Effect of A-site cation size mismatch on charge ordering behavior in (La1-xYx)(0.5)(Ca1-ySry)(0.5)MnO3

Electron energy-loss spectroscopy (EELS) has been used to determine the val ence state of manganese for all the samples at low temperature (93 K). EELS results indicate that the valence state of manganese keeps constant (simil ar to3.5) for all the samples. The effects of the A-site cation size mismat ch on the charge ordering (CO) behaviors in the manganites (La1-xYx)(0.5)(C a1-ySry)(0.5)MnO3 are studied by transmission electron microscopy (TEM). TE M images show that the size mismatch and disorder of A-site cations have a suppression effect on the CO transition. The schematic models are proposed for the incommensurate CO modulation in the samples with size mismatch sigm a (2)less than or equal to0.003. The disappearance of the CO transition in the sample with the largest mismatch (sigma (2)=0.005) is explained by the random arrangements of Mn3+ and Mn4+