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Structure and microstructure of superconductor Sr2CuO3+δ (nominal δ = 0.4) prepared under high pressure

A single-phase superconductor Sr2CuO3+δ with nominal δ = 0.4 (detected from x-ray powder diffraction (XRD) data) (Tc = 75 K) has been synthesized using a high-pressure technique. The structure and microstructure of this material have been carefully studied by means of transmission electron diffraction (ED), high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS) and energy dispersive analysis of x-ray (EDX) techniques. Interestingly enough, ED and HRTEM investigations reveal that almost all grains of the tetragonal form Sr2CuO3+δ exhibit modulated features. This result strongly suggests that the superconductivity at 75 K in the material is associated with the modulated phases. Two types of modulated phases having different primitive cells but equal in volume are found in the sample. One is the face-centred orthorhombic modulated phase with a space group Fmmm and lattice constants of . The other is the base-centred monoclinic modulated phase with a space group C 2/m and unit-cell parameters of , bm = cp, and β = 101.3°. The relative ratio of the former to the latter phase contents is about 4:1. Oxygen K absorption edges from EELS measurements suggest that the two modulated phases have the same hole intensity. Defects such as twin boundaries, small-angle asymmetrical tilt boundaries and dislocations are frequently observed in the C 2/m monoclinic modulated phase, and some of them are carefully studied by means of HRTEM, EELS and EDX.