A. Sequeira

Low-temperature phase transitions in LiKSO4: A neutron study

A single crystal neutron diffraction study of the low temperature structural phase transitions in LiKSO4 has been carried out. Detailed temperature dependence of the Bragg peak profiles and integrated intensities for a group of about 20 reflections has been investigated upon cooling and heating in the temperature range 300 K to 96 K. The crystal undergoes three clear phase transitions at 205 K (from sp. gr. P63 to sp. gr. P31c), at 189 K and at about 135 K on cooling. The corresponding temperatures are 250 K, 190 K and 138 K on the heating cycle. These transitions are extremely sluggish, the kinetics depending on the thermal treatment of the crystal.

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 BiCaSrCuO 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.

Kinetics and symmetry changes in low-temperature phase transitions in LiKSO4

The kinetics and the symmetry changes associated with the low-temperature structural phase transitions in LiKSO4 have been investigated by single crystal neutron diffraction. The study indicated three clear phase transitions at 205 K (from sp. gr. P63 to sp. gr. P31c), at 189 K (to sp. gr. Cc) and 135 K in the cooling cycle; a phase change is also indicated at 165 K. All these transitions show thermal hysteresis. The full crystal does not undergo transformation at any of the transition temperatures but exhibits mixed phases. The kinetics of the phase transitions are characterised by prolonged equilibration times (days) which strongly depend on the thermal history of the sample. With the results of the structural investigations in the temperature range 300 K to 100 K, an attempt has been made to explain the anomalies and variant results observed in the physical properties of LiKSO4 at low temperatures.

Observation of librational modes of water molecules in single crystal hydrates by neutron scattering

Abstract The librational motions of water molecules in single crystals of Ba(ClO3)2.H2O and K2C2O4.H2O are studied by the technique of neutron scattering. The assignments of the observed peaks in the two cases are discussed with reference to features affecting the intensity, namely, the orientation of the crystal and the polarisation vector of the librational motions. Unambiguous assignments have been made for the rocking modes of vibration.

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.