Research On The Relationship Between Cluster Effect And Variation Of Valence Electron Density In Materials

Positron annihilation technology is a new kind of detection technology, it is very sensitive to microscopic defect in material, the body life variance in different site reflects the defect distribution,defect size and local valence electron density. Therefore, positron annihilation technology plays an important role in the study on microstructure of materials.For most solid material, element substitution is a kind of common research methods. When the alternative elements rip into solid material, it will take up the specific position, and influence the material structure, electronic behavior and defects. We measured the variable parameters by XRD, positron annihilation technology etc, then we can study the solid material properties and physical mechanisms of some phenomena. Large number of experimental results show that, with the increase of alternative quantity, the substitutable element in matrix tend to form clusters.Generally speaking, the atom radius of substitutable element is similar to matrix atoms, but the valence state will change in most cases. As the doping ions ripping into the matrix, part of matrix atoms are expelled, and the valence electron density of alternative location will change due to the difference of valence state between doping ions and matrix ions. In low doping concentration, the alternative ions exist in isolation. When the valence of alternative ion is higher than the replaced element, the lost valence electron near the replace area will increase accordingly, the capture rate of positrons is reduced, then the local covalence density step down, with the increase of alternative content, part of substitution ions will gather and form clusters. This is the request of lowest energy principle. Based on the consideration of valence balanced, the clusters formed by high valence ions will attract more anions into the lattice, the local covalence density of clusters increased. Conversely, in the case of low valence alternative, as the valence of alternative elements is lower, the lost valence decreases, the local covalence density rising up, the body life of positron in material get shorter, while doping concentration reaches a certain value, low-valence alternative ion will also form clusters. Nevertheless, because part of anions will be expelled, in the process of forming clusters, and lead to the lower local covalence density compare with the electronic density in pure samples.In high temperature superconductors, the typical substitution in Cu sites consist of magnetic ions such as Fe, Co, Ni and nonmagnetic ions Al, Zn, etc. When they ripping to the lattices and alternative the Cu sites, there will cause some change in electronic distribution, carriers concentration and defect distribution, and then influence the superconductivity. When measuring with positron annihilation experiment, the body life of positron will tend to be saturated gradually with the doping quantity. What result in the phenomenon?Over a very large number of experiments, we found that the phenomenon of covalence density and positron life saturation are almost existed in all kinds of materials, but there is still no one in-depth study on the nature of this phenomenon. Some people think this is caused by the material defects, but the longer part of positron life do not support the explanation. In addition, we can clearly see that the covalence density in materials is saturated after a fluctuate period. This shows that there are at least two interaction mechanisms in material, they are competing with each other. In certain doping concentration later, two mechanisms achieve a balance, both the influence on covalence electron density cancelled out. And then they can achieve the balance.As is known to all, clusters are widespread in all kinds of materials, when the same or similar elements gather together in certain concentration, they will attract each other and form the clusters. Due to the particularity of structure, clusters have some strange properties, such as quantum size effect, etc. According to the analysis of experimental results, clusters effect is the deciding factor of covalence density saturated detected in all kinds of materials. This paper selects several comparatively typical materials, such as high Tc superconductors YBCO, BaTiO3 ceramics, photoelectric ceramics PLZT, heavy metal oxide glass,10% PbS sulfide glass, TiAl alloy, we analyzed the lifetime changes in these doped materials respectively, discussed the possible influencing factors of covalence electron density. Finally, with the competing theories of clusters effect and isolated ion effect, we explained the reason of covalence density and positron body life saturation phenomenon in various materials reasonable.