Bi 2−Z Pb z Sr 2−x (La/Nd) x CuO y (z=0 and 0.4, x=0.2–1.0) was synthesized in air and nitrogen atmosphere. Valence analyses by dichromate titration and iodometry showed Bi valence larger than 3.0 and Cu valence close to 2+ over the wide range of x, being difficult to explain superconductivity. The Tc depends on substituent lanthanoid, La or Nd. The maximum Tc was achieved at x=0.4–0.5 in all these series.

Superconductivity of Bi2−xPbxSr2−xLnxCuOy(Ln=La,Nd) and valence analysis of Bi and Cu

The structure and superconductivity in the single CuO layer bismuth (2021) system can be stabilized also by the doping of Na 1+ ions. Contrary to the La-doped Bi 2 (Sr,La) 2 CuO 6+ δ system with T c enhancement up to 35 K. T c remains around 10 K for the Na-doped Bi 2 (Sr 2− x Na x )CuO 6+ δ system (0.0 ≤ x ≤ 0.4). The variation of T c with the CuO bond length r CuO in the CuO layer for the Bi family which consists of the systems (Bi,Pb) 2 Sr 2 CuO 6+ δ , Bi 2 (Sr,Na) 2 CuO 6+ δ , Bi 2 (Sr,La) 2 CuO 6+ δ and (Bi,Pb) 2 (Sr,La) 2 CuO 6+ δ indicates an optimum r CuO value of 1.906–1.913 A with maximum T c of 35 K for these Bi(2021) compounds.

The correlation between superconductivity and CuO bond length for the single CuO layer compounds with the Bi2Sr2CuO6+δ-type (2021) structure

Vortex lattice melting for temperature near transition temperature T C was observed for 32 K superconductor (Bi 1.6 Pb 0.4 )(Sr 1.5 La 0.5 )CuO 6+; through the temperature dependence of magnetization in static applied field B a for both field-cooled (FC) and zero-field-cooled (ZFC) measurements. The irreversibility line (de-Almeida-Thouless line) H * (T) in the low field region (B a c ) 3/2 . Low-field magnetic phase diagram shows very small lower critical field H cl (T) with H cl (0 K) = 26 G. The effects of grain geometry and magnetic relaxation are discussed.

Magnetic phase diagram of 32 K superconductor (Bil.6Pb0.4)(Sr1.5La0.5)Cu06+δ

Single crystals of Bi 2 Sr 2 CuO 6+δ were synthesized and characterized by X-ray diffraction. The Bi(2201) sample remains not superconducting or magnetically ordered down to 5 K. With an applied magnetic field perpendicular or parallel to the c -axis, Magnetic measurements yielded anisotropic magnetic susceptibilities ϰ c ( H ⊥ c ) and ϰ ab ( H ‖ c ). Both can be fitted to the sum of a temperature-independent ϰ 0 and a Curie-Weiss term C /( T +θ p ) in the temperature range between 5 and 300 K, with a common Curie constant C = 1.0 × 10 −2 cm 3 K /mol and Curie-Weiss paramagnetic intercept θ p =+1.1 K . Since the resulting Curie-Weiss term is the same for both directions, the overall anisotropy Δϰ ≡ ϰ c — ϰ ab = Δϰ 0 is also temperature independent. Its magnitude, 3.3×10 −5 cm 3 /mol, however, is lower than the corresponding values for Bi(2212) (2.3×10 −4 cm 3 /mol) and Bi(2223) (5.8×10 −4 cm 3 /mol).

Anisotropic magnetic properties of Bi2Sr2CuO6+δ single crystal

Oxygen contents of Bi-2201, Pb substituted Bi-2201 and T1-2201 were determined by iodometric titration and the relationship between Tc and oxygen content was examined. Oxygen number of Bi-2201 series increases with La substitution for Sr and reaches 6.52 with Lanthanum content of 1.0 in air annealed specimen. Lattice parameter, a, changes with La and Pb substitution, indicating its dependence on Cu charge. La and Pb atoms act as phase stabilizer of 2201 through charge control and structural stabilization respectively.

J. B. Shi

Papers

Superconductivity Enhancement in the Bi, (Sr2-x Lax)CuO6-δ, System