Effect of Ni and Zn substitutions for Cu in the 25 K phase of Bi-Sr-La-Cu-O superconductors

We investigate the effect of Ni and Zn substitutions for Cu on the lattice parameters, phonon vibrational feature, electronic structure, as well as transport properties in Bi₂Sr_1.8La_0.2CuO_y. The experimental data show that both Xi and Zn are soluble in the Bi₂Sr_1.8La_0.2CuO_y material up to about 10% of the Cu content. In Bi₂Sr_1.8La_0.2Cu_1-xNi _xO_y system, the superconducting critical temperature T_c decreases linearly with Ni concentration and the rate of T_c decrease is around 2.5K/at.%. while for Bi₂Sr_1.8La_0.2Cu_1-xNi _xO_y system the dependence of T_c on the Zn dopant concentration deviates from a linear behavior and the Zn substitution suppresses the T_c much stronger than the Ni substitution. The comparison of the phonon vibrational feature of the apical oxygen atoms in the CuO₆ octahedron between the Bi₂Sr_1.8La_0.2Cu_1-xNi _xO_y and Bi₂Sr_1.8La_0.2Cu_1-xNi _xO_y systems shows that the Zn substitution leads to a more serious structure distortion than the Ni substitution. In addition, the electronic structures of the two doped systems were compared using XPS spectrum. The results show that the Zn substitution reduces Cu oxidation level more remarkably than the Ni substitution. Some possible explanations for the stronger suppression of T_c by the Zn substitution than by the Ni substitution are provided, based on these experimental results.