A systematic view stemmed for the most important parts from the authors’ own research results is presented for tailoring high-Tc superconductive copper oxides from the standpoint of materials chemistry. The present article first elucidates that, in the multi-layered superconductive copper-oxide structures presently known to exist, the distribution of charge/charge carriers is inhomogeneous over several different spatial dimensions throughout the crystal. At the same time useful guiding rules are derived based on the concepts of tolerance parameter and bond-valence for controlling the charge distribution. Then experimental and semi-experimental evidences are presented to establish the relationships between the inhomogeneous distribution of holes and superconducting properties. To reach this goal, phenomenological aspects of the high-Tc superconductive copper oxides are described systematically, and selected important techniques for estimating the local carrier concentration and distribution are discussed. That is, the following points are included in the present article: (i) general crystallographic categorization and naming scheme of multi-layered copper oxides based on the concept of homologous series, (ii) crystallographic and chemical factors to control the oxygen non-stoichiometry and charge distribution, (iii) techniques for probing the charge/carrier distribution in the layered copper-oxide crystal, and (iv) empirical relationships between the carrier distribution and superconducting properties including the superconductivity transition temperature, Tc, the magnetic irreversibility field, Hirr, and the so-called peak effect/fishtail phenomenon. Finally, the unified view is summarized and some indespensable questions to be still answered in future work are mentioned.

Control of the charge inhomogeneity and high-Tc superconducting properties in homologous series of multi-layered copper oxides

Nearly single-phase polycrystalline samples of HgBa2Ca2Cu3O8+δ were successfully synthesized under relatively simple and unprecedented synthesis conditions, i.e. at a “low” temperature (<700°C) and for a “long” period of time (5–168h), using mono-oxide powders of Hg, Ba, Ca, and Cu as starting materials. A pressed powder mixture was heated at 665°C for ≈75 h in an evacuated fused quartz tube, and then post-annealed in flowing O2 gas to exhibit a Meissner signal below 135.4 K. Zero resistance of this sample was observed at 133.9 K.

Encapsulation method for the synthesis of nearly single-phase superconducting HgBa2Ca2Cu3O8+δ with Tc⩾ 135 K

Several methods of chemical analysis, for example various redox titrations and thermogravimetric approaches, have been adapted to establish the oxygen content in superconducting cuprates and to determine the higher oxidation states of copper and other mixed-valent metals. The present review discusses and complements the authors' own experimental results on the oxygen content and metal valence analyses, but literature data from about 150 other references are reviewed as well.

https://iopscience.iop.org/article/10.1088/0953-2048/9/3/001/pdf

Determination of oxygen content and metal valences in oxide superconductors by chemical methods

MgO particles were introduced into (Bi,Pb)2Sr2Ca2Cu3O10+δ powder in various weight fractions. Bulk samples were made from the Bi2223/MgO powder and heat-treated at various temperatures. The phase formation and the microstructure were studied for different amounts of MgO and for different sintering conditions. It was found that, below 845 °C, the ratio of Bi2223 phase decreased when the amount of MgO increased. The addition of MgO caused the Bi2201 phase content to decrease via an increase of the liquid viscosity. The presence of MgO influenced the microstructure of the samples and decreased the mean size of Bi2223 grains. The effects of the MgO addition on the intergranular critical current density, Jc as a function of magnetic field was investigated by magnetisation hysteresis measurements. MgO presence significantly expands the magnetisation hysteresis loop and thus increases Jc for amount of MgO, which does not exceed 15 wt.%.

The effect of MgO addition on the formation and the superconducting properties of the Bi2223 phase

We have investigated the effect of addition of Gd in Bi 1.8 Pb 0.35 Sr 1.9 Ca 2.1 Cu 3 Gd x O y superconductor with x =0, 0.1, 0.2, 0.3, 0.4 and 0.5. The samples were prepared by standard solid-state reaction methods. The investigation consisted of X-ray diffraction (XRD), scanning electron microscopy (SEM), dc electrical resistivity, hole concentration and transport critical current density measurements. Transport measurements indicated that the superconducting transition temperature, transport critical current density and hole concentration values of the samples strongly depend on the Gd addition. The values of T c , and J c of the samples decreased with the increase in Gd addition. When Gd addition ratio was increased, surface morphology and grain connectivity of the samples were observed to degrade from SEM investigations and phase ratio of the high- T c (Bi-2223) phase to the low- T c (Bi-2212) phase decreased (XRD measurements). The possible reasons for the observed degradation in superconducting and microstructure properties of Bi-2223 due to Gd addition were discussed.

