Self-doping Effects on the Superconducting Properties of Cu0.5Tl0.25M0.25Ba2Ca2Cu3O10−δ (M = Bi, Hg, Nb, Pd, Li, Na, K)

The effect of self-doping and substitution of elements of higher and lower electronegativity, such as Bi, Hg, Nb, Pd, Li, Na, K, on the superconducting properties of Cu_0.5Tl_0.5−x M _x Ba₂Ca₂Cu₃O_10−δ with x=0.25 is investigated. These experiments demonstrated that the elements of lower electronegativity such as Li, Na, and K can easily liberate their outer most s-electron that could be supplied to the conducting CuO₂ planes of Cu_0.5Tl_0.5−x M _x Ba₂Ca₂Cu₃O_10−δ superconductor, and as a result, we get enhanced superconducting properties. However, highly electro-negative elements hinder the transfer of carriers from charge reservoir layer to the conducting CuO₂ planes and promote inferior superconducting properties. In the present studies, we have investigated the effect of post-annealing in nitrogen and oxygen atmospheres for optimizing the carriers in conducting CuO₂ planes of Cu_0.5Tl_0.5−x M _x Ba₂Ca₂Cu₃O_10−δ (M=Bi,Hg,Nb,Pd,Li,Na,K) superconductor. These studies are important since the density of carriers in the conducting CuO₂ planes determines the Fermi-vector k _F and Fermi velocity v _F of the carriers, which ultimately brings about the final superconducting state of the system.      

Improvement of Superconductivity with the Modification of Charge Reservoir Layer in (Cu0.5Tl0.5?xMx)Ba2Ca2Cu3O10?δ

The relatively higher electronegative elements (M = Pd, Nb, Bi, Hg) have been partially doped at Tl sites in Cu_0.5Tl_0.5−x M _x Ba₂O_4−δ (x=0,0.25) charge reservoir layer of Cu_0.5Tl_0.5−x M _x Ba₂Ca₂Cu₃O_10−δ superconductor. These elements may retain more oxygen in the charge reservoir layer due to their higher electronegativity as compared to Tl, and the higher population of oxygen in the charge reservoir layer can optimize the charge carriers’ density in the conducting CuO₂ planes. The optimum density of mobile charge carriers in the conducting CuO₂ increases Fermi wave-vector K _F and Fermi velocity v _F of the carriers, which results in the improvement of superconducting properties of the material.     

Effect of cationic size in Hg(Tl/Bi)Ba2Ca2Cu3O8+δ on superconducting and microstructural characteristics

In this paper we have reported investigations on the effect of simultaneous substitution of Bi and Tl at the Hg site in the oxygen deficient HgO_δ layer of HgBa₂Ca₂Cu₃O_8+δ cuprate superconductor. Bulk polycrystalline samples have been prepared by the two-step solid state reaction process (precursor route). It has been observed that the as grown HgBi_0.2?xTI_xBa₂Ca₂Cu₃O_8+δ (withx = 0.00, 0.05, 0.10, 0.15, 0.20) corresponds to the 1223 phase. It has been found that the T_c varies with the average cationic size < R_d > of the dopant cations. The optimumT _c of ～ 131 K has been found for the composition HgBi_0.15TI_0.05Ba₂Ca₂Cu₃O_8+δ This composition leads to the average dopant cation size of ～1.108 å which is very close to the size of Hg²⁺ (～ 111 å). The microstructure for HgBi_0.15TI_0.05Ba₂Ca₂Cu₃O_8+δ has been found to be most dense and this phase exhibits the highest stability. The J_c of the optimum material HgBi_0.15TI_0.05Ba₂Ca₂Cu₃O_8+δ is found to be ～ 1.29 × 10³ A/cm² at 77 K.     

ac MH loop measurements on Mn doped YBa2Cu3O7-δ superconductors

Isothermal ac MH (magnetization-field) loops for varying field amplitudes were recorded at 77 K on YBa₂(Cu_1-xMn_x)₃O_7-δ with x = 0, 0.010, 0.015, 0.020, 0.025, 0.035 and 0.050, YBa₂(Cu_o.o75Fe_0.025)₃O_7-δ, YBa₂(Cu_0.075Ni_0.025)₃O_7-δ and YBa₂(Cu_0.075Zn_0.025)₃O_7-δ samples up to a maximum field amplitude of 80 Oe. Flat band susceptibility, ac losses and flux profiles were deduced from the ac MH loops. The undoped sample exhibited a minimum weak link ac loss and the 5.0% doped sample showed maximum weak link ac loss. Ni and Fe doped samples showed higher granular losses.J _cg estimated from the flux profiles decreases monotonically with increasing concentrations of Mn up to 2.5%.     

