STOICHIOMETRY AND SUPERCONDUCTIVITY IN SINGLE LAYER BI2+XSR2-YCUO6+/-DELTA

An insulating polymorph with a composition close to Bi2Sr2CuO6 complicates the synthesis of the single copper layer superconductor Bi(2+x)Sr(2-y)CuO6+/-delta. We concentrate on the region where the material is single phase in air synthesis (2.18 < x < 2.29, 1.44 < y < 1.7). The physical properties from insulating to marginally conducting as the composition moves toward x = y = 0. Like to two-layer compound, the material has a superlattice due to a buckling of a planar structure, however the superlattice coherence is degraded by defects at finite values of x and y. Density measurements indicate that the nonstoichiometry of Bi(2+x)Sr(2-y)CuO6+/-delta is not accompanied by the formation of a large number of vacancies. The insulating polymorph, Bi2Sr2CuO6, can be suppressed by synthesis in modest oxygen pressures (above 5-10 atm). The result is a bulk superconductor (T(c) almost-equal-to 10 K).