Characterization of heavily Cu-doped La2CuO4+δ by transmission electron microscopy and electron energy loss spectroscopy

The techniques of transmission electron microscopy and electron energy loss spectroscopy (EELS) have been used for characterizing heavily Cu-doped polycrystalline La2CuO4.003 (LCO) samples. Semiquantitative and spatially resolved EELS analysis reveals that Cu may be doped into LCO lattice and that Cu doping introduces effectively holes into the sample. At room temperature, the LCO grains were found to phase separate into hole-poor and hole-rich grains, and in hole-rich grains holes were found to have a strong tendency to concentrate and form one-dimensional static ordering with planar domain wall promoted by high-energy electron beam. Two kinds of modulation vectors have been found, they are 1/4b*±1/3c* (with a wavelength ∼18.9 A) and 1/6a*±1/3b*±1/2c* (with a wavelength ∼12.7 A). At lower temperatures, sharper and higher-order superlattice reflections were observed, indicating an enhanced charge ordering, and superlattice reflections originating from 1/4b*±1/3c* were found to dominate those originating from 1/6a*±1/3b*±1/2c* with a shorter modulation period