Search

YBa2(Cu1−xFex)3O7−d annealed in an Ar atmosphere, then reoxygenated at various temperatures, has been studied by Mossbauer spectroscopy at 300 K and low temperature with and without an applied field of 5.5 T. The results are interpreted as being due to Fe clustering remaining in the chains rather than a significant transfer to the planes.

YBa2(Cu1−xFex)3O7−d (0.02 ≤ x ≤ 0.04) annealed in an Ar atmosphere, then reoxygenated have been characterized (X-ray diffraction, Tc) and studied by Mossbauer spectroscopy at room and low temperatures. The results are interpreted as due to Fe clustering rather than to a significant occupation in the plane.

The preparation, X-ray diffraction (XRD), resistivity, AC susceptibility and Hall constant of Y 1− x Ca x SrBaCu 2.7 Co 0.3 O 6+ z ( x =0, 0.1 and 0.2) are reported. All the samples crystallized with a tetragonal symmetry. The room temperature resistivity decreased from 27 mΩ cm to about 2 mΩ cm when x increased from 0 to 0.2. The critical temperature T c increased remarkably from 22 K (for x =0) to 40 and 58 K for x =0.1 and 0.2, respectively. A relation between the Ca concentration, T c and the hole density derived from Hall measurements was established. The data are compared to those earlier reported in the case of (0.3) Al substitution. These results suggest that Ca injects holes into Cu-O planes.

X-ray diffraction, iodometric titration, resistivity and SQUID magnetometry measurements have been performed on samples of YSrBaCu2.6Fe0.4O6+z in which yttrium has been partially replaced by calcium. The calcium-free compound is semiconducting but superconductivity is restored for x>0.05. The normal-state resistivity decreases and Tc increases with increasing Ca concentration. The effective valence of copper decreases with increasing calcium content and there is also some loss of oxygen. The observed behaviour is quite similar to that previously reported for samples which were doped with Al but values of Tc are generally lower with Fe doping. This is thought to be due primarily to the fact that the Fe partially occupies the Cu(2) sites in the superconducting Cu-O planes.

We report X-ray diffraction, resistivity and Mossbauer studies on several substituted 2201 Bi cuprates, Bi2Sr2Cu1−xFexOy(O≤x≤1) and Bi2Sr1.6La0.4Cu1−xFexOy and Bi1.6Pb0.4Sr1.6La0.4Cu1−xFexOy (O≤x≤0.04). The first series is not superconducting and a new phase appears for x=1. For La substituted samples, Tc increases up to 35K and decreases strongly with x. All Mossbauer spectra were fitted well with four quadrupole doublets, with relative population depending on samples.

We report on the preparation, X-ray diffraction, resistivity, hole density (p) and ac susceptibility measurements of Y 1−x Ca x SrBaCu 2.6 Al 0.4 O 6+z . For x=0, superconductivity (SC) is destroyed and p reaches a value of 1.5 10 21 /cm 3 . SC is dramatically restored when Ca is substituted. T c values are 29 and 47 K for x = 0.1 and 0.2 respectively. A relation between p, T c and x is observed. Similar data are obtained in the case of Fe(0.4) substituted samples though the maximum value of T c is around 30 K when 0.15 Ca is co-substituted.

We have studied the Mossbauer spectra of YBa2Cu3−xFexO7−d (11 compositions) and present here the results obtained from quantitative analyses of the paramagnetic and magnetic spectra.

Hall constants (R H) at 300 K of three families of CuO-based superconductors are presented—the 123 YBaCuO with Fe substitution, the 2212 and 2223 Bi systems with Pb, Gd, Nd. Sm substitutions. In all these systems.R H is positive indicating hole conductivity.T c increases initially as the hole density (p=1/R He) increases and shows a maximum. Each family shows a maximum value ofT c but at different values ofp. These data along with the observation ofn-type conductivity in NdCeCuO are discussed in the light of certain models.

Structural, electrical, magnetic and Mossbauer studies of several Fe substituted high T c Bi-compounds with low and high substitution rate are presented. A new Fe 2201 phase is described. At low Fe concentration, T c is lowered and a destabilization of certain phases is observed.

High statistics room temperature Mossbauer spectra have been performed on ten YBa2Cu3−x Fe x O7−δ (0.01≦x≦0.15) samples. The different fit solutions (including broadening of the lines) are tested with “χ2” factor analysis. Only two solutions are valid with almost similar parameters. The results on present samples and our previous results on various materials made under different heat treatments show thatT c is governed byx and the relative population of one of the iron species. These species can be related to highly coordinated irons in the chains.

