R.S. Liu

Superconductivity with Tc(zero) above 105 K in Tl-containing septenary oxides with Y1Ba2Cu3Oy-like structure

Abstract A family of high- T c superconductors with T c(zero) above 105 K is described. The compounds are seven-element blends with the general formula Tl 0.5 Pb 0.5 Ca 0.8 A 0.2 Sr 2 Cu 2 O x ( A = Y ), and rare-earth elements La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb). The results represent one of the first cases where chemical substitution has significantly raised T c in a known compound. The septenary compounds are tetragonal in structure with almost identical lattice parameters to those of now-famous Y 1 Ba 2 Cu 3 O y structure. The samples were highly reproducible, stable and remarkably homogeneous both in composition and structure. The preparative conditions were found to be much less stringent than those of other copper-based high- T c superconductors.

Crystal structure of the (Pb, Hg)Sr2(Ca, Y)Cu2O7-δ superconductor

Abstract Bulk superconductivity up to 90 K has been achieved in the Hg containing cuprate (Pb, Hg)Sr 2 (Ca 1- x Y x )Cu 2 O 7- δ when x =0.3, which has the highest T c among the Pb based materials. The crystal structure of the newly discovered Hg containing Pb based 1212 phase has been investigated by Rietveld analysis of X-ray powder diffraction (XRD) data, selected area electron diffraction (SAED) and high-resolution electron microscopy (HREM). We have identified the phase responsible for superconductivity in the so-called 1212 phase with a space group of P4/mmm and lattice constants of a=3.8166(1) A and c=11.9484(4) A for the x =0.5 sample. A disordering of the hole reservoir layer (Pb, Hg) due to the displacement of the oxygen ions and Pb/ Hg ions from their ideal positions of (0.5, 0.5, 0) to (0.39, 0.5, 0) and from (0, 0, 0) to (0.044, 0, 0), respectively, was found in the XRD refinement of the x =0.5 sample. This displacement may have given rise to the appearance of superlattice spots along the [104] ∗ direction with a commensurate modulation factor of 8 in the electron-diffraction pattern viewed down [010]. Moreover, mutual substitutions of cations (≈19% Cu in the Pb/Hg sites and ≈9% Hg in the Ca/Y sites) have been found in the sample.

Superconductivity up to 32 K in a new family of the Hg-containing (Pb, Hg) (Sr, La)2CuO5−δ (1201) system

Abstract A new family of superconductors with a maximum Tc of 32 K were observed in the Hg-containing (Pb0.5Hg0.5) (Sr2−xLax) CuO5−gd (1201) system for the composition range 0.8 ⩽x⩽ 1.0. The chemical composition and crystal structure of this system have been investigated by X-ray powder diffraction including the Rietveld refinement, electron diffraction, and high-resolution transmission electron microscopy. We have identified the phase responsible for the superconductivity to be similar to (Pb, Cu) (Sr, La)2CuO5 or (Ti, Pb) (Sr, La)2CuO5 (the so-called 1201 phase) with a space group of P4/mmm and lattice constants of a=3.7908(2) A and c=8.6915 (6) A for the x=0.8 sample in (Pb0.5Hg0.5 (Sr2−xLax)CuO5−δ. The disordering of the hole reservoir layer of (Pb, Hg)O due to the displacement of the oxygen ion from its ideal position (0.5, 0.5, 0) to (0.33, 0.5, 0) was found by Rietveld refinement for powder X-ray diffraction data of the x=0.8 sample. This displacement may give rise to the appearance of the superlattice spots along the [102]∗ direction with a modulation factor close to 4 in the electron diffraction pattern viewed down [010].

63Cu NMR shift and relaxation behavior in Tl2Ba2Ca2Cu3O10−δ (Tc=125K)

Abstract Extensive 63 Cu NMR measurements have been carried out on a magnetically aligned sample of the triple-layered superconductor Tl 2 Ba 2 Ca 2 Cu 3 O 10−δ ( T c =125K). The 63 Cu shifts in the three Cu-O layers all have a similar temperature depen dence to that of 17 O, showing that they probe the same static spin susceptibility. The magnetic properties of this material were found to resemble those of underdoped YBa 2 Cu 3 O 7−δ . The Knight shift decreases dramatically with decreasing temperature below 150 K in the normal state, whereas the nuclear spin lattice relaxation rate (1/ T 1 T ) shows a broad maximum at ≈ 165 K, indicating an opening of a spin gap well above T c .

