H. Rajagopal

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.

On the coordination of bismuth in Bi2CaSr2Cu2O8 - A2aa/Amaa structures revisited

Abstract A re-examination of the profile refinement of the neutron data of Bi 2 CaSr 2 Cu 2 O 8 according to A2aa/Amaa structures leads to new and unusual features in the coordination of Bi. Our analysis points to the formation of closely packed near-linear BiO 2 moieties stacked parallel to the a -axis with unusually short contacts between oxygens of neighbouring BiO 2 groups.

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.

Ambient pressure synthesis and neutron structure analysis of YBa2Cu4O8

Abstract The high- T c superconductor YBa 2 Cu 4 O 8 has been synthesized by the solid state reaction of Y 2 O 3 with a precursor matrix under ambient oxygen pressure. The final product is single phase 124 which shows zero resistance at 78.5 K ( ΔT c =2.0 K ). DC magnetic susceptibility studies confirm the presence of a single bulk superconducting phase ( T c ≅82.5 K ) and absence of 123 or 247 phases as impurities. Neutron profile refinement studies of the structure of the sample indicate vacancies at 9% of the linear chain oxygen sites, O(1); increased puckering of Cu(1)-0(1) layers, and a much reduced short-contact distance between neighbouring 0(1)-anions. Other notable features of the structure are vacancies (14%) at Ba-sites and unusually short bridging bond-length Cu(1)-0(4) and Cu(2)-0(4).

Neutron diffraction evidence for oxygen dimers in Bi-Ca-Sr-Cu-O superconductors

A comparative neutron diffraction study of Bi2CaSr2Cu2O8+δ, Bi2Ca2SrCu2O8+δ and Bi2Ca1.5Y0.5SrCu2O8+δ has not only shown the presence of considerable oxygen excess in the Bi layers, but also evidence for oxygen pairing giving rise to O1−2 or O2−2 type species, probably the latter. The proportion of the paired species increases when Y partly replaces Ca. Furthermore, the Tc decreases with an increase in paired species.

Neutron diffraction and Hall effect measurements on YBaCu(Mn)O system

Abstract Effect of Mn doping on the superconducting properties of YBa2Cu3-x MnxO∼7 system has been studied. The Hall effect measurements show that Mn reduces mobile charge carriers without affecting the 1 T relationship. The Rietveld analysis of neutron diffraction data shows that Mn prefers Cul site and produces chain oxygen disorder in the structure. An attempt has been made to understand the Tc depression with Mn doping from our neutron diffraction and Hall effect results.

A neutron diffraction study of the structure of Bi1.6Pb0.4Ca1Sr2Cu2Oy

Abstract Neutron profile refinement analysis conducted on Bi 1.6 Pb 0.4 Ca 1 Sr 2 Cu 2 O y (Space Group Amaa) points to Pb-ions substituting at Bi-sites as Pb 4+ . A comparative study of air-annealed and vacuum-annealed samples of Bi 1.6 Pb 0.4 Ca 1 Sr 2 Cu 2 O y indicates a slight reduction in the cell parameters upon annealing in vacuum in line with the observation of a reduction in oxygen-content per formula unit ( y ) from 8.72 for the air-annealed sample to 8.39 for the vacuum-annealed one. We propose that the rather high value of y (=8.72) for the air-annealed sample is due to the formation of dimerised oxygen-species in the bismuth layer. The significant tilting of Cu-O 5 pyramids observed for air-annealed Bi 1.6 Pb 0.4 Ca 1 Sr 2 Cu 2 O y is nearly eliminated upon vacuum-annealing treatment which is suggestive of a reduction in the hole-density as confirmed by a marginal drop in T c ( R =0) from 93 to 89 K after vacuum-annealing treatment.

Ageing effects in high-Tc YBa2Cu3O7−x superconductor a neutron diffraction study

Abstract Effect of ageing on the high-Tc superconductor YBa 2 Cu 3 O 7−x has been studied using neutron diffraction at room temperature. Significant changes in line-width and diffuse scattering background were observed when the samples were kept in open environment. The structural changes have been analysed using the Rietveld profile refinement. The deterioration in structural parameters was however, found to be much reduced when the samples were kept sealed in quartz.