TEM and EELS characterization of a Sr2CuO3+δ superconductor post-annealed at different temperatures:: enhancement of Tc by apical oxygen reordering

A 75 K Sr2CuO3+delta superconductor ( nominal delta = 0.4), detected to be a K2NiF4- type tetragonal single phase from x-ray powder diffraction (XRD) data, was annealed in a N-2 atmosphere at different temperatures, and with raising the annealing temperature, the superconducting transition temperature (T-c) is found to increase from 75 K (as-prepared), in turn, to 89 K (post-annealed at 150 degrees C), 95 K ( post-annealed at 250 degrees C), and then to disappear when further increasing the annealing temperature above 250 degrees C. Transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) techniques are utilized to characterize both as-prepared and post-annealed samples in order to further identify the superconducting phases. TEM investigations reveal that almost all grains in the as-prepared sample exhibit modulated structures having the K2NiF4- type tetragonal structure of Sr(2)CuO(3+)d as their basic substructure, and two types of modulated phase, i.e., Fmmm modulated phase ( a approximate to b = 5 root 2a(p) and c = c(p)) and C-2/m modulated phase (a = 5 root 2a(p), b = c(p), c = root 26 root 2/2a(p) and beta = 101.3 degrees), are found in the sample. With increasing annealing temperature, successive structure phase transitions starting from the C2/m modulated phase are observed, i.e., C2/m modulated phase -> Cmmm modulated phase (a = c(p), b = 5 root 2a(p) and c = 5 root 2a(p)) ( annealed at 150 degrees C) -> Pmmm modulated phase ( a approximate to b = 4 root 2a(p) and c = c(p)) ( annealed at 250 degrees C). unmodulated orthorhombic structure ( annealed at 350 degrees C), while no obvious changes are found for the Fmmm modulated phase up to 250. C. The experimental results strongly suggest that the C2/m modulated phase is responsible for Tc at 75 K, the Cmmm modulated phase for Tc at 89 K, and the Pmmm modulated phase for Tc at 95 K, while the Fmmm modulated phase is nonsuperconducting. The absence of superconductivity in the Fmmm modulated phase can be explained by the oxygen vacancies located at the Cu-O planes, based on the previous results. For the superconducting modulated phases, the oxygen vacancies can only be located in the Sr-O layers ( i.e., in the apical sites) due to the requirement of oxygen vacancy-free CuO2 planes for superconductivity, and thus their formations are naturally explained by the ordering of apical oxygen vacancies. Besides a minor effect of the hole doping level, the reordering of apical oxygen in those superconducting modulated phases is revealed to be the key factor resulting in the enhancement of Tc.