Physica C 173 (1991) 37-50 North-Holland Stoichiometry and superconductivity in single layer Bi2+xSr2_yCuO6_+ R.M. Fleming, S.A. Sunshine, L.F. Schneemeyer, R.B. Van Dover, R.J. Cava, P.M. Marsh, J.V. Waszczak, S.H. Glarum and S.M. Zahurak AT&T Bell Laboratories, Murray Hill, NJ 07974, USA F.J. DiSalvo Department of Chemistry, Cornell University, Ithaca, NY 14853, USA Received 11 September 1990 An insulating polymorph with a composition close to Bi2Sr2CuO 6 complicates the synthesis of the single copper layer supercon- ductor Bi2+xSr2_yCuO6e~. 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= O. Like to two-layer compound, the material has a superlattice due to a buckling of the planar structure, however the superlanice coher- ence is degraded by defects at finite values ofx and y. Density measurements indicate that the nonstoichiometry of Bi2 +~Sr2_yCuO6_+ 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 arm ). The result is a bulk superconductor ( Tc~ 10 K). I. Introduction A superconductor with a single copper-oxygen layer such as Bi2+xSr2_yCuO6_+ ~ is interesting for several reasons. For example, the low transition tem- perature (10 K) seen in Bi2+xSr2_yCuO6_+6 allows one to measure normal state transport properties to lower temperatures than in other high-To materials [1]. In addition, it may be possible to dope Bi2+xSra_yCu06±6 samples arbitrarily close to su- perconductivity allowing one to study the transition from an insulating to a superconducting phase [2]. At the same time, however, the preparation of Bi2+xSr2_yCuO6_+ 6 has been remarkably frustrating because of the difficulty in obtaining single phase material. All of the early reports of superconductiv- ity in this material have been on multiphase ce- ramics [2-10] or on crystals prepared at a point where the equilibrium properties of the structure are multiphase [ 1,2,11-14]. We now know that the syn- thesis is complicated by an insulating phase with a composition that is very close to the stoichiometric composition Bi2Sr2CuO6 [ 14,15]. The insulating phase is stable with air synthesis, but is suppressed at higher oxygen pressures [16]. In this paper we have confined the study of Bi2+xSr2_yCuO6_+~ to sin- gle phase material and we have systematically ex- plored the relationships between the structure and the properties of this material. An issue that is central to understanding the car- rier doping mechanism in Bi2+xSr2_yCuO6+_6 is the role of the superlattice. Superlattices are common to oxides in general and are present in most high-To materials. In some materials such as oxygen defi- cient Ba2YCu3OT_+~ [17-20] or Nd2_xCexCuO4+a [20,21] a compositional modulation results from oxygen defect ordering. In Ba2Yfu307±6, interplay between oxygen ordering and doping of two differ- ent Cu sites leads to a complicated and controversial relationship between the superlattice and supercon- ductivity [22,23]. In other materials like Bi2Sr2CaCu208, a displacive modulation plays a more straightforward role by being the sole mechanism for adding extra oxygen to the structure [20,24]. In all of these materials the role of the superlattice is cou- pled with the question of the relative importance of 0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)