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Abstract

Sizable Bi2Sr2Ca2Cu3O10+delta (Bi-2223) single crystals were grown by
the travelling solvent floating zone technique and subsequently
annealed in 02 and/or Ar flow for 120-500 h in a temperature range of
430-850degreesC. 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+delta (Bi-2212), Ca2CuO3 and CuO phases co-existing with
the Bi-2223 phase. Annealing these multi-phase crystals in O-2 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 less than or equal to30% 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 I to 550 bar)
causes an alteration in oxygen content, resulting in a systematic
change in the c-axis lattice parameters and superconducting transition
temperature T-c. Interestingly, T-c is found to increase with
decreasing c-axis lattice parameters reaching similar to110 K, but
change very little with the further decrease of c-axis lattice
parameters, exhibiting a broad plateau on the plot of T-c 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
T, remains essentially unchanged. The anomalous saturation of T-c 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.