Phase diagram, crystal growth and superconductivity of CaLaBaCu3O7−Y crystal

CaLaBaCu₃O_7–y (CLBCO) is a high T _c superconductor with unique tetragonal structure and cross-cationic substitution sites. In this paper, we present our study on the BaO—CuO—CLBCO phase diagram and its application to the crystal growth of CLBCO. The eutectic composition of BaO—CuO system is confirmed by the improved thermogravimetric analysis ITGA and is used for the crystal growth as a self-flux system. A pseudo-phase diagram of CLBCO—eutectic flux system is established by differential thermal analysis DTA which enables the selection of a suitable starting composition and temperature program for the crystal growth. We explore the relation between crystal growth, cell parameters and the main superconducting transition temperatures T _c. The T _c of CLBCO crystals depends strongly on the Ca/La ratio in the starting materials. As for YBa₂Cu₃O_7–y YBCO, a small amount of BaF₂ additive greatly increases the main transition temperature of CLBCO crystals. This can be understood by the cluster model we proposed previously for YBCO crystal growth. Based on the research on the phase diagram, crystal growth and the superconductivity, high T _c crystals of CLBCO with size up to 6.5 × 6.5 × 0.4³ have been grown from the eutectic self-flux system with BaF₂ as additive. T _c was determined by an a.c. susceptibility measurement, which shows the main transition temperature up to 60 K for the as-grown crystals. After annealing in flowing oxygen at 600 to 400 °C for two days, T _c increased to 73 K.     

Formation of oxide phases in the system Fe2O3−Gd2O3

The formation of oxide phases in the system (1 -x) Fe₂O₃ +xGd₂O₃ was investigated for 0 x 1. On the basis of XRD measurements the distribution of oxide phases, -Fe₂O₃, Gd₃Fe₅O₁₂, GdFeO₃ and Gd₂O₃ was determined, as a function ofx. No solid solutions were observed with certainty even at the very ends of the concentration range. This was also confirmed by⁵⁷Fe Mössbauer spectroscopy. New accurate crystallographic data for Gd₃Fe₅O₁₂ are given. The formation of oxide phases in the system Fe₂O₃- Gd₂O₃ is compared with the data for analogous system Fe₂O₃-Eu₂O₃.     

EPR in superconducting and nonsuperconducting Gd0.02Y0.98Ba2Cu3O7−δ. Resolved and unresolved fine structure of Gd3+

Electron Paramagnetic Resonance (EPR) measurements at various temperatures are reported for Gd_0.02Y_0.98Ba₂Cu₃O_7– single crystals, before annealing (superconducting sample) and after annealing (nonsuperconducting). A resolved fine structure from Gd³⁺ is observed below 100 K, in annealed samples only; at high temperatures and before annealing, the EPR lines coalesce into a single narrowed line, with g_eff=2.05±0.01, which is attributed to the effect of the Gd³⁺-conduction electrons coupling (Plefka–Barnes mechanism). The superconducting phase is characterized by an important field dependence of the base line just below T_C, and by the appearance of the noise on the spectrum.     

Long range coupling in YBa2Cu3O7−δ/Y1−xPrxBa2Cu3O7−δ multilayers

YBa₂Cu₃O_7–/(Y_1–xPr_x)Ba₂Cu₃O_7– multilayers have been used to probe coupling through (Y_1–x:Pr_x)Ba₂Cu₃O_7– alloys. We observe that the coupling between ultrathin YBa₂Cu₃O_7– layers, 12 or 24 Å thick, survives through several hundred Å of (Y_1–xPr_x)Ba₂Cu₃O_7– with x=0.4 and 0.55. T_c versus the thickness of the spacer-alloy, and activation energies for flux motion, with fields parallel and perpendicular to the c-axis, have been used to probe this long range coupling. All these experiments point to an unusually large coupling length for these two alloy compositions. In the x=0.55 case this result is particularly surprising since the alloy material display a semiconducting behaviour for this composition. T_c measurements, activation energies, and a study of the vortex dynamics in these coupled multilayers is presented along with new results obtained on a series of multilayers built with a more insulating alloy, x=0.7.     

