Phase evolution, structural and superconducting properties of Pb-free Bi₂Sr₂Ca₂Cu₃O₁₀₊ single crystals

Abstract

Sizable Bi2Sr2Ca2Cu3O10+δ (Bi-2223) single crystals were grown by the travelling solvent floating zone technique and subsequently annealed in O2 and/or Ar flow for 120–500 h in a temperature range of 430–850 °C. The effect of annealing on the phase evolution as well as the structural and superconducting properties was studied using x-ray diffraction (XRD) and magnetization measurements as well as ellipsometric measurements of the far-infrared c-axis conductivity. The results show that some of the as-grown Bi-2223 crystals are nearly phase pure, while others contain a certain amount of Bi2Sr2CaCu2O8+δ (Bi-2212), Ca2CuO3 and CuO phases co-existing with the Bi-2223 phase. Annealing these multi-phase crystals in O2 flow at high temperature can lead to a phase transformation from Bi-2212 to Bi-2223, which can be understood by a layer-intercalation mechanism. For the samples consisting of Bi-2212 phase, nearly phase-pure Bi-2223 crystals can be obtained by high-temperature, long-time annealing. Annealing phase-pure crystals in various atmospheres, temperatures and pressures (ranging from 1 to 550 bar) causes an alteration in oxygen content, resulting in a systematic change in the c-axis lattice parameters and superconducting transition temperature Tc. Interestingly, Tc is found to increase with decreasing c-axis lattice parameters reaching K, but change very little with the further decrease of c-axis lattice parameters, exhibiting a broad plateau on the plot of Tc versus c-axis lattice constant. The magnetization and ellipsometric measurements on high-oxygen-pressure annealed Bi-2223 crystals show much stronger Josephson coupling between the CuO2 layers, indicating a progressively higher hole doping upon increasing annealing oxygen pressure, although Tc remains essentially unchanged. The anomalous saturation of Tc on the overdoped side is quite unique compared with the mono- or bi-layered Bi-based cuprates. It is possibly related to the significant difference in the hole doping between the crystallographically inequivalent inner and outer CuO2 planes in the multi-layered cuprate system.