Oxycarbonate cuprate called Sr₂CuO₂(CO₃) was reported in 1988. After the report, it was found that the crystal structure has CuO₂ plane which consists of Cu and O like copper oxide superconductor, and stacks Sr₂CO₃ and CuO₂ plane alternately. Since the discovery of oxycarbonate superconductor, (Ba_0.55Sr_0.45)₂Cu_1.1O_2.2+δ(CO₃)_0.9 with 30 K of superconductive transition temperature, our laboratory has developed oxycarbonate superconductors. Here, let us introduce the oxycarbonate superconductors discovered by this laboratory.

1. 1992   (Y,Ca)Sr₂Cu_3-x(CO₃)_xO_y        T_c=63K

2. 1992   (Bi,Pb)-Sr-Cu-(CO₃)-O       T_c = 41K , 54K

3. 1993   Ca(Ca,Sr)₂Cu₂(CO₃)_1-x(BO3)_x        T_c=33K , 55K , 105K , 115K

4. 1993   Tl(Ba,Sr)₄Cu₂(CO₃)O_7-δ        T_c=73K

5. 1993   Hg(Ba,Sr)₄Cu₂(CO₃)O_6-δ        T_c=67K

1. (Y,Ca)Sr₂Cu_3-x(CO₃)_xO_y

Development of the oxycarbonate superconductors began with the concept of the block layer by Tokura and Arima at the University of Tokyo. Our laboratory regards Sr₂CO₃ as a new block layer after the discovery of Sr₂CuO₂(CO₃), and composed oxycarbonate superconductors under partial pressure of a carbonic acid. Fig.1 is the crystal structure of (Y,Ca)Sr₂Cu_3-x(CO₃)_xO_y. Replacing Y³⁺ by Ca²⁺, this substance shows the superconductor with T_c = 63 K. Moreover, as it replaces Y by lanthanoids, such as Er, Ho, Dy, Gd, Eu, and Sm, it becomes a superconductor with T_c ~ 50 K. As for the crystal structure, some Cu-O chains are replaced with the carbonate ion, because the size of carbonate ion and the size of the copper-oxide perovskite are almost the same. Therefore we expect many cuprates show superconductors composed under partial pressure of a carbonic acid.

2. (Bi,Pb)-Sr-Cu-(CO₃)-O

The feature of oxycarbonate superconductors is combination of the block layers which supply apex oxygen in spite of the fact that it was considered cuprates could not keep such block layers. Therefore in case of oxycarbonate superconductors, any combination of block layers is permitted. It is expexted that many approaches for depeloping new superconductors. Fig.2-1 and 2-2 are the crystal structures of (Bi,Pb)₂Sr_2+nCu_1+n(CO₃)_nO_6+2n. Fig.2-1 corresponds to the same crystal structure of Bi-2223 and it is the case of n=1. Moreover, the case of n=2 is shown in Fig.2-2, it is the same structure with Bi-2245. CuO₂-plane without apex oxygen is replaced by the carbonate ion. These have unstable structure stacking CuO₆. In the chemical viewpoint, it is not known whether the apex oxygen shared with this carbonate ion is regarded as normal apex oxygen. Therefore, this is very interesting that such structures are permitted. This system shows superconductivity in replacing Bi³⁺ by Pb²⁺. This superconductive transition temperature is about 41[K] in the case of n=1 ((Bi,Pb)₂Sr₃Cu₂(CO₃)O₈) and is about 54[K] in the case of n=2 ((Bi,Pb)₂Sr₄Cu₃(CO₃)₂O₁₀). Although it is a low value compared with the usual transition temperature such as Bi-2223, this never means that the carbonates never show high temperature superconductivity. The answer is shown in the next chapter about replacement by borate.

3. Ca(Ca,Sr)₂Cu₂(CO₃)_1-x(BO₃)_x

In case of exygen deficit type block layer structure in Ca system, the multilayer structure is permitted. It is possible to increase the number of CuO₂ planes in a unitcell. Our laboratory succeeded in compounding a substance Ca(Ca,Sr)_2+nCu_1+n(CO₃)_1-x(BO₃)_xO_y (n=1,2,3) with stacking structure as shown in Fig.3. The carrier control is performed by replacing CO₃ by BO₃. The chemical bond and the size of carbonate ion are similar, but the balence between the two is different. Therefore this is a very effective method. The superconductive transition temperature is 55K (n=1), and 105K (n=2) and 115K (n=3), respectively. In the case of n=1, if we compose not using Ca, but using Ba, the superconductive transition temperature goes up to 60K. This is because the CuO₂-plane is smoothed by replacing by a large ion radius element, Ba. In the case of n=3, it realizes high transition temperature, and it is one of the highest T_c superconductors, excepting for those include dangerous elements such as Hg or Tl. It is thought that this system is suitable for carrier control because carbonate ion can be replaced by borate ion. It is expected that the copper oxide superconductor containing carbonate ion with multilayer structure will exceed the transition temperature of Hg system.

4. Tl(Ba,Sr)₄Cu₂(CO₃)O_7-δ

Although Tl is very a strong toxic element, it is known that the copper oxide containing Tl shows very high transition temperature. Therefore the material design in Tl system has been performed earnestly. As for superconductors containing carbonate ion, Tl_0.5Pb_0.5Sr₄Cu₂(CO₃)O_y was discovered. The superconductive transition temperature is 70 [K], and this discovery enhanced the material design containing the carbonate ion in Tl system. Our laboratory discovered a superconductor with very unique super-lattice structure, Tl(Ba,Sr)₄Cu₂(CO₃)O_7-δ. This substance tends to be overcareer because of much oxygen deficit. Therefore without a carrier doping, it has 73[K] of superconductive transition temperature. Fig.4 is the crystal structure. It is found that the substance has the periodic structure called (C-C-C)-(Tl-Tl-Tl)-(C-C-C) in the direction of a-axis. This periodic structure is realized by the amount of substitution of Ba. Moreover, as Sr and Ba is a divalent, Ba substitution does not result in carrier control. However, transition temperature changes with the amount of Ba substitution. This is because the amount of oxygen changed with substitution. This is very interesting because the periodic structure is caused by Ba substitution and crystal structure is changed by the amount of oxygen.

5. Hg(Ba,Sr)₄Cu₂(CO₃)O_6-δ

The new superconductor containing Hg, HgBa₂CuO_4+δ, is discovered in 1993. This substance shows the superconductivity at 94 [K]. It has only one CuO₂-plane in unit cell. HgBa₂Ca₂Cu₃O_y, which can increase the number of CuO₂-plane shows superconductivity at 134[K] and has the present highest transition temperature. This substance increases the transition temperature up to 164[K] by applying pressure. This is because the new type block layer HgBaO_2+δwhich lacks most of the oxygen in HgO. With the new block layer this HgBaO_2+δ and the block layer containing carbonate ion called Sr₂CO₃, our laboratory discovered HgSr₂Ba₂Cu₂(CO₃)O_6+δ shown in Fig.5. CuO₂-plane is not a perfect 2-dimensional plane, but very rippled structure. By the sample which administered oxygen annealing, superconductive transition temperature is about 67[K], and it has become a comparatively low value in Hg system. However, it is also possible that it has the high transition temperature 67[K] although CuO₂-plane is rippled. We can say that the substance containing carbonate ion in Hg system has high transition temperature, judging from the result of the usual Hg system copper oxide superconductor.