Structural characterization of a TlCaBaCu oxide in Tc onset=155 K and Tc zero=123 K superconducting specimens

The crystal structure of a Tl‐Ca‐Ba‐Cu‐O compound in Tc onset=155 K and Tc zero=123 K superconducting specimens was characterized by transmission electron microscopy. Convergent beam electron diffraction analysis established that the phase is of P4/mcc space group. The lattice parameters of the phase were determined to be a=b=0.394 nm and c=3.95 nm. The phase was found to be primarily of a ten‐subcell structure by both high‐resolution lattice imaging and diffraction pattern analysis. The composition of the grains with the ten‐subcell structure being predominant was measured by energy dispersive spectrometry to be TlCa2Ba3Cu4Ox. Polytypes of the phase with four‐ and five‐subcell structures were also observed. The five‐subcell structure was identified to be of P4/mmm space group with a=b=0.394 nm and c=1.97 nm. The compositions of the specimens were measured to vary in composition ratios of Tl, Ca, Ba, and Cu from 1:2:2:3 to 1:2:3:4. The results strongly suggested that the TlCa2Ba3Cu4Ox phase is a high Tc s...

Epitaxial growth of high-Tc superconducting Tl-Ca-Ba-Cu-O films by liquid phase epataxial process

Abstract Tl-Ca-Ba-Cu-O epitaxial films have been successfully grown on (001) MgO substrate by liquid phase epitaxial (LPE) process. The as-grown films showed an onset of superconductivity at about 140 K and zero resistance at 111 K. X-ray diffraction analysis suggests the films to be highly preferentially oriented with the c -axis perpendicular to the film surface. Observation from transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) revealed the presence of both Tl-Ca 2 Ba 2 Cu 3 O y and TlCa 2 Ba 3 Cu 4 O y phases.

Further measurements on the Tl0.5Pb0.5Sr2(Ca1−yYy)Cu2O7−δ system Pb NMR and magnetic susceptibility

Abstract Susceptibility data are presented for the septenary alloy system Tl0.5Pb0.5Sr2(Ca1−yYy)Cu2O7−δ (0≤y≤0.5), together with Pb NMR. The earlier hypothesis that over the metallic alloy range at room temperature the spin susceptibility might be constant is shown to be incorrect. In correlating these new data with earlier copper and yttrium NMR data a consistent picture emerges of a one-component systems, but with a small (∼10%) anisotropy of the spin susceptibility of the copper planes. We show also that there is 89Y p character admixed into the copper hole wavefunctions. The susceptibility shows a pre-cursor drop in all samples at temperatures close to 120 K, well above Tc, perhaps an indication of spin pairing well into the non-superconducting region. This may be spin-fluctuation diamagnetism.

A study of the orientation dependence of the 63Cu nuclear magnetic resonance in the (Tl0.5Pb0.5)Sr2(Ca1−yYy)Cu2O7-δsystem

Abstract Nuclear magnetic resonance (NMR) results for copper-63 nuclei in the Tl-based 1212 system (Tl 0.5 Pb 0.5 )Sr 2 (Ca 1- y Y y )Cu 2 O 7-δ (0.0⩽ y ⩽0.8) are reviewed. We report the NMR spectra both on random powder samples and partially aligned powder samples. After deconvoluting the random fraction from the partially aligned spectra, we obtain the resonance frequency as a function of θ, the angle between c -axis and the applied magnetic field B . Using the theoretical equation to fit the experimental data, we calculate the isotropic Knight shift, K iso , the axial Knight shift, K ax , and the quadrupole resonance frequency v Q for each value of y (0⩽ y ⩽0.8). Nuclear quadrupole resonance (NQR) on the y =0.0 sample was performed. The measured quadrupole resonance frequency is in very good agreement with the result from the NMR experimental data. We correlate these dependences on y of the resonance parameters with other structural trends.

Synthesis of a 90 K Y2Ba4Cu7O15−x superconductor under ambient pressure by triethylammoniumoxalate co-precipitation

Abstract A 90 K superconducting Y 2 Ba 4 Cu 7 O 15− x (247) powder with fine and homogeneous grain size was prepared by oxalate/triethylamine co-precipitation of a stoichiometric amount of mixed cations of yttrium, barium and copper from their aqueous solution of nitrate salts. Characterization results of the 247 superconductor by X-ray diffraction, electrical resistivity and magnetization measurements are reported. The intragrain critical current density ( J c ) of the as-prepared 247 superconductor was derived from magnetic hysteresis to be 8.5 × 10 6 A/cm 2 at 5 K and 1 T. This new route may have considerable potential in the large-scale preparation of Y 2 Ba 4 Cu 7 O 15− x .

Effects of bismuth replacement for copper on the microstructure and superconductivity of YBa2Cu3Oy

The partial replacement of copper with bismuth in YBa2Cu3Oy had led to a new phase instead of crystal site substitution. The resulting YBa2BiO6 could also be found whenever YBa2Cu3Oy reacted with Bi2O3. The YBa2BiO6 was formed either at the expense of YBa2Cu3Oy or by decomposing it, together with the liberation of copper oxide. Electrical measurements revealed the trend toward increased semiconducting normal state resistivity for bismuth-replaced samples. The insulating YBa2BiO6 phase was responsible for the deterioration of both superconductivity and mechanical strength.