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 lat tice parameters, phonon vibrational feature, electronic structure, as well as transport properties in Bi2Sr1.8La0.2CuOy. The experimental da ta show that both Ni and Zn are soluble in the Bi2Sr1.8La0.2CuOy mater ial up to about 10% of the Cu content. In Bi2Sr1.8La0.2Cu1-xNixOy syst em, the superconducting critical temperature T-c decreases linearly wi th Ni concentration and the rate of T-c decrease is around 2.5 K/at.%, while for Bi2Sr1.8La0.2Cu1-xZnxOy system the dependence of T-c on the Zn dopant concentration deviates from a linear behavior and the Zn su bstitution 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 Bi2Sr1.8La0.2Cu1-xNixOy and B i2Sr1.8La0.2Cu1-xZnxOy systems shows that the Zn substitution leads to a more serious structure distortion than the Ni substitution. In addi tion, 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 pos sible explanations for the stronger suppression of T-c by the Zn subst itution than by the Ni substitution are provided, based on these exper imental results.