A new class of superconducting material - PbSrYCaCuO - has been recently synthesized. Energy Dispersive X-ray analyses of a sample prepared in our laboratory indicate that 80 % of the matrix is constituted by the Pb 2.25Sr1.95Y0.72Ca0.28Cu2.95O 8.5 compound, 12 % represents random compositions and 8 % corresponds to (Ca, Sr) CuOx compounds. Chemical cartography and maps correlation illustrate such conclusions. Some new electron diffraction data are correlated to these analyses.

We report Hall-constant ({ital R}{sub {ital H}}) measurements on the Bi-Sr-Ca-Cu-O system. Two systems, 2:2:1:2 and 2:2:2:3, have been studied with a variety of heat treatments and cationic substitutions (Pb, Gd, Nd, Sm). The carrier density ({ital n}=1/{ital eR}{sub {ital H}}) at 300 K as a function of {ital T}{sub {ital c}} for these polycrystalline samples show two distinct maxima---one corresponding to the low-{ital T}{sub {ital c}} (2:2:1:2) phase and the other to the high-{ital T}{sub {ital c}} (2:2:2:3) phase. The data point to compelling evidence of correlations between charge carriers and superconductivity confirming similar Hall data on Y- and La-based cuprates. A discussion of some relevant models is presented.

Al (y=0.1, 0.2) substitution decreases T c in YSrBaCu 3−y Al y O but when Ca is substituted at the Y site T c increases depending on the Ca concentration(x). A relation between T c , x and hole density is shown.

0.5 • 0.5 • 2 : 2 and heated in an alumina crucible in air at 950°C for about 12h. The product was reground and stoichiometric ratios of In2• 3 and 1 PbO were added to give three samples whose nominal compositions were In~_xPb~Sr2Y0. 5 T'-" Ca0.sCu20 z (x =0.5, 0.6 or 0.7). These were ~ 0 pressed into small pellets and placed individually in a gold tube. The tube was pinched tight and heat ~ LC) -~ treated in flowing oxygen at 920 °C for 18 h. The samples were not very well sintered but were hard ,~ enough to make electrical contacts. Indium was -e ultrasonically welded to one of the pieces and the 0 a.c. resistivity (p) was measured using a four-probe technique. The field-cooled (FC) and the zero-field- cooled (ZFC) susceptibility Z of the samples were 1

We report on the X-ray diffraction, resistivity and magnetic susceptibility (SQUID, H = 5 Oe) data of (M)BiSrCaCuO (M = Pb, Nd, Sm, Gd) system. The nominal compositions studied were (2212), (2223), (4334) and (2224). The superconducting properties are influenced very much by the annealing conditions. For M = Pb (0.4) in (2223) and (2224), a three layer CuO structure (n = 3 with c = 37 A ) appears with the zero resistivity at 100 Tc 10K. Whereas for 10% of M = Nd, Sm, Gd in (2212), (2223) or (2224) a two layer structure ( c = 30 A ) is obtained and Tc is less than 90K. The data seem to indicate the role played by BiO planes as pointed out by band structure calculations.

We present and discuss experimental results of resistivity (ρ) and Hall coefficient (RH) measurements on ceramic samples of the BiSrCaCuO system, and compare them with our previously reported data on YBa2Cu3O7 − x. We also report data on single crystals of the latter, both parallel and perpendicular to the ab plane. Relatively sharp superconducting transitions, as well as a reasonably high resistivity ratio ( ρ 296K ρ 117K = 2.6 in the BiSrCaCuO system, and ρ 296K ρ 96K = 2.9 in YBaCuO), indicate that the samples are of good quality. In the BiSrCaCuO system (Tonset = 112 K), the Hall coefficient is positive (with a room-temperature value of 4.18 × 10−3 cm3 C−1) and shows a temperature dependence similar to that observed in the ceramic samples of YBaCuO ( R H(117K) R H(296K) = 3.0 in BiSrCaCuO and R H(96K) R H(296K) = 2.9 in YBaCuO). We discuss these results in terms of the anisotropic nature of these compounds. The roles played by the CuO planes and the CuO chains in YBaCuO are also discussed.

The magnetisation as a function of magnetic field (0

Systematic resistivity, DC susceptibility, X-ray diffraction, electron microscopy and diffraction studies in Bi-Sr-Ca-Cu-O system are reported as a function of different cationic ratios and as a function of heat treatment from 850°C. Imaging and diffraction electron microscopy studies of the crystals show very few faults and there is no crystallographic phase transition during cooling down to 15K. Current voltage characteristics at 77K reveal Josephson junctions with voltage jumps as large as 8 mV.