Deformation Effects on the Structure and Properties of Y1−x Ca x Ba2Cu3O7−δ (x=0;0.3) Tapes Produced by OPIT Method in the Ag-Tube

Samples of superconducting tapes with two type cores (YBa₂Cu₃O_7−y and Y_0.7Ca_0.3Ba₂Cu₃O_7−δ ), obtained by the OPIT method under two different regime of rolling—cold (20 °C) and hot (825 °C) are investigated. The superconducting core materials were prepared by standard solid-state technology and fill in the high purity (4N) Ag-tubes. The effect of cold and hot rolling deformation on the orientation of supercoducting crystals were studied by XRD analysis. It was found that a higher degree of texturing in the out of plane direction is obtained after hot rolling deformation in tapes with overdoped core. As a result, the highest critical current density (J _c ) at 77 K and zero magnetic field was achieved in them. J _c increases with the time in the superconducting tapes, which is connected probably with the releasing of stress between the superconducting core and Ag sheat.     

High-pressure synthesis of a new homologous series of superconductors, (Hg1−xTIx)2Ba2Can−1CunOy (0.3 ≤ x ≤ 0.6,n=3,4)

We have recently synthesized new superconducting cuprates containing a mercury-based bilayer, i.e. (Hg,Tl)₂Ba₂(Ca,Y)Cu₂O_y and (Hg_1–x Tl_x)₂Ba₂ Ca_n–1 Cu_nO_y (x 0.4, n = 3,4) by a high-pressure technique. These compounds are the n=2, 3 and 4 members of a new homologous series, (Hg, Tl)-22(n–1)n. The effects of Tl substitution for Hg on the formation and superconductivity of the (Hg, Tl)-2223 and -2234 phases are investigated. Both the phases are stabilized for the Tl content x of 0.3–0.6. T_c of the as-synthesized (Hg,Tl)-2223 compound increases from 45 K to 81 K with an increase in x. In contrast, T_c of the (Hg,Tl)-2234 compound is almost independent of x (114 K).     

Effect of Cd and Cd-Ca Substitution on the Superconductivity in Y1−xPrxBa2Cu3O7−δ Superconductor

The structural and superconducting properties of Y_1−v−η Pr_NCd{η}¨zCa _z Ba₂Cu₃O_7−δ system are investigated using X-ray diffraction, ac susceptibility, dc resistivity, and oxygen content measurements. The effect of increasing Cd concentration substituting the Y-site in Y_0.8Pr_0.2Ba₂Cu₃O_7-δ suppresses the superconducting transition temperature and lowers the hole concentration, unlike that of Ca substitution in Y_0.8Pr_0.2Ba₂Cu₃O_7-δ which increasesT _c due to hole doping by Ca. The suppression ofT _c due to Cd substitution can be counterbalanced by simultaneous hole doping by Ca which increases the T_c with increasing Ca concentration. In spite of similarity in the ionic radii and valency, the role played by Cd and Ca substitution at the Y-site in the Y_0.8Pr_0.2Ba₂Cu₃O_7-δ system is opposite in nature as Cd doping helps in T_c suppression due to the Pr effect, suggesting that Cd does not provide the necessary holes like Ca substitution which helps to increase the T_c by the hole doping mechanism.     

A new high-Tc superconductor containing thallium and its crystal structure: The “1212” phase (Tl1-xBix)1.33Sr1.33Ca1.33Cu2O6.667+δ

