in inorganic crystals, which provides a useful guide to the design of new materials.Based on the electrostatic potential of elements in covalent crystals, an EN model of elements in covalent crystals is proposed. According to this model, the EN values of 58 elements with different bonding electrons and coordination numbers are determined, which are satisfactorily used to predict the elastic moduli and electronic polarizabilities of covalent crystals. This scale is an important supplement of the ionic EN scale, which is very helpful to the study of the structure-property relationship of covalent materials.According to Sanderson's idea of EN equalization, the definition of bond EN is proposed. Based on the electron-holding energy per unit volume, atomic stiffness, ionic stiffness and bond hardness are defined, and the nature of material hardness is investigated at these three microscopic levels. It is found that the hardness of materials is essentially determined by the electron-holding energy of its constituent chemical bonds per unit volume, by which a microscopic model for identifying the hardness of materials is established. By using this model, the hardness of various materials can be satisfactorily predicted solely in terms of EN of constituent atoms and crystal structure data. Moreover, the elemental combinations which may form superhard materials are selected, which is of great significance to the design of novel superhard materials. Furthermore, the hardness of various phases of group IVA and IVB nitrides is systematically studied on the basis of current model for hardness. It is found that for compounds made of light elements such as boron, carbon and nitrogen, the diamondlike structure is the hardest one among all possible structures, whereas high coordination number is the general requirement for heavy-element compounds to achieve high hardness. Finally, the hardness model is extended based on bond density, bond strength and degree of covalent bonding, which are three determinative factors for the hardness of materials. The newly developed model for hardness can also be well used to predict hardness.