Structure and microstructure of superconductorSr2CuO3+δ (nominal δ = 0.4) prepared under high pressure

Abstract

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 microstructureof this material have been carefully studied by means of transmission electron diffraction(ED), high-resolution transmission electron microscopy (HRTEM), electron energy-lossspectroscopy (EELS) and energy dispersive analysis of x-ray (EDX) techniques. Interestinglyenough, ED and HRTEM investigations reveal that almost all grains of the tetragonal formSr2CuO3+δ exhibit modulated features. This result strongly suggests that the superconductivity at 75 Kin the material is associated with the modulated phases. Two types of modulated phaseshaving different primitive cells but equal in volume are found in the sample. One isthe face-centred orthorhombic modulated phase with a space group Fmmm andlattice constants of . The other is the base-centred monoclinic modulated phase with a space groupC 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. OxygenK absorption edges from EELS measurements suggest that the two modulatedphases have the same hole intensity. Defects such as twin boundaries, small-angleasymmetrical tilt boundaries and dislocations are frequently observed in theC 2/m monoclinic modulated phase, and some of them are carefully studied by means of HRTEM,EELS and EDX.