Study On The Relationships Between Chemical Bond And Properties Of Rare Earth Garnets And Lanthanide Layered Superconductors

The dielectric theory developed by Phillips, Van Vechten and Levine has been extended to complex crystals and lots of successful examples have proved the correction of the theory when we applied it to the calculation of chemical bond parameters and estimation of the macroscopic properties. Based on the theory, the quantitative relationships between chemical bond parameters with hardness and isomer shift were established, and the variational regularity of macroscopic properties induced by the changing of microscopic properties were studied.According to the dielectric theory of complex crystals and microscopic theory of hardness, the chemical bond parameters and hardness of yttrium aluminum garnet were calculated and the hardness value was 12.04 GPa. For comparison, we measured the microhardness of yttrium aluminum garnet, which was synthesized by sol-gel method, and the measured value was 14.05GPa. There was a relatively good agreement between these two values. We also calculated chemical bond parameters and hardness of other rare earth garnets. The results showed that there were close relationships between bond length, ionicity and hardness, i.e.,a shorter bond length and smaller ionicity of chemical bond would lead to a higher hardness. The relationships between bulk modulus, bond order and hardness were discussed. They showed the similar tendencies, in other words, the hardness and bulk modulus values increased as the unit cell volume of the rare earth garnet structures decreased. All of these relationships were strongly associated with the concrete crystal structures.A calculation method which was more suitable to calculate the chemical bond parameters of layered crystals was proposed based on the dielectric theory of complex crystals. Using the method, we calculated the chemical bond parameters and M?ssbauer isomer shift values of lanthanide layered superconductors and a few of ferric compounds. Besides, the chemical bond parameters and refractive index values of other crystals with different structures have also been calculated. The quantitative relationship between isomer shift, refractive index and chemical bond parameters were established. The calculated results indicated that the change of crystal parameters and bond length will alter the covalency of chemical bond and peripheral electron density of Fe2+ nuclear, which in turn leads to the change of the isomer shift and refractive index. The good agreement between the calculated results and available experimental data proved the accuracy and universality of the theory.