P.V.P.S.S. Sastry

Zero resistance at 120 K in Bi(Pb)-Ca-Sr-Cu oxide

Abstract Bulk Bi 1.6 Pb 0.4 Ca 2 Sr 2 Cu 3 O y samples synthesised by a precursor matrix reaction method show a zero resistance state at 104 K after long-term sintering at 1140 K. Samples subjected to an additional heating close to the melting point, for short durations, attain a record zero-resistance temperature of 120 K. Superconducting behaviour of these samples is stable and does not show any degradation upon thermal cycling.

On the preparation and structure of the 81 K single phase superconductir in the BiCaSrCuO system

Abstract A new and elegant method to synthesize a single phase 2122 compound in the Biue5f8Caue5f8Srue5f8Cuue5f8O system is described. Electrical resistivity measurements show the superconducting transition temperature ( R = 0) at 81 K. X-ray and neutron diffraction analysis indicate that the single phase compound has cell parameters a = 5.362(6) A , b = 5.443(6) A and c = 30.77(2) A and space group Fmmm. Results of neutron diffraction profile analysis show that the orthorhombic distortion is significantly larger than hitherto reported and also indicate possible substitution of Bi for Sr/Ca.

Evidence for K-substitution in the Tl-sites of superconducting Tl2CaBa2Cu2Oχ: Neutron diffraction studies

Abstract A comparison of the Rietveld profile refinement of neutron diffraction data of the superconducting compounds Tl-2122 (i.e. Tl 2 CaBa 2 Cu 2 O χ ) and Tl 1 K 1 Ca 1 Ba 2 Cu 2 O y provides evidence that nearly half of the Tl-sites belonging to the pure Tl-2122 structure get substituted by K-ions in Tl 1 K 1 Ca 1 Ba 2 Cu 2 O y . The consequence of this is reflected in a substantial movement of Ba-ions towards the plane of the O(2)-ions in the K-substituted compound.

Novel structural features of Pb-stabilised Bi-2223 high-Tc phase from neutron-diffraction study

Abstract Detailed structural features of the Pb-stabilised Bi-2223 superconductor (Tc = 110 K) are obtained, for the first time, from profile refinement of neutron diffraction data on single-phase polycrystalline samples of Bi1.7Pb0.3Ca2Sr2Cu3Oy (nominal). The refined structure (space group Amaa, a=5.399(6) A , b=5.413(6) A and c=37.13(2) A ) reveals that Bi-layers are free of Pb and contain nearly 30% vacancies at Bi-sites. The basal Cu(2)-O bonds are tilted to such an extent that they lead to a significant occupancy of oxygens in the Ca-plane, feature not observed thus far in Bi- or Tl-based high-Tc oxides. This also brings about a bridging between Cu(2)- and Cu(1)-layers. Detailed analysis of the structural features suggests an occupancy of ≈ 10% of the Ca-sites by Pb4+-ions.

Towards the synthesis of the single-phase Bi-2223 superconductor from stoichiometric (Bi, Pb)2Ca2Sr2Cu3Oy compositions

Abstract In order to establish the ideal nominal composition and processing conditions for the synthesis of the single-phase Bi-2223 superconductor in Bi(Pb)-Ca-Sr-Cu-O system, we have synthesised samples with stoichiometric composition Bi 2− x Pb x Ca 2 Sr 2 Cu 3 O y ( x = 0.0, 0.2, 0.3, 0.4 and 0.5) using a precursor matrix method. A single phase Bi-22230998 product is obtained for the composition corresponding to x = 0.3. The best superconducting characteristics (viz. T c = 121 K) are obtained for the single-phase sample if it is subjected to a short-term partial melting step during synthesis. The compositions corresponding to x = 0.2, 0.4 and 0.5 show evidence of contamination by the lower- T c Bi-2122 phase although the R ( T ) plots depict a single transition with a zero-resistance state at 98 K, 120 K and 114 K, respectively. The R ( T ) curve for the Pb-free ( x = 0) sample shows a significant drop at ≈ 110 K although the sample contains Bi-2122 as a majority phase. The transport J c value is at maximum (172 A/cm 2 ) for the single-phase ( x = 0.3) sample.