Physical, electrical, transport and magnetic properties of Nd(Ba,Nd)2.1Cu3O7−δ system

In this study, the superconducting Nd(Ba,Nd)_2.1Cu₃O_7?δ system has been prepared using conventional solid-state reaction technique. Transport properties including structural/microstructural evolution, electrical, magnetic and critical current density properties were investigated. After high temperature heat treatments at over 1,000 °C, large and strongly connected grains were obtained but weakly connected and small in size granular formation were obtained for the low temperature heat treated samples at around 900 °C. The best T _c and T ₀ values were obtained as 93 and 89 K respectively for the sample prepared at 1,020 °C for 24 h, which is very close to peritectic temperature of YBCO material. Magnetization of the sample heat treated at 1,020 °C was investigated in detail. The magnetization hysteresis loops are expounded to be the product of Nd-123 grains and unscreened Nd³⁺ ions within intergranular boundaries and vortex cores. The peak effect on the magnetization curves was described by the extended critical state model. Temperature dependencies of the irreversibility field, the peak field and the full penetration field correlate and there is scaling behavior of the pinning force as well. Thermoelectric power data was analyzed by “Modified two band model with linear T-term for superconductors”. Temperature dependence of thermal conductivity of the samples showed small peak with broad maximum just below the T _c value. Thermal conductivity of samples prepared was calculated by using “The Modified Callaway Model and Wiedermann–Franz law” and results obtained discussed.     

Suppression of superconductivity in the Er1− x Pr x Ba2Cu3O7−δ

Measurements of electrical resistivity. X-ray diffraction patterns, magnetic susceptibility and thermoelectric power of the Er_1-x Pr _x Ba₂Cu₃O_7- system have been made. The superconducting transition temperature was found to decrease monotonically with praseodymium concentration, x. From the susceptibility data, it was determined that the valence of praseodymium lies between +3 and +4. The thermoelectric power was found to increase with x, and the slopes of dS/dT were negative except for the case x= 0. The. tendency of the thermopower to change with increasing praseodymium concentration has been qualitatively explained using the theory for strongly correlated systems.     

Interdiffusion of rare-earth cations in Ln1−x ′Ln x ″ Ba2Cu3O7 − δ (Ln′, Ln″ = Y, Dy, Ho) High-T c superconductors

The Y³⁺/Ho³⁺, Y³⁺/Dy³⁺, and Dy³⁺/Ho³⁺ interdiffusion coefficients in the YBa₂Cu₃O_{7 ? δ}-HoBa₂Cu₃O_{7 ? δ}, YBa₂Cu₃O_{7 ? δ}-DyBa₂Cu₃O_{7 ? δ}, and DyBa₂Cu₃O_{7 ? δ}-HoBa₂Cu₃O_{7 ? δ} systems have been determined in air at a temperature of 1233 K by the Boltzmann-Matano method from the yttrium/holmium, yttrium/dysprosium, and dysprosium/holmium concentration profiles across the diffusion zone between two pellets of the high-T _c superconductors. The composition dependences of the interdiffusion coefficients in the solid-solution systems are presented.     

Nd substitution effects in YBa2Cu4O8 prepared using a conversion reaction from Y1−xNdxBa2Cu3O7−δ and CuO

Synthesis, crystal structure and magnetic properties of samples with partial substitution of Nd for Y in YBa₂Cu₄O₈ are considered. The samples with nominal composition Y_1−xNd_xBa₂Cu₄O₈ (x=0.0–0.75) were prepared using a solid state conversion reaction from Y_1−xNd_xBa₂Cu₃O_7−δ and CuO using LiF as a flux. The annealing was performed at 790°C in a flowing oxygen atmosphere. The products with x=0.0–0.5 showed thermal stability up to 660°C in N₂(g) atmosphere revealing a single phase composition. The success of the synthesis is considered in relation to the effective ionic size of the Nd³⁺ ion. The crystal structures of the compounds are described on the basis of powder neutron diffraction measurements. The T_c values of the Nd substituted samples were noticeably lower than for the pristine YBa₂Cu₄O₈. Possible reasons for the decrease in T_c are discussed on the basis of calculated values for hole concentrations in the Cu–O₂ planes obtained via the BVS (bond valence sum) method, and on the basis of some Nd³⁺ ions being introduced at Ba²⁺ ion sites in the lattice.     

X-Ray and High-Resolution Neutron Diffraction Studies on Nd x Y1−x Ba2Cu3O7−δ Superconductors

A synthesis of neodymium-substituted YBCO superconductor Nd _x Y_1?x Ba₂Cu₃O_7?δ (x=0,0.25,0.5,0.75,1) has been done using a dissolved method in order to obtain homogeneous crystals and higher critical current density. The effects of the substitutions on the structural and magnetic properties of the superconductors after sintering at 970 ^°C have been examined. Crystallinity of the synthesized powders was confirmed using X-ray and high-resolution neutron diffraction (XRD and HRPD) techniques. Rietveld analyses for both diffraction data sets gave increasing lattice parameters with addition of Nd content and decreasing orthorhombicity. Such addition also caused a decrease in occupancy of the oxygen in the O(4) site. Further investigation using SQUID showed critical temperature of the superconductors between 90.9 and 92.0 K. The critical current density (J _c ) was calculated from the magnetic hysteretic loop at 5 K as 40 kA?cm^?2 for Nd_0.25Y_0.75Ba₂Cu₃O_7?δ sample and 100 kA?cm^?2 for Nd_0.5Y_0.5Ba₂Cu₃O_7?δ sample. We also found that increasing Nd content on the Nd _x Y_1?x Ba₂Cu₃O_7?δ superconductor samples can improve their resilience of superconductivity and critical current density.     

