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

Two families of mercury-based superconductors, Hg 0.4 Pr 0.6 Sr 2− x Pr x and Hg 0.3 Pb 0.7 Sr 2− x La x CuO 4+δ , with a T c onset of 41 K, have been isolated. The praseodymium phase exhibits a narrow existence range x ⩽ 0.1; in the lead phase, lanthanum can be substituted for strontium up to x = 0.65. The X-ray diffraction and high-resolution electron microscopy analyses show that they both belong to the “1201” series; the structures can be described from the intergrowth of single perovskite layers with double oxygen-deficient rock-salt layers built up from the mixed layers [Hg 0.4 Pr 0.6 O δ ] ∞ and [Hg 0.3 Pb 0.7 O δ ] ∞ . Evidence is found for ordering between mercury and praseodymium and a structural model is proposed.

New mercury-based superconductors with the “1201” structure Hg0.4Pr0.6Sr2−xPrxCuO4+δ and Hg0.3Pb0.7Sr2−xLaxCuO4+δ

A new superconductor with an original structure, Sr4−xBaxTlCu2CO3O7 has been isolated for 1≤x≤2. It crystallizes in a A-type space group with a=3.84 A ≈a p , b≈8 × a p , c≈17.0 A . The HREM study of this new curprate shows that it derives from the Tl0.5Pb0.5Sr4Cu2CO3O7 structure by a shearing mechanism. Indeed, it can be described as ribbons of the latter structure, four octahedra thick, shifted c/2 with respect to each other. It results in infinite single perovskite layers waving along the (001) plane and connected through flat mixed layers [(TIO)4(CO)4]∞. This oxycarbonate can also be described as a 1201-type structure in which the [TIO]∞ layers are replaced by mixed [(TIO)4(CO)4]∞ layers. The susceptibility measurements show that this cuprate exhibits a significantly higher critical temperature than the 1201-cuprates. The Tc onset of the synthesized phase is 56 K and it can be increased up to 62 K by hydrogen annealing; in the same way, one observes a large Meissner volume fraction of 35% at 4.2 K.

A 62 K superconductor with an original structure: Sr4−xBaxTlCu2CO3O7

It is shown that the superconducting state can be stabilized ( T c =9.5 K) by slow-cooling TlBa 2 CuO 5 in a nitrogen environment. Interestingly, partial substitution (50%) of Ba 2+ by Sr + 2 yields a zero-resistance state at 33 K in TlBaSrCuO 5 , even for air-quenched samples. T c could be further enhanced to 43 K ( R =0) by a slow cooling treatment in flowing nitrogen. Contraction of the lattice for TlBaSrCuO 5−δ in comparison with TlBa 2 CuO 5 , supports the substitution of Sr 2+ -ions at Ba-sites.

Stabilization of superconductivity in TlBa2CuO5−δ at 9.5 K and its enhancement to 43 K in TlBaSrCuO5−δ

Crystal chemistry of superconducting copper oxycarbonates

Stable and reproducible superconductivity in the Tl(Ba2-xLax)CuO5-δ (0.0≤x≤0.6) system with the tetragonal TlBa2CuO5-δ-type (1021) structure was reported. A Prototype compound TlBa2CuO5-δ had shown a metastable superconducting onset around 25 K, with zero resistivity at 10 K. With partial substitution of La for Ba ions, Tc (50% resistivity drop) increases to 45 K, Tc0 (zero resistivity) to 42 K and onset around 50 K. A diamagnetic signal was observed with onset as high as 57 K. Tetragonal lattice parameters decrease with the increasing La concentration due to the partial replacement of larger Ba2+ ions by smaller La3+ ions. The Pairing field energy of 170 K and electron-elementary excitation coupling constant λ of 0.76 were derived from the BCS-like Tc formula through comparison with other single Tl-O layer systems TlCan-1Ba2CunO2n+3-δ.

https://ir.nctu.edu.tw:443/bitstream/11536/4355/1/A1989AD72800011.pdf

The occurrence of superconductivity in the TlBa2CuO5−δ-type (1021) system

The magnetic properties of the monophasic superconductor Tl0.5Pb0.5Ca0.8Er0.2Sr2Cu2Oy with a TlCaBa2Cu2O7-type (1122) structure have been studied using a combination of magnetisation and ESR (Electron Spin Resonance) measurements. This new material exhibits a superconductive transition (Tc) around 110 K, above which the temperature dependence of the magnetic susceptibility can be described by a Curie-Weiss law. Below Tc the co-existence of superconductivity and paramagnetism was observed.

T. J. Watson-Yang

Papers

The occurrence of superconductivity in the TlBa2CuO5−δ-type (1021) system

Magnetic Properties of the High-Tc Superconductor Tl0.5Pb0.5Ca0.8Er0.2Sr2Cu2 Oxide