A variety of synthetic strategies has been employed to prepare pure monophasic cuprates of different families with good superconducting properties. Besides the traditional ceramic method, other methods such as coprecipitation and precursor methods, the sol-gel method, the alkali flux method and the combustion method have been employed for the synthesis of cuprates. Depending on the requirements, varying conditions such as high oxygen or hydrostatic pressure and low oxygen fugacity are employed in the synthesis. In this review, the authors discuss the synthesis of the various types of cuprate superconductors and point out the advantages and disadvantages of the different methods.

https://iopscience.iop.org/article/10.1088/0953-2048/6/1/001/pdf

Synthesis of cuprate superconductors

The one-dimensionally modulated structure of the high-Tc superconductor Bi2Sr2CaCu2O8+x with a0=5.407(2), b0=5.412(3), c0=30.1771(8) AA, q=0.210(2)b0* has been analysed in the commensurate approximation using the conventional three-dimensional space group Pnnn and utilizing single-crystal X-ray diffraction data. A reliable model (R=0.07, Rw=0.079 for 1011 unique reflections) has been obtained. The extra oxygen atoms have been found inside and between the Bi-O-Bi chains of the BiO layer. The modulation displacement functions of the atoms have been obtained by means of harmonic analysis of the modulation displacements. It has been shown that the topology of Bi-O-Bi chains. The modulation of the structure and the form of the modulation displacement functions can be explained by the shifts of the oxygen atoms in the BiO layer correlated according to the antiferroelectrical law. The cationic vacancies in the Sr sites provide the hole conductivity in the copper-oxygen plane and lead to the formation of the percolation picture in this plane.

https://iopscience.iop.org/article/10.1088/0953-8984/6/19/009/pdf

Causes of modulation and hole conductivity of the high-Tc superconductor Bi2Sr2CaCu2O8+x according to X-ray single-crystal data

The discovery by J. G. Bednorz and K. A. Muller in 1986 that the superconducting state can exist in oxides at temperatures above 30 K stimulated research in the field of superconductivity and opened a new field of research. Within a few years a large number of cuprate superconductors with transition temperatures well above the boiling point of liquid nitrogen have been found. In this chapter, an overview of the major families of high-temperature superconductors and their physical properties is presented.

High-Temperature Superconductors

The 110 K superconductor of the (Bi,Pb)‐Sr‐Ca‐Cu‐O system is prepared in single‐phase form at the composition Bi1.6Pb0.4Sr2.01Ca2.71Cu3.93Oy. Disks of the material with favorable texture can be obtained in relatively short durations in an O2 poor atmosphere. The literature on the compositions of the 110 K superconductor is reviewed to reveal that the material can be synthesized in single‐phase form in a range of compositions, which is poorer simultaneously in Bi and Sr with respect to the nominal composition (Bi,Pb)2Sr2Ca2Cu3Oy, (2:2:2:3). At compositions close to 2:2:2:3, intermediate cold pressing steps between heat treatments are necessary to promote the formation of single‐phase material. We find that at compositions which are considerably poorer in Bi and Sr with respect to 2:2:2:3, no such steps are necessary. In this paper, the microstructural reasons for this difference in behavior are also investigated.

On the composition of 110 K superconductor in a (Bi,Pb)‐Sr‐Ca‐Cu‐O system

Four-circle x-ray diffraction measurements were carried out on a single crystal of Bi 2.1 Sr 1.9 Ca 2 Cu 3 O 10+δ with doping levels from underdoped to overdoped. The excess charge of each inequivalent CuO 2 plane was estimated using the obtained inter-atomic distances in two ways by means of the bond–valence-sum method. The copper valence and the valence sum of the copper and the surrounding oxygen ions gave contradictory results for the inhomogeneous charge distribution of the inequivalent CuO 2 planes.

Structural Study of Inhomogeneous Charge Distribution of Inequivalent CuO2 Planes in Bi2.1Sr1.9Ca2Cu3O10+δ Single Crystals

The high-${\mathit{T}}_{\mathit{c}}$ (110-K) superconducting phase in the Pb-Bi-Sr-Ca-Cu-O has been isolated and its structural and physical properties investigated. Transmission-electron-microscopy results show that the structure belongs to a monoclinic system that is different from reports by other groups and in agreement with our x-ray-diffraction analysis. The refined crystal data are a=3.779 \AA{}, b=3.834 \AA{}, c=37.150 \AA{}, \ensuremath{\alpha}=\ensuremath{\beta}=90\ifmmode^\circ\else\textdegree\fi{}, \ensuremath{\gamma}=90\ifmmode^\circ\else\textdegree\fi{} 51', V=538.5 A${\mathrm{\r{}}}^{3}$, space group I112. Based on the resistivity, magnetization, critical-field, and specific-heat measurements, various parameters characterizing the monophase superconductors were determined or derived. Some of them are listed as follows: ${\mathit{T}}_{\mathit{c}\mathit{e}}$ (endpoint)=107 K, ${\mathit{T}}_{\mathit{c}\mathit{m}}$ (midpoint)=110 K, \ensuremath{\Delta}C (specific heat jump at 107 K)=5.3 J/mol K, \ensuremath{\gamma} (the Sommerfeld constant) =34.6 mJ/mol ${\mathrm{K}}^{2}$, FTHETA (Debye temperature)=418 K, ${\mathit{H}}_{\mathit{c}1}$(0)=900 G, ${\mathit{H}}_{\mathit{c}2}$(0)=184 T, and ${\mathit{J}}_{\mathit{c}}$(1.5 K, 0 T)=4.8\ifmmode\times\else\texttimes\fi{}${10}^{4}$ A/${\mathrm{cm}}^{2}$, etc. Morphology, thermal analysis, and pressure effects of the sample are also mentioned.

F. Shi

Papers

Preparation process, crystal structure, and physical properties of the 110-K single-phase Pb-Bi-Sr-Ca-Cu-O superconductor.