M. Schlichenmaier

New family of superconducting copper oxides: Bi cuprates of type 1212

Abstract Bi cuprates of type 1212 have been synthesized above 970°C from the proper starting materials. The oxides of idealized composition (Bi 0.5 Cu 0.5 )Sr 2 YCu 2 O 7− z crystalline in the tetragonal space group P4/mmm ( a=3.815(5), c=11.73(1) A ) and show solid solution behaviour. The as-prepared materials are non-superconducting above 5 K. However, post-treatment in flowing oxygen at moderate temperature (≈500°C) introduces superconductivity. The highest superconducting transition temperature is situated at 68 K for a sample of nominal composition (Bi 0.5 Cu 0.5 )Sr 2 Y 0.8 Cu 2.2 O 6.95 .

Structural changes of the superconductor YBa2Cu3O7 by cobalt substitution. A high-resolution neutron powder diffraction study

Abstract The structure of cobalt substituted YBa 2 Cu 3− x Co x O 7± z compounds with x =0.05, 0.1, 0.2, 0.3 and 0.65 was determined by high-resolution neutron powder diffraction at 5 and 300 K. The samples are superconducting up to x =0.3. YBa 2 Cu 2.95 Co 0.05 O 7 has orthorhombic structure with space group Pmmm; all compounds with x ⩾0.1 have tetragonal structure with space group P4/mmm. The Rietveld refinements show clearly that the Co-atoms occupy only the Cu(1)-sites at (0, 0, 0), i.e. copper chain sites in the orthorhombic structure, for the whole composition range. The characteristic structural changes as a function of cobalt concentration and temperature are discussed. Along the c -axis the Cu(1) to O(2) distance is decreasing while the Cu(2) to O(2) distance is increasing as in the Co-free, oxygen deficient compounds. A charge transfer from the Cu/Co chains to the Cu planes is discussed. For the nonsuperconducting YBa 2 Cu 2.35 Co 0.65 O 7.23 some magnetic peaks are observed.

System (AO)mM2B2n − 2FenO4n + z with m = 2 and n = 1,2 for A ≡ Bi, Pb and M, B ≡ Sr

Determination des parametres cristallins par diffraction RX pour BiPbSr 2 FeO 6,04 , Bi 1,5 Pb 0,5 Sr 4 5Fe 2 O 10,04

Superconducting properties of the lead-containing 2212 phase Bi1.75Pb0.25Sr2CaCu2O8 + z with Tc in the range 70–98 K

Abstract The lead-containing, nearly single-phase 2212 material of nominal composition Bi 1.75 Pb 0.25 Sr 2 CaCu 2 O 8 + z has a T c of about 70 K after air quenching. By controlling post-treatment, the transition temperature can be raised to about 98 K. Since no significant changes in the oxygen content are observed, we conclude that the function of post-treatment is to produce a more favorable ordering of oxygen in the lattice. Accordingly, all dimensions are influenced by the post-treatment. Additional results for air-quenched 2212 material with variations in the Bi:Pb:Sr:Ca ratio show that differences in the position of T c are also reflected in variations of the cell dimension, thus pointing to an extended field of existence for the 2212 phase in the system BiPbSrCaCuO with phase boundaries which are still unknown.

The onset of oxygen diffusion in YBa2Cu3O7 − δ: An investigation by neutron diffraction

Abstract In order to obtain information on the oxygen mobility in orthorhombic YBa 2 Cu 3 O 7 − δ , we carried out in situ neutron diffraction measurements at the D2B high resolution diffractometer at Grenoble in the temperature range 300 K ⩽ T ⩽ 1000 K under a constant pressure of 1 atm O 2 . From the data evaluation using the Rietveld method we conclude a maximum oxygen occupancy at T ≈ 600 K with δ = 0.09. The mean square amplitude B 11 of the O4 site rises almost linearly up to 800 K and then increases sharply. We take this temperature as the onset of enhanced oxygen diffusion occurring at the order-disorder phase transition within the orthorhombic phase. We use the temperature dependence of the mean square delocalization of O4 to obtain an estimate of the energy barrier ΔV PER hindering diffusion. ΔV PER starts to decrease appreciably as soon as the temperature exceeds 800 K and comes down to a value ΔV PER ≈ 1200 K at a temperature of 950 K. Thermal activation is therefore strong enough for diffusion to occur.

Influence of the preparation technique on the electrical properties of the 2212 high Tc superconductor Bi1.75Pb0.25Sr2CaCu2O8 + Z

Irrespective of the preparation technique, the optimal hole concentration is always conserved. More probably the increase in T c is due to an improvement in texture and crystalline order of the 2212 system, due to a reduction in stacking faults

The high Tc phase (n = 3) in the BiPbSrCaCuO system

Abstract The influence of the Bi : Pb : Sr : Ca : Cu ratio on the electrical properties of the lead-rich high Tc phase (n = 3) of ideal composition Bi2−xPbxSr2Ca2Cu3O10+z (x ≈ 0.25) is studied. The highest transition temperature Tc(mid) = 114 K is obtained for x = 0.25 after heat treatment in sealed silica ampoules. Deviations from the ideal stoichiometry may reduce the total annealing times at the expense of the superconducting properties.

Structural and electric properties of the 2201 phase in the system Bi/Pb-Sr-Cu-O

Abstract The effect of lead substitution on the structure, the oxygen content and the electrical properties of the 2201 system is studied. The “ideal” 2201 phase is absent for the ideal stoichiometry Bi2−xPbxSr2CuO6+z. However, a substitution of lead for strontium (system Bi2Sr2−xPbxCuO6+z) as well as a combined substitution of lead and bismuth for strontium (Bi2Sr2−x−yPbxBiyCuO6+z or lead for strontium and bismuth (B2−wSr2−xPbw+xCuO6+z) results in the formation of the 2201 phase. With increasing lead content the tetragonal subcell becomes orthorhombic and the specific resistivity increases. For a minimal lead content of x = 0.25 no transition to superconductivity was observed; however, in the semiconducting regime the temperature dependence of the specific resistivity is complex. Any attempt to prepare calcium containing 2201 phases results in the formation of a phase mixture between 2201 and 2212.

Stacking polytypes in the systems Bi-Pb-Sr-Ca-T-O with T = Mn, Fe, Co, Ni, Cu

The influence of chemical substitution on the properties of the stacking polytypes (AO) m M 2 B n−1 T n O y (A, B, M = (Bi, PB), Sr, Ca) with T = Mn, Fe, Co, Ni, Cu is studied for the systems Cu-2212 (Bi 1 6 Pb 0.4 Sr 2−x Ca 1−y Nd x+y Cu 2 O 8+z Bi 2−x Pb x Sr 2 Ca 1−y Gd y Cu 2 O 8+z ); T-2201 (BiPbSr 2 TO 6+z TO 6+z ; T=Fe, Co, Ni) as well as for the series T-2212 and T-3212 with T = Mn, Fe.