Superconductivity of Bi-2201 (n=1) as influenced by the substitution of Pb and/or rare-earths (R=La, Nd and Pr)

Abstract Preparation conditions for stabilizing the bulk superconductivity (T c=9.5 K ) in the nominal Bi 2 Sr 2 CuO ∼ 6 composition corresponding to the n=1 phase in the Bi 2 Sr 2 Ca n−1 Cu n O 2n+4 system are described. The samples Bi 2.1 Sr 1 .9 CuO y and Bi 2.2 Sr 1.8 CuO y become superconducting at 10 K and 12 K, respectively. Single-phase compounds (space group Amaa) could be synthesized for the range 0⩽x⩽1.0 in the series Bi 2 Sr 2−x La x CuO 6+y but the orthorhombicity decreased gradual ly with increase in x and the structure became tetragonal at x=1.0. The T c (R=0) value increased to a maximum of 19 K for x=0.4, corresponding to a value of ∼2.2 for the formal valence of copper. For x⩾0.5 a sudden loss of superconductivity is observed. Simultaneous substitution of Pb for Bi, and, La for Sr yields further improvement in superconducting characteristics, hitting a peak of 26 K (R=0) for x=0.4 in the series Bi 2−x Pb x Sr 2−x R x CuO y (R=La). The R=Nd- and and Pr-s ubstituting compounds showed lower values of T c for the same extent of doping (x=0.4). Interestingly, doping of Pb alone (i.e., Bi 2−x Pb x Sr 2 CuO y ) results in suppression of superconductivity altogether. The electrical resistivity, magnetic susceptibility and the cell parameter data are presented for varying levels of different substituents and discussed.

Synthesis and properties of a 125 K superconductor in the Tl-Ca-Ba-Cu-O system

A method for the synthesis of a 125 K superconducting phase having a nominal composition TlCaBaCu2 Ox is described. It is shown that by sintering at 1200 K in flowing oxygen containing a controlled amount of water vapor, a zero electrical resistance state (Tcf) at temperatures as high as 125 K can be obtained. A transport critical current density (Jc ) of ∼240 A/cm2 at 77 K was obtained in this 125 K material.

Superconducting transition temperature of single-phase Tl-2223: Crucial role of Ca-vacancies and Tl-content

Abstract Detailed investigations on the synthesis and phase-indentification of a number of Tl-Ca-Ba-Cu oxides show that single-phase Tl-2223 superconductor with the best transition temperature ( R = 0 at 130 K) can be prepared from a thallium-deficient starting composition, namely Tl ≈1.0 Ca 2 Ba 2 Cu 3 O x . A number of other starting compositions containing Tl: Ca T c -values between 115 K and 130 K. It is argued that the existence of vacancies at Ca-sites is crucial to the production of pure 2223-phase and the highest T c (130 K) is obtained when the number of such vacancies reaches an optimum. Starting compositions deficient in Tl lead to such a possibility by aiding the migration of some Ca-ions to vacant Tl-sites. Under standardised preparation conditions single-phase Tl-2223 structure is obtained for a combined nominal stoichiometry 3 ⩽ (Tl + Ca) ⩽ 4.

Superconducting behaviour of Bi1.7Pb0.2Sb0.1Ca2.0Sr2.0Cu2.8Ox

Abstract Bi 1.7 Pb 0.1 Sb 0.1 Ca 2.0 Sr 2.0 Cu 2.8 O x samples prepared by matrix reaction method show a sharp drop in electrical resistance between 145K and 124K after partial melting and annealing for 50 hours. The dc magnetic susceptibility shows evidence of a bulk superconducting phase at T c ⋍ 109 K but the R(T) curve does not show zero-resistance at this temperature and depicts a foot below 124K. The observed superconducting behaviour is stable upon thermal cycling between 77K and 300K.

Effect of argon-annealing and subsequent oxygen-annealing on the superconductivity and structure of Tl2CaBa2Cu2O8-x

Abstract Annealing the single-phase Tl 2 CaBa 2 Cu 2 O 8 superconductor ( T c =108 K ) in argon environment at 950 K for 24 h brings the T c down to ≅80 K. Re-annealing the samples in oxygen under similar conditions restores the T c to 105±5 K. Rietveld profile refinement of the neutron diffraction spectra of the argon-annealed samples indicates that some Tl-ions, presumably Tl 1+ , move from the Tl double-layers to occupy Ca-sites. Upon subsequent annealing in oxygen, however, these Tl-ions escape from the lattice. Implications of this and other structural features are described.