The current status of oxide superconductors is briefly reviewed from a crystal structure perspective. Tokura et al. recently classified high-temperature superconductors using block layer concept. We propose a new “layer” concept and discuss the possibility of new block layers by reconstructing the layers. As an example, we present new high-Tc materials containing CO3 by combining an “old” and new block layers recently discovered. Finally, main roles for realizing the high-Tc superconductors are discussed from the structural point of view.
Cupric-oxide superconductors are complex oxides in which counter cations like rare-earth and alkaline-earth ions are involved. The counter cations and cupric ions compromise with each other to coexist in a crystal by adjusting their coordination numbers, bond lengths, and bond angles. Pressure and temperature can evidently influence these parameters, and so it is expected that, depending upon pressure and temperature, different counter cations would be stabilized in a given structure and different structures would appear for a given composition. By applying 6GPa to the alkaline-earth-copper-oxygen system a new superconductor, (Ca1-y Sry) 1-xCuO2, with the highest Tc of 110K has been found.
Sr2 (Y0.65Ca0.35) Cu2GaO7 and (Ba1-xSrx) 2Cu1+yO2+2y+z (CO3) 1-y contain novel structural blocks which have not been found in other superconductors. The crystal structure of Sr2 (Nd0.75Ce0.25) 2Cu2NbO10-z is also unique in that the Nb ion fully occupies a site corresponding to the B site in perovskite-type compounds. Pressureinduced structural changes in Ba2YCu4O8 and Tl2Ba2CuO6+z were studied by TOF neutron powder diffraction under high pressure. Marked pressure dependences of Tc in these superconductors are ascribable to the transfer of holes from charge reservoirs to CuO2 conduction sheets.
Superconducting La1-xSrxCaCu2O6 samples with x in the range of 0.1≤x≤0.4 were synthesized by means of O2-HIP (hot isostatic press) technique. Superconducting properties were optimized at the composition of La1.8Sr0.2CaCu2O6, when the sample was O2 HIP annealed for 50h at 1070°C in the mixture of 80%-Ar and 20%-O2 gas of the total pressure of 100MPa. A joint neutron x-ray diffraction study was carried out to analyze both cation ordering and oxygen stoichiometry. This revealed the prefernital occupation of Ca for 2a site and of both La adn Sr for 4e site. A comparison of the structure of La1.8Sr0.2CaCu2O6 with those of non-superconducting or weakly superconducting doped samples suggests structural differences that may be responsible for the observed differences in behavior.
The crystal chemistry of lead-based copper oxide superconductors is discussed on the basis of their crystal data obtained by X-ray and neutron powder diffraction. The block layers of the quenched specimens of Pb2Sr2YCu2Oy (Pb3212) and PbBaSrYCu3Oy (Pb2212) consist of independent PbO and Cu layers, whereas those of the oxygen-annealed specimens of Pb2212 and (Pb, Cu) Sr2 (Y, Ca) Cu2Oy (Pb1212) contain double or single (Pb, Cu) Ox layers with disordered metal configurations. The structure of the block layers for oxygen-annealed Pb3212 varied with annealing conditions. Displacement of oxygen atoms from ideal positions in the (Pb, Cu) layer was considered to affect the crystal structure of Pb3212 and Pb2212.
Modulation structures of (1) Bi2+xSr2-xCuOz (2201) type of compounds doped with Fe and (2) Bi2Sr2CoOz doped with Pb, are examined by electron diffraction and high-resolution transmission electron microscopy. It is found that modulation periodicity decreases by increase of Bi3+ and Few replacements of the Cu sites of the 2201 compounds. Modulation periodicity increases by increase of Pb2+ replacements of the Bi sites in Bi2Sr2CoOz, and finally modulation disappears by 35% or higher replacements. These results are compatible with the expectations based on “excess oxygen model” on the origin of the modulation structures.
Critical current density of the superconductor is not an intrinsic property, but it is strongly dependent on the microstructure. Hence, microstructural control is very important in order to achieve large critical currents. In this review, I will present several defects which are considered to be responsible for flux pinning enhancement in oxide superconductors.