Theoretical Study On Chemical Bond Of Complex Layered Ferroelectric And Superconductor

Theoretical studies can provide reliable guidance for developing excellent materials experimentally.The bismuth-layered structure ferroelectrics,copper oxide superconductors and iron-based superconductors were studied in this dissertation,all of these compounds have complex layered structures.Start from the crystal structure data and based on the space group theory,the valence bond parameters of chemical bonds were studied by atomic environment calculation（AEC）and empirical electronic theory of solids and molecules（EET）,and combined with Phillips–Van Vechten–Levine dielectric method（PVL）and Brown bond valance model,the relationship between bonds parameters and physical properties of crystals were further discussed.According to the space group theory of crystal,the tetragonal SrBi₂Ta₂O₉、Bi₄Ti₃O₁₂ and SrBi₄Ti₄O₁₅ crystals were constructed on the basis of their orthorhombic crystal structure parameters.Atomic coordinates of the reconstructed structure were consistent with the experimental data,and the atomic displacements in the phase transition were calculated by the atomic coordinate analysis.Based on the atomic displacements in the phase transition,it is determined that the spontaneous polarization of SrBi₂Ta₂O₉、Bi₄Ti₃O₁₂ and SrBi₄Ti₄O₁₅5 were occurs mainly in the ab plane,and the direction of polarization is approximately parallel to the axis a.Furthermore,the effective valence electrons numbers of each atom in crystal were calculated by the valence electron structure parameter n_αand identical bond number I_αbased on the EET.According to the relationship between the ferroelectric spontaneous polarization and the atomic displacements,the calculated spontaneous polarization along a axis in SrBi₂Ta₂O₉,Bi₄Ti₃O₁₂ and SrBi₄Ti₄O₁₅5 were 18.14μC/cm²、41.67μC/cm² and 25.81μC/cm²,Respectively.The results are in good agreement with the experimental values and other theoretical calculated values.Base on the EET electronic theory,Brown bond valence theory and PVL dielectric theory,as examples of bismuth layered ferroelectrics(SrBi₂Ta₂O₉,Bi₄Ti₃O₁₂),copper oxide superconductors(YBa₂Cu₃O_7-δ)and iron-based superconductors（LaOFeAs,BaFe₂As₂,perovskite-like blocks intercalation iron-based superconductors）,the influences of ion doping and atomic site occupation probability on the crystal structure and physical properties were mainly discussed.The PVL calculations show that the ions doping have less effect on ionicity of chemical bond in bismuth layered ferroelectrics Bi_4-xLa_xTi₃O₁₂2 and Sr_1-xBi_2+xTa₂O₉,and relative variation is no more than 1%.On the basis of the bond dipole moment,the relationship between the spontaneous polarization and ionicity of chemical bond was established,and the spontaneous polarizations of Bi_4-xLa_xTi₃O₁₂ and Sr_1-xBi_2+xTa₂O₉ along a axis were calculated.The results show that the more ions doped in the A site atom of bismuth layered ferroelectrics,the less spontaneous polarization obtained.The symmetry of the local cluster structures and the influence of that on superconductivity were comparatively analyzed based on the crystal structure and atomic local cluster structures of YBa₂Cu₃O_7-δ.The effective valence electrons numbers of each atom in crystal were calculated by EET theory,the results were consistent with the bond valence sum（oxidation state）of Brown bond valence theory.The change of O content in YBa₂Cu₃O_7-δresulted in the change of its valence electron structures.The change trend of effective valence electrons numbers of Cu2 and Y withδand the change trend of Tc withδshown a certain regularity,the result indicates that superconducting properties of YBa₂Cu₃O_7-δcrystal is closely associated with the atomic charge distribution in conducting layer.The impact of structural phase transition,temperature variation and ion-doped on the valence electron structure of iron-based superconductors were discussed based on the EET calculation.The results show that the structural phase transition and temperature have little effect on valence electron structure parameters,but the ion doping has a significant influence on it.The effective valence electrons numbers can be used to explain the doped mechanism.Since the F^-doped in LaOFeAs,the number of effective valence electrons were increased,it is equal to introduction more electrons in LaOFeAs,called as electron-doping.Sr²⁺doped in LaOFeAs,the number of effective valence electrons were decreased,which brings more holes into the system,called as hole-doping.K⁺doping in BaFe₂As₂ decreased the number of effective valence electrons of each atom,that means increased the number of holes in the crystal.The ion-doped iron-based superconductors show the superconductivity under the superconductor critical temperature,which indicate an increase in the number of electrons or holes in crystal influences the superconductivity to some extent.Converting the occupation probability problem of atomic doping and substitution to the problem of crystal chemical composition,which can provide a theoretical reference for experiment and process research and provide reliable information for design and development of new materials.