Hole filling and hole creation in the superconducting compounds Bi2Sr2-xRxCuO6+y (R=La, Pr, Nd, and Sm)

The superconducting properties, hole content, and lattice constants of several rare-earth-doped compounds Bi2Sr2-xRxCuO6+y have been investigated as a function of rare-earth doping and oxygen content. The maximum superconducting transition temperature, Tc, obtained for each of the rare-earth dopants was 23 K (La), 25 K (Pr), 21 K (Nd), and 12 K (Sm). The Tc's of these compounds exhibited a parabolic dependence on the La and Pr concentration (0<x<1), with the maximum Tc occurring for x=0.4 for samples with low oxygen content. For La- and Pr-doped samples treated in pure oxygen, the position of the maximum shifted to x=0.6 and 0.5, respectively, with a concurrent reduction in the maximum value for Tc (17 K for La and 18 K for Pr). For x>0.2, the samples were single phase and exhibited unusually sharp superconducting transitions (width of ac-susceptibility transition less than 2 K). The high quality of the samples allowed a precise study of the variation of the orthorhombic cell parameters with the rare-earth and oxygen content. The oxygen content of the samples was systematically varied using oxygen partial pressures that ranged from 100 ppm to 100% oxygen during the heat treatments. Absolute values of the oxygen content and the changes of the oxygen content with heat treatments were determined using thermo- gravimetric analysis. The qualitative features of the dependence of Tc on x and y can be understood by assuming an excess of holes in the Cu-O planes of the undoped compound that are filled by the extra electrons contributed with rare-earth doping or increased with the addition of oxygen. For La-doped samples treated in pure oxygen there was a smooth variation of Tc with hole content, with the maximum Tc (18 K) occurring at a hole content of 0.20.02. For La-doped samples treated in 0.1% oxygen, the maximum Tc occurred at the same hole content (0.2), but the value of Tc was significantly higher (23 K). These data illustrate that, even within one alloy system, composition can affect Tc as well as hole content. Finally, lattice-constant and oxygen-content data from the Pr-doped materials suggest that Pr substituted onto the Sr site has a formal valence of approximately 3.3.