Doping of Nb to the Ba and Cu Sites in the Y0.6Gd0.4Ba2Cu3O7−δ

Polycrystalline samples of Y_0.6Gd_0.4Ba_2?x Nb _x Cu₃O_7?δ and Y_0.6Gd_0.4Ba₂Cu_3?x Nb _x O_7?δ with different Nb contents (x=0.025, 0.075, 0.125, 0.175 and 0.225) were prepared using the solid state reaction method. Structural and electrical properties of new compounds were investigated with optical microscope, scanning electron microscopy (SEM), X-ray diffraction (XRD), Four Point Probe (FPP). The results indicated that Nb constituted YBa₂NbO₆ structure instead of substituting to the Y_0.6Gd_0.4Ba₂Cu₃O_7?δ structure. YBa₂NbO₆ structures gathered between grains. Transition temperature did not change significantly, but critical current (J _c) values decreased with increase of Nb concentration. The underlying reason is that the oxygen concentration of compounds remained unchanged due to Gd; thus, T _c values did not change significantly.     

Phase relations in the systems ZrO2Y2O3Nd2O3 and ZrO2Y2O3CeO2

The phase relations of ZrO₂Y₂O₃Nd₂O₃ and ZrO₂Y₂O₃CeO₂ systems have been studied at 1100–1600°C. The single region of the fluorite phase was intensively examined using the relation between lattice parameter and composition. In the ZrO₂Y₂O₃Nd₂O₃ system, 37 mol% Nd₂O₃ is soluble in Y₂O₃-stabilized zirconia (fluorite phase) at 1100°C and 42 mol% Nd₂O₃ at 1600°C. In the ZrO₂Y₂O₃CeO₂ system, 40 mol% CeO₂ dissolves into the stabilized zirconia at 1600°C.     

Anomalous Behavior in Y1−x Pr x Ba2Cu3O7−δ

It is found that the samples of x=0.25 and 0.5 in the series samples of Y_1?x Pr _x Ba₂Cu₃O_7?δ demonstrate some anomalous behavior. First, the changes of T _c value of them are a bit different from their neighbor samples. Second, under a small magnetic field, i.e. 5–100 Oe, a little rise in the T _c value with the rise of the field is observed. The anomalous behavior is primitively explained by the local structure changes and the magnetic properties. It is attributed to local structural changes, i.e. the chemical bonds in the Cu–O₂ plane.     

Preparation and thermophysical properties of nano-sized Ln2Zr2O7 (Ln?=?La, Nd, Sm, and Gd) ceramics with pyrochlore structure

Rare earth zirconates (Ln₂Zr₂O₇, Ln = La, Nd, Sm, and Gd) with pyrochlore structure were prepared by hydrothermal method with polyethylene glycol as surfactant. X-ray diffraction, thermogravimetric analysis/differential scanning calorimetry, Fourier transform infrared spectroscopy, Raman spectroscopy, and field emission scanning electron microscopy were utilized to characterize the phase structure, thermal decomposition, and morphology of the products. Qualitative analysis indicates that the as-prepared zirconates are pyrochlore-type structures. The specific surface area, lattice parameter, and average crystallite size of the as-prepared products are closely related to the ionic radius. The activation energy of crystal growth shows an increasing trend with the decrease in ionic radii. The sintering behavior of compacted body was also investigated, revealing that the sintering-resistance properties of Ln₂Zr₂O₇ are descending as the order of La₂Zr₂O₇, Nd₂Zr₂O₇, Sm₂Zr₂O₇, and Gd₂Zr₂O₇.     

Mechanism of high-temperature superconductivity in YBa2Cu3O7−δ: Crucial role of oxygen

Oxygen atoms in the middle CuO layer of YBa₂Cu₃O₇ consisting of strings of corner-connected (CuO₄)_∞ units are shown to be crucial for superconductivity. Importance of hole-hole pairing giving rise to OO bonds is also indicated.     

Cation composition variation observed in Y1Ba2Cu3O7−z

The cation concentration ratio in the individual grains (1 to 25 μm) of four Y₁Ba₂Cu₃O_7−z pellets was investiga spectroscopy. A spread in the cation composition was observed even in a sample showing a narrow superconducting transition. The concentrations of Y, Ba and Cu were in the ranges 12–20, 30–35 and 48–53 at%, respectively. It is, therefore, likely that the Y₁Ba₂Cu₃O_7−z structure covers a finite area on the YO_1.5-BaO-CuO phase diagram.     

