Advanced Electron Microscopy Group, National Institute for Materials Science,
Charge ordering (CO) is an important phenomenon of physics in the correlated electron systems . In manganites, the CO state can be controlled by the solid solution of the perovskite A-site where rare-earth (RE) ions and alkaline-earth (AE) ions locate. The CO correlation is suppressed by the A-site randomness that originates from ionic-radius-variance-induced local distortions . In this work, we have investigated the relation of the CO state and the variance σ in A-site ionic radii in RE2-xAExNiO4 (x = 1/3) (RE: Pr, Sm; AE: Sr, Ca). The single crystals were prepared by the floating-zone method . For TEM observations, the samples were thinned by argon-ion milling. Selected-area electron diffraction (SAED) patterns were obtained by the transmission electron microscope (Hitachi HF-3000S). By analyzing the SAED patterns of RE2-xAExNiO4 (x = 1/3), we found that the CO state depends strongly on the variance in A-site ionic radii. The modulation wave number δd and the half-width at half-maximum (HWHM) of the superlattice spot have been plotted as a function of the variance in A-site ionic radii σ. It is obvious that the HWHM increases with increasing the σ, while δd exhibits the contrary behavior. The results indicate that the CO instability and correlation length decrease when σ increases. Namely, the CO state is suppressed by the A-site randomness in the nikelates, just as observed previously in manganites.
 Colossal Magnetoreasistive Oxides, edited by Y. Tokura (Gorden and Breach Scirnce Publisher, 2000).
 Y.Tomioka et.al, Phys. Rev. B. 70 (2004) 14432.  S. Seki et al, to submit.