The influence of Gd addition on microstructure and transport properties of Bi-2223

(Bi,Pb)-2223 superconductors with nominal composition Bi1.85Pb0.35Sr2Ca2Cu3.1Oy were synthesized as almost single phase samples, with a Tc onset of 110 K. By applying a vacuum encapsulation and low temperature post-annealing process, Tc onset increased to 115 K, and Tc zero exceeded 110 K. If samples were encapsulated in an oxygen atmosphere, Tc did not enhance after post-annealing. The powder X-ray diffraction analysis showed that, with the enhancement of Tc, the lattice parameters a and c increased. From thermoelectric power measurements and the evolution of the lattice constants, it is suggested that the increases in Tc may be due to the decreases in hole concentration.

Enhancement of Tc in (Bi,Pb)-2223 superconductor by vacuum encapsulation and post-annealing

The oxygen contents and metal valences have been determined in various (Bi, Pb) 2 Sr 2 (Ca, Y)Cu 2 O z , Bi 2 Sr 2 Ca 2 Cu 3 O z and Pb 2 (Ba, Y, Ca) 3 Cu 3 O z samples by a two-step coulometric titration method. The first titration experiment, which consists of the reactions of Cu(III), Bi(V) and Pb(IV) with CuCl followed by an anodic oxidation of the excess Cu(I), reveals the oxygen content of the sample. In the second experiment the valences of bismuth and lead are distinguished from the Cu-valence by taking advantage of the reaction with Fe(II). By performing the first experiment without adding CuCl it was even possible to evaluate the distinct valence values for crystallographically different copper atoms when the sample contained both Cu(III) and Cu(I) in separate layers. The applicability of the employed analysis procedures as well as the results obtained for the studied materials are discussed.

Valence studies on various superconducting bismuth and lead cuprates and related materials

The origin of superconductivity in samples of (Bi 0.3+ m Cu 0.7 )Sr 2 -Y 1- m Cu 2 O z ( m = 0.2 and 0.3) was investigated and was found to be due to a small amount of Bi-2212 phase. A virtually single phase of Bi-based 1212-type materials was prepared with the nominal composition of (Bi 0.3 Cu 0.7 )Sr 2 YCu 2 O z , which was insulating after sintering in air. Following high oxygen pressure annealing, the oxygen content z was greatly increased, and superconductivity was induced at T onset c = 18 K, and T R =0 c = 15 K for z = 7.125. The superconducting volume fraction was found to be small, and the conductivity of non-metallic regions in the annealed samples follows the variable range hopping law, suggesting that carrier localization is still preponderant and more mobile holes are needed for getting both higher T c and higher volume fraction in this system.

Synthesis and high oxygen pressure annealing effects in (Bi,Cu)-1212 cuprates

(Bi,Pb)-2223 superconductors with nominal composition of Bi1.85Pb0.35Sr2−xCa2+xCu3.1Oy (x=0 and 0.1) were synthesized. They are virtually single-phase and show Tc (mag.) around 109K~110K. By applying an encapsulation and low temperature post-annealing process, Tc increased to 115 K by SQUID measurement. From the resistivity, thermoelectric power measurements and the evolution of the lattice constants, it is suggested that the increases in Tc may due to the decreases in hole concentration.

Effect of post-annealing on Tc in Pb-doped Bi-2223 superconductors

Almost single-phase (Bi, Cu)-1212 samples, (Bi 1- x Cu x )Sr 2 YCu 2 O y with various oxygen contents were synthesized only in very narrow ranges of the composition around x ≈0.7, and with the sintering temperature around 980°C. Although the resistivity decreased with increasing oxygen content, all the samples did not show metallic conductivity nor superconductivity. Compared with Bi-2212 cuprates, it was estimated that the density of doped hole carriers in the samples with high oxygen content was high enough to show superconductivity. It was confirmed that the doped carriers were localized by the observation of variable-range hopping conductivity and positive magnetoconductivity. The origin of the localization in layered cuprates is discussed.

Jie Wang

Papers

Enhancement of Tc in (Bi,Pb)-2223 superconductor by vacuum encapsulation and post-annealing

Valence studies on various superconducting bismuth and lead cuprates and related materials

Synthesis and high oxygen pressure annealing effects in (Bi,Cu)-1212 cuprates

Effect of post-annealing on Tc in Pb-doped Bi-2223 superconductors

Synthesis and transport properties of (Bi, Cu)-1212 cuprates evidence of the localization of the doped holes