New oxide superconductors in the Sr-Bi-Cu-O system

New oxide superconductors in the Sr-Bi-Cu-O system were prepared by the conventional processing techniques consisting of mixing, pressing and sintering. The compositions of the superconductors are located in a range surrounded by CuO, SrCu₂O₃ and Sr_0.9Bi_1.1O_2.55. No distinct compositional change in the critical temperature,T_c, is observed and the value is 8 to 9 K at onset and 4 to 5 K at zero resistance. The zero resistance value increases on replacement of strontium by sodium or potassium which have different atomic valences and a nearly equal ionic radii compared to strontium, accompanied by an increase in the positive slope of electrical resistance and reaches a maximum value of 6.7 K for Sr_0.9Na_0.1 Bi₁Cu₂O_4.45 and 5.9 K for Sr_0.9K_0.1Bi₁Cu₂O_4.45. The further increase in sodium and potassium content brings about the disappearance of superconductivity, along with an extreme increase in the negative slope of electrical resistance. The increase inT_c for the Sr-Bi-Cu oxides with the replacement by sodium or potassium is probably because the replacement gives rise to a change in the valence of copper atoms and/or the formation of an oxygen-deficient structure.