Polymorphism in Sr3Fe2O7−x

The compound Sr₃Fe₂O_7–x , with variable iron valence, was investigated by X-ray powder techniques, both at room and at high temperatures. If the material is examined in massive form, a single phase called -Sr₃Fe₂O_7–x appears as previously reported in the literature. This -phase is tetragonal and exhibits the lattice parameters: a=3.874 and c=40.314 Å. Two other phases, called and -Sr₃Fe₂O_7–x , respectively, can be obtained on heating the finely powdered material when laid on a flat platinum support. The form is stable up to 1275° C, while the form is revealed only above 1275° C and changes always into -Sr₃Fe₂O_7–x when quenched. Both and phases are tetragonal, with a=4.001 and c= 58.251 for the form and a=4.013, c=57.092 Å for the form. The transition involves a true phase equilibrium, while the transformation is possible only by means of a suitable mechanical treatment of the material.     

Domain growth in partially melted bulk Nd1+xBa2−x Cu3O7−δ

Domain growth of bulk partially melted Nd_1+xBa_2–xCu₃O_7– in air was found to be severely limited by liquid-phase segregation and losses. The use of a reduced oxygen pressure greatly improved the stoichiometry control, leading to the formation of well-developed domains and enhanced superconducting properties. Direct nucleation of Nd_1+xBa_2–xCu₃O_7– from the liquid phase by dissolution of Nd₂BaCuO₅ and a spiral growth mechanism in three perpendicular directions of the platelets stacked in the domains, are suggested.     

Formation temperature of bosons in high-T c YBa2Cu3O7−y systems

At different equilibrium temperatures, a precise measurement of the electrical resistivity (R) is performed for high T _c polycrystalline YBa₂Cu₃O_7–y systems. T ² law is applied in the part of the R-T relationship far above the superconducting transition temperature. Since Landau and Pomeranchuk predicted the T ² law arising from electron-electron scattering, a higher critical temperature of T ² law is defined as a start point (T _es) of electron-electron scattering. On the other hand, we defined a lower critical temperature of T ² law as the formation temperature (T _ee) of an electron-electron pair, i.e. the formation temperature of boson. T _ee relates to the offset temperature of the superconducting transition .     

Preparation and characterization of high-T c superconducting YBa2 Cu3 O7−x filaments

High-T _c superconducting filaments of the Y-Ba-Cu-O system were prepared using a low-cost suspension spinning method, where the solvent was removed by a phase inversion technique. The Y-Ba-Cu oxide precursor, containing polysulphone (PSF), was spun as a filament into a precipitating medium, removing the solvent by phase inversion and using water as a non-solvent. The green product filament was washed, dried and subjected to a heat treatment to remove the binding material and generate the oxide in, the appropriate superconducting phase. Stoichiometry, porosity and grain size were investigated by scanning electron microscopy, energy-dispersive X-ray analysis and electron probe micro analysis, while crystal structure was checked by X-ray diffraction analysis. Resistivity measurements as a function of temperature were performed by the four-point method and typicalT _c values of 88 K were observed, whileJ _c in the range of 125 A cm^–2 at 77 K and zero field.     

Effects of Zn doping on crystal structure,Raman spectra and superconductivity of SmBa2Cu3O7−δ systems

Polycrystalline SmBa₂Cu_3?xZn_xO_7?δ (SBCZO) (x = 0.0–0.4) samples are prepared by the usual solid-state reaction technique. The effects of Zn doping on the structure, the grain morphology, Raman spectra and electronic transport properties of SBCZO systems have been investigated. The orthorhombic structure of the samples does not change remarkably. The samples become denser and grain boundary becomes unclear with the increase of Zn content. Raman spectra exhibit different features with increasing Zn content which shows that Zn ions act as strong scattering centers to the charge carriers in the CuO₂ planes, enhance the disorder of the CuO₂ planes and increase oxygen depletion in CuO chains. The measurements of the resistivity show that the superconducting transition temperature T_c decreases rapidly and the superconducting transition width increases gradually with increasing Zn contents. Furthermore, the changes of the samples’ normal state resistivity from metallic to semi-conducting behavior show the increase of heterogeneities with increasing Zn content which causes inter-grain or intra-grain disorders. All the results suggest that lattice disorder in the CuO₂ planes, the oxygen content change in CuO chains and local weak superconductivity regions due to the substitution of Zn for Cu are related to the suppression of T_c in the SBCZO systems.