In YBa2Cu2.985Fe0.015O7, the iron substitution and (consequently) the effect on superconductivity (SC) are low. Magnetically split Mossbauer spectra are obtained in the presence of an external field of 5.5 T at low temperatures, and they have been fitted to spin relaxation spectra appropriate for Fe3+ ions in the high-spin state. The analyses show near-perfect diamagnetic shielding of a component spectrum at low temperatures, characterised by a low spin relaxation frequency and consequently a larger SC band gap Delta . It appears that planes containing Cu2 ions are mainly superconducting, though SC extends to a smaller degree to the Cu1 sites. The authors argue that the results do not imply coexistence of magnetism and SC but of isolated magnetic ions and SC.

Preparation, electron microscopy, resistivity (ζ) and field cooled susceptibility (χ) of Pb 2 Sr 2 Y 1- x Ca x Cu 3 O 8+ δ ( x =0.5; 0.25) are reported. They crystallise in an orthorhombic structure. ζ shows a two step transition with distinct onsets at 86 and 45 K with zero resistivity at 15 K. χ data show a diamagnetic onset at 74 K. These data while confirming the results of Cava et al (Nature 336, 211 1988) point out the influence of the preparation conditions on the superconducting properties calling for a systematic study.

We present X-ray, resistivity ρ, Hall effect and susceptibility /gc data for YBa 2 Cu 3 − x M x O 7 − δ ( M ≡ Al , Ga , Fe ; 0 x x ≈ 0.1, the structure changes from orthorhombic to tetragonal, although T c > 77 K . The current density derived from the magnetization data decreases faster for aluminium than for iron and gallium.

X-ray, dc susceptibility and magnetization data of YBa 2 Cu 3−x Fe x O 7−δ (0 86 K. The superconductivity may thus be preserved through the orthorhombic-tetragonal (t) transition which takes place near x = 0.10. The critical current J c derived from the magnetization data decreases very little for x = 0.02 with respect to x = 0. For x>0.02, J c decreases faster than T c does. The effect of heat treatment is pointed out. The data are briefly discussed taking into account the small coherence length and the presence of orthorhombic domains in t phase.

Complete wave vectors for the d5 energy levels were computed on the basis of the 252‖SLJMJ〉 kets of the configuration for D4h, C4v, and C2v symmetries. The interelectronic repulsion Hamiltonian with B=800 cm−1, C=3.7B, the spin–orbit coupling Hamiltonian with ζ=400 cm−1, and a crystal field Hamiltonian written in Wybourne’s tensor operator formalism with the five parameters B40, B44, B42, B20, B22, were used in combination. The free atom parameters used correspond to Fe3+ in solids. Maps of the ground state multiplicity versus B40, B44, and B20, (C4v) were established. Two zones, where the three multiplicities (2S+1) =2, 4, and 6 meet are apparent, one corresponds to low values of B20 and medium values of B40, B44 (30 000 cm−1). The positive D factor for zero‐field splitting of the 6S5/2 state was computed as a function of crystal field parameters. g∥ and g⊥ were computed for the low‐spin domain in C4v; gx, gy, gz were calculated in C2v symmetry in the low‐spin domain near the border with four and six mul...

The authors report on the preparation, structural, electrical and magnetic properties in the Y-Ba-Cu-O system. The appearance of superconductivity in this system is dictated by the preparation conditions. The single phase YBa2Cu3O8 shows a superconducting transition at Tc=91 K. Replacing Y with 20% Eu does not change Tc sensibly.

Resistivity, D.C. susceptibility and Mo¨ssbauer spectra (MS) of YBa2, 985Fe0, 015O7−δ have been obtained. The spectra in zero field show the presence of three inequivalent sites. Spectra at 4.2, 11, 25, 47 and 155 K in field of 5.5 Tesla reveal that in superconducting (SC) state the diamagnetic shielding of external field (Hext) is different for the different sites of iron. Ionic spin relaxation (ISR) effects are present at low temperature.

The variation of the resistivity and the Hall coefficient in ceramic samples of YBa2Cu3O7−x as a function of oxygen content is reported and discussed in terms of a simple two-band model.

Resistivity, magnetic susceptibility, X-ray, electron diffraction and EDX analyses of the title compound are presented. The preparative conditions for this compound are more stringent than for YBaCuO and BiSrCaCuO.

High statistic Mossbauer spectra have been performed at 300K on ten samples of YBa 2 Cu 3−x Fe x O 7−† (0.01 ≦ x ≦ 0.15). Only two fit solutions hold from χ 2 factor analysis. It is shown that T c is governed by x and by the population of one iron species which could correspond to high coordinated irons in chains.

Hall constants (RH) of a variety of samples of Bi2−xMxSr2Can−yMyCumOz (M = Pb, Hg, Gd, Sm, Nd; x,y ⩽ 0.4; n = 1, 2; m = 2, 3, 4) are reported. The data confirm the simultataneous role in the high Tc superconductivity of the (CuO2 planes and effective free carriers (holes), by showing an universal curve Tc = f (1/RHe). The results can be understood qualitatively in terms of a bi-polaron model.