| The applications of the energy bond theory and quantum chemistry approach have been limited to simple and small compound systems. Since the multicomponent compound systems are dominant in the novel materials, it is necessary to provide the theory suitable for these systems. The concept of chemical bond in crystals has been applied in chemistry and materials science. Especially noteworthy is the dielectric theory of chemical bond developed by Phillips and Van Vechten, which has been applied in the design of binary compounds. This thesis provides a method in which the issue of characterizing a complex compound with several bonding types can be transformed into one of characterizing a series of separated binary compound with single bonding by introducing bonding formula, describes systematically a dielectric theory of chemical bond suitable for complex crystals, which makes us treat and analyze conveniently the bond characteristics in complex crystal. Considering most of the functional materials containing some transition-metal ions with d electrons which contribute to their bond characteristics, a calculation method of chemical bond parameters for 3d transition-metal compounds was presented by using a correction factor to consider the specific effects of 3d electrons on the bonding.According to the empirical rules of influence of the chemical bond on spectrum, electron spectroscopy and Mossbauer spectroscopy. the chemical environmental factor uas further defined with the covalency and polarizability of a bond. Since the factor can effectively describe the change trends of the chemical shifts in the above spectra, the bond nature of the chemical shifts was revealed. The clear quantitative relations between the environmental factors and Mossbauer isomer shift of various ions such as ?7Fe and Sn were obtained, we can predict the isomer shifts in all kinds of complex crystals by using this theory. On the one hand, these relations will be useful for us to analyze Mossbauer spectroscopy, or otherwise, the experiments in Mossbauer spectroscopy can verify validity of the chemical bond dielectric theory for the complex crystals.Hexagonal ferrites are perdendicular magnetic recording and microwave-absorbing materials with potential applications. M- and W-type hexagonal ferrites were prepared by using chemical precipitation method, and characterized by DTA, SEM, XRD. TEM and Mossbauer spectroscopy. By using the chemical bond dielectric theory for complex crystals, we studied the properties of the chemical bonds and Mossbauer isomer shifts inthe M-, W- and R-type hexagonal ferrites, solved the issue in the calculation of Mossbauer spectra resulted from various crystallographic positions in hexagonal ferrites. The calculated results of Mossbauer isomer shifts are in agreement with their experimental values. This theory provides us a new and valid method further to study the bond properties of the complex hexagonal ferrites.By using the chemical bond theory for complex crystal, the chemical bond properties of Y-, Bi-, Tl- and Hg-based superconductors were studied. The results show that the CuO bonds manifest more covalent character, which was in agreement with that from the effective valence and electronic charge-density figure in the literature. However, our quantitative calculations make images of the chemical bonding more clearly. Mossbauer isomer shifts of "9Sn doped in 123, 214, Bi-2212, Bi-2223, Tl-1223 and Hg-1223 superconductors were calculated by using the chemical environmental factor. We found a very good agreement between the theoretical and the corresponding experimental results. This illustrates the reasonableness of the calculated chemical bond parameters. In addition, Mossbauer isomer shifts of 57Fe doped in 123, 214, Bi-2201, Bi-2212, Bi-2223, Ti-2212, Tl-2223, Tl-1223 and Hg-1223 superconductors were also calculated by using the chemical environmental factor. The calculated results of main Doubles were agreed to the reference values. The assignments of the other small Doubles were also discussed in de...
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