Joining YBa₂Cu₃O_6.5+ (123 phase) and Bi₄Sr₄Ca₂Cu₄O₁₆₊ (4424 phase) as structurally characterized high-T _c , superconductors, the thallium-containing superconductor (Tl_.75Bi_.25)_1.33Sr_1.33Ca_1.33Cu₂O_6.667+ with the ideal stoichiometry (Tl,Bi)₁Sr₂Ca₁Cu₂O_6.5+ (1212 phase) is reported here. As prepared from the component oxides, 1212 has an initial deviation from resistance linearity at 120 K, a superconducting onset temperature of 92 K, and zero resistance at 75 K. The tetragonal unit cell (P4/mmm, a=3.800 Å;c=12.072 Å, deduced from powder data) contains double copper oxygen sheets (like 4424 and 123) that alternate withsingle thallium-bismuth oxygen sheets (in contrast to 4424, which containsdouble bismuth oxygen sheets), resulting in a total of three stacked perovskite-like cells (as in 123). The copper oxide sheets (with intersheet spacing 3.38 Å) are separated by Ca²⁺ and the Cu oxide sheets and (Tl,Bi) oxide sheets (with spacing 4.35 Å) are separated by Sr²⁺, Ca²⁺, and excess (Tl,Bi)³⁺. The 1212 cell constitutes the building block for the centered, more complex 4424 cell. The 1212 structure persists to Bi contents as low as 1% and can also be stabilized by Pb instead of Bi; Tl cuprates also form other superconductors with lowerT _c .     

Enhanced Superconductivity in?(Cu0.5Tl0.25M0.25)Ba2Ca2Cu3O10?δ Samples

The elements (M = Li, Na, K) of group I-A of the periodic table having lower electronegativity as compared to that of Tl have been partially doped at Tl sites in the (Cu_0.5Tl_0.25M_0.25)Ba₂O_4−δ charge reservoir layer of (Cu_0.5Tl_0.25M_0.25)Ba₂Ca₂Cu₃O_10−δ superconductor. The objective was to investigate the role of electron–phonon interactions and mobile charge carriers in the oxide high temperature superconductors. It has been observed from these studies that the superconductivity parameters were improved after the doping of lower electronegative elements in the charge reservoir layer. The substitution of lower electronegative elements retains smaller amount of oxygen in (Cu_0.5Tl_0.25M_0.25)Ba₂O_4−δ charge reservoir layer, which in turn facilitates the transfer of mobile charge carriers to the conducting CuO₂ planes.     

Effect of substitution at the barium site in YBa2Cu3O7-δ

The oxygen content in YBa₂Cu₃O_7-δ has been varied by substitution at Ba site with Sr or K and its influence on the structural and superconducting properties has been studied. These properties are compared with that of unsubstituted YBa₂Cu₃O_7-δ prepared by quenching. The increase inδ value of YBa₂Cu₃O_7-δ decreases theT _c drastically, whereas in YBa_2-χK_χCu₃O_7-δ system increase in theδ value (as high as 0·92) does not affect the superconducting transition temperature, which remains above 77 K. In the YBa_2-χSr_χCu₃O_7-δ system, as Sr content increases there is marginal decrease in the oxygen content as well as in the superconducting transition temperature.     

Preparation and characterization of Ag-added Bi1.84Pb0.4Sr2Ca2.2Cu3O10+x bulk tube conductors for cryogen free superconducting magnet

Bulk tube conductors of Bi _1.84 Pb _0.4 Sr ₂ Ca _2.2 Cu ₃ O _10+x with addition of silver varying from 0 to 25 wt% (not reported earlier) were systematically studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), electrical transport and a.c. susceptibility techniques. The tube conductors formed by cold isostatic pressing (CIP) of the powders obtained from spray drying method have been made successfully. It was found that Ag addition has not only affected the formation of the desired Bi-2223 phase and the microstructure of these large bulk tube samples thereby influencing on the critical current ( I _c ),it also reduces the contact resistance to minimize the cryogen losses. These variations have been found to be Ag content dependent. An optimized value of 10 wt% Ag has been found to produce the best quality tubes showing reproducible I _c value > 120 Amp at 77 K which is in general a requirement to energies of the cryogen free conventional/HTSC superconducting magnets below 20 K.     

Effect of praseodymium substitution on the growth and properties of Y1−x Pr x Ba2Cu3O7−δ (o <x < 0·4) single crystals

In the present paper, a modified self-flux technique has been successfully employed for the growth of pure and praseodymium substituted (partially) large single crystals of high temperature superconducting Y_1−x Pr _x Ba₂Cu₃O_7−δ (x = 0·0,0·2,0·4). Typical sizes of the platy and bulky crystals of pure YBCO(123) material are ≈ 2 × 2 × 0·1 mm³ and 4 × 1 × 1 mm³, respectively. In case of Pr-substitution, the typical sizes of platy and bulky crystals of Y_0·8Pr_0·2Ba₂Cu₃O_7−δ and Y_0·6Pr_0·4Ba₂Cu₃O_7−δ materials are ≈ 2 × 3 × 0·1 mm³ and 5 × 1 × 1 mm³ and ≈ 1 × 1·5 × 0·1 mm³ and 7 × 0·2 × 0·1 mm³, respectively. The morphology and growth habit of the as-grown single crystals and the critical transition temperature (T _c) of the oxygenated crystals were found to depend on the Pr-content. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

Cu0.5Tl0.5Ba2Ca3Cu4−y Zn y O12−δ (y=0, 1.0, 2.0, 3.0, 3.5): Superconductor with Four ZnO2 Planes

A new Cu_0.5Tl_0.5Ba₂Ca₃Cu_4−y Zn _y O_12−δ (y=0, 1.0, 2.0, 3.0, 3.5) superconductor with four ZnO₂ planes is reported. The structure of the material remains tetragonal for all Zn doping concentration. The substitution of Zn at CuO₂ planar site was carried out following Cu_0.5Tl_0.5Ba₂Ca₃Cu_4−y Zn _y O_12−δ (y=0, 1.0, 2.0, 3.0, 3.5) formula. Contrary to all previous studies of Zn doping in all copper oxide high temperature superconductors, the zero resistivity critical temperature T _c(R=0), critical current density and quantity of diamagnetism increase with increased Zn concentration. The onset temperature of superconductivity in these samples was observed at 128 K and T _c(R=0) at 122 K for y=3.5. The volume of the unit cell observed through X-ray diffraction scan is found to decrease with increase Zn doping; promoting an increase in Fermi vector K _F and effective density of states which results in enhanced superconductivity parameters. The synthesis of Cu_0.5Tl_0.5Ba₂Ca₃Cu_4−y Zn _y O_12−δ material by this method is highly reproducible.      

Effect of CuO addition in Bi-Sr-Ca-Cu-O and Y-Ba-Cu-O ceramics

The effect of adding CuO matrix to Bi_1·6Pb_0·4Sr₂Ca₂Cu₃O_δ and Y₁Ba₂Cu₃O_δ superconductors was investigated through resistivity, XRD, SEM, thermoelectric power (TEP), and ac magnetization studies. Samples with as low as 20% (by weight) of the superconducting phase showed superconducting transition in resistivity-temperature (R-T), with the superconducting zero around 77 K in thermoelectric power, critical current (J _c) values were evaluated by applying Bean’s critical state model. CuO could be added to the superconducting material without any significant deterioration in the quality of the samples, up to a concentration of as low as 40% (by weight) of the superconducting material.     

Optical absorption and electron spin resonance studies of Cu2+ in Li2O-Na2O-B2O3-As2O3 glasses

The local structure around Cu²⁺ ion has been examined by means of electron spin resonance and optical absorption measurements in xLi₂O-(40-x)Na₂O-50B₂O₃-10As₂O₃ glasses. The site symmetry around Cu²⁺ ions is tetragonally distorted octahedral. The ground state of Cu²⁺ isd _x ²−y ².The glass exhibited broad absorption band near infrared region and small absorption band around 548 nm, which was assigned to the ²B_1g →²E_g transition.     

High-temperature ceramic oxide superconductors

High-temperature superconductivity in oxides of the type La_2−x Ba _x (Sr _x )CuO₄, YBa₂Cu₃O_7−δ , La_3−x Ba_3+x Cu₆O₁₄ and Bi(Tl)-Ca-Sr(Ba)-Cu-O systems is discussed, with special emphasis on the experimental findings from the author’s laboratory. The importance of holes on oxygen and of the Cu¹⁺ (d ¹⁰) state is examined. A transition is shown to occur from chain- to sheet-superconductivity in YBa₂Cu₃O_7−δ accompanying a change in oxygen stoichiometry. Some of the important material parameters and technological applications are briefly presented. There is every hope that materials withT _c close to room temperature will be discovered in the near future. All the highT _c oxides found hitherto have perovskite-related structures with two-dimensional Cu-O sheets. Contribution No. 485 from the Solid State and Structural Chemistry Unit.     

Spectroscopic investigations of Cu2+ in Li2O-Na2O-B2O3-Bi2O3 glasses

Pure and copper doped glasses with composition,x Li ₂ O-(40-x)Na ₂ O-50B ₂ O ₃-10Bi ₂ O ₃,have been prepared over the range 0 ≤ x ≤ 40. The electron paramagnetic resonance (EPR) spectra of Cu²⁺ ions of these glasses have been recorded in the X-band at room temperature. Spin Hamiltonian parameters have been calculated. The molecular bonding coefficients, α² and β², have been calculated by recording the optical absorption spectra in the wavelength range 200–1200 nm. It has been observed that the site symmetry around Cu²⁺ ions is tetragonally distorted octahedral. The density and glass transition temperature variation with alkali content shows non-linear behaviour. The IR studies show that the glassy system contains BO₃ and BO₄ units in the disordered manner.     

Ion Beam Analysis and Physical Properties Measurements of (Tl0.8Hg0.2−x Sb x )Ba2Ca2Cu3O9−δ

The superconducting phase (Tl_0.8 Hg_0.2−x Sb _x ) Ba₂Ca₂Cu₃O_9−δ , with 0.0 ≤x≤0.2, was characterized by means of X-ray powder diffraction (XRD), scanning electron microscope (SEM), proton induced X-ray emission (PIXE), Rutherford backscattering spectrometry (RBS), electrical resistivity (ρ) and transport critical current density (J _ct). The tetragonal structure of (Tl, Hg)-1223 remains unchanged with antimony-substitution, whereas the lattice parameters were changed. The actual elemental-content of Tl, Hg, Sb, Ba, Ca and Cu, determined from PIXE, were very close to those of the nominal elemental-content. The oxygen-stoichiometry of the prepared samples was determined by non-Rutherford backscattering cross section at 3 MeV H⁺. A little increase in the oxygen stoichiometry was observed as Sb-content increased. The superconducting transition temperature T _c, determined from electrical resistivity, increased from 124.8 K to 131.5 K as x increased from 0.0 to 0.1 and then it decreased with the further increase in x. The same behavior is obtained for J _ct vs. x. This indicates that the optimal Sb-substitution in (Tl, Hg)-1223 is at about x=0.1.     

Ca doped YBCO on the Ba site

The effect of partial, nominal replacement of Ba with Ca on the properties of the YBa₂Cu₃O_6+gd superconductor varies with the concentration of Ca. For nominal concentrations in Ca up to 50% a two phase system is formed: the known superconducting phase Y_1–yCa_yBa₂Cu₃O₆₊ and a new, non superconducting phase of composition Y_1.14Ca_1.58Cu_3.27O_6.45. The x-ray diffraction pattern of this new phase matches the Bragg peaks, as well as the commensurate and incommensurate peaks of a partially indexed pattern of the phase Y_2–xCa_2+xCu₅O₁₀. For nominal Ca concentrations 40 and 50%, microprobe analysis shows that the superconducting phase becomes a mixture of Y_1–yCa_yBa₂Cu₃O_6+° and YBa_1.8Ca_0.2Cu₃O_6.5, where Ca enters the structure in the Ba site. The microraman spectra indicate a small substitution of Ba by Ca, while the new phase shows novel characteristics.     

Investigations of chemical interaction between Bi-based 2212 and (RE)Ba2Cu3O7 high Tc superconducting materials

Composite materials have been synthesized by mixing 90% (or 95%) YBa₂Cu₃O₇ and 10% (or 5%) Bi₂Sr₂Ca₁Cu₂O₈ by weight, and firing at 900°C to promote grain growth by inducing a liquid phase (Bi₂Sr₂Ca₁Cu₂O₈) in the system. The influence of the amount of liquid phase on the X-ray diffraction data and electrical properties is reported. Energy dispersive X-ray (EDX) analyses are also reported. The YBiBa₂O₆ phase is formed during the heat treatment and introduces additional chemical heterogeneities at the grain boundaries. A previously reported 2212-related superconducting phase, Bi₂(Sr,Ba)₂(Ca,Y)Cu₂O_8+y, could also be formed during the synthesis process, and its effect on the electrical resistance versus temperature measurements is discussed. Attempts to substitute RE ions (Dy³⁺, Er³⁺, Ho³⁺) for Y³⁺ in YBiBa₂O₆ have been successful and are reported in an appendix section. X-ray diffraction data are also reported. EDX analyses have been performed specifically for a typical ErBiBa₂O₆ compound and reveal the presence of a new Er₂Ba₄O₇ phase.