| In the cuprate superconductors, YBCO system has the typical CuO2 plane and the Cu-O chain structure, thus it is an ideal object in the basic research and attracts a lot of attentions, In order to clarify the physical mechanism of high Tc superconductivity, element substitution plays an important role in the investigation of cuprate superconductors. For the Cu sites substitution, typical magnetic ions Fe, Co, Ni, and nonmagnetic ions Al, Zn have been studied. They enter CuO2 planes or Cu-O chains depending on their valence state. In order to make clear the doping effect of these ions, simulation calculations and positron annihilation technology were used to investigate systemically the high-Tc cuprates YBa2Cu3-xMxO7- (M=Fe, Co, Ni, Zn and Al; x=0~0.5) system, and some significant results are obtained in the present dissertation as follows:I . Based on the Born model, simulated calculations were performed tostudy the formation of doping ion clusters and their distribution characteristics.In order to understand how the doped ions enter the different Cu sites, simulations were made. The calculations are based on the crystal lattice structures with energy minimization principle and formulated within the framework of Born model. Simulations determine quantitatively the effect of doping, the total defect energy and average binding energy in the clusters. As well known, the larger the average binding energy is, the more stable the cluster combines. Consequently, the doped ions should prefer to form the cluster with largest average binding energy. The present results indicate that the doped ions may combine into different clusters, including 2, 4, 6 or 7 ions. +2 valence ions prefer to gather into the cluster of 7 ions as Double Square and +3 valence ions prefer to do the cluster of 6 ions as Hexamer. The cluster effect ishelpful to understand how the superconductivity suppressed by ion substitution. Some previous unclear phenomena may be explained: firstly, this is why the Tc falls faster and faster with the substitution content x increasing; secondly, why the local electron density ne approaches saturation at higher doping content.II. The site occupation and displacemant of the Ni doping ions in YBCO system is clarified. In Ni doped YBCO systems, Ni ions have been thought generally to occupy Cu(2) sites, but various experimental results did not achieve a definite and consistent conclusion. The present work performed systemic study on YBa2Cu3.xNixO7.6(x=0~0.4) cuprates with the x-ray diffraction (RXD), positron annihilation technique (PAT), simulated calculations. The results show that Ni ions occupy Cu(2) sites at lower doping content, and with the doping content increasing, part of Ni ions begin to occupy Cu(l) sites and the occupation rate increases gradually.III. This work proved that the cluster effect is also an important factor to suppress the superconductivity, besides the site occupation of doping ions. It is avital factor that the doped ions in Cu(l) or Cu(2) sites determine their capability in suppression of the superconductivity. At the same time, cluster effect is also an important factor in suppressing the superconductivity, which has not been paid much attention before. The doped ions enter Cu(l) or Cu(2) sites is only determined by their valence state rather than their magnetic or nonmagnetic property. Generally, +2 valence ions enter Cu(2) sites, such as Zn2+ and Ni2+, a small doping content x will induce the increasing of local electron density ne and a drop of the Tc. With the doping content x increases, cluster effect becomes more important, and ne approaches its high saturation. While +3 valence ions enter Cu(l) sites, such as Fe3+, Co3+and Ni3+, a small doping content x will make the decreasing of local electron density ne and a fast drop of the Tc. Accordingly, with the content x increases, cluster effect is more marked, and ne approaches its low saturation. Cluster effect can explain the followingcharacteristics: Firstly, Al ions occupying Cu(l) sites, and the cluster effect of...
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