Single crystals of triple-layered cuprates Bi 2Sr 2Ca 2Cu 3O 10+ δ: growth, annealing and characterization

Abstract

The growth of Pb-free Bi 2Sr 2Ca 2Cu 3O 10+ δ (Bi-2223) single crystals has been investigated using the travelling solvent floating zone technique under mixed gas flow of argon and oxygen. Slow growth rates of 0.20, 0.10, 0.06 and 0.04 mm/h were applied to four growth runs. Nearly single-phase Bi-2223 crystals sized up to 10×6×0.5 mm 3 were obtained using a slow growth rate of 0.04 mm/h and subsequent annealing. The crystals were characterized by energy dispersive X-ray, X-ray diffraction (XRD), transmission electron microscopy, magnetic susceptibility and resistivity measurements. It is found that the growth rate affects the phase formation of Bi-2223 crystals. Both XRD and magnetic susceptibility measurements show that the as-grown crystals obtained at a slow growth rate of 0.04 mm/h consist of >90% Bi-2223 phase, whereas as-grown crystals obtained at rates of 0.10 and 0.20 mm/h contain predominantly Bi-2212, Ca 2CuO 3 and a small quantity of Bi-2223 phase. Post-annealing experiments were carried out at 500–850 °C for 120–500 h in an effort to improve the crystal quality with respect to phase purity and oxygen homogeneity. A phase-transformation process from Bi-2212 to Bi-2223 was observed. The results of XRD and susceptibility measurements show that the relative fraction of the Bi-2223 phase increases upon increased annealing temperature and/or prolonged annealing time while the Bi-2212 phase diminishes gradually, resulting in crystals that were of 98–99% Bi-2223 phase. Such a phase transformation from Bi-2212 into Bi-2223 might be explained via a layer-intercalation mechanism. The refinement results of XRD data show that pure-phase Bi-2223 crystals have orthorhombic symmetry with lattice parameters of a=5.408(2) Å, b=5.413(7) Å, and c=36.868(1) Å. Narrow full width at half maximum of 0.16–0.20° determined from X-ray rocking curve measurements demonstrates the high crystallinity of Bi-2223 crystals. The superconducting transition temperature T c increases from 102 K for as-grown crystals to 110 K for crystals annealed at 850 °C for 500 h in O 2 flow. The in-plane and out-of-plane resistances as a function of temperature measured on annealed crystals show that the general features of the resistivity of Bi-2223 crystals are similar to those of Bi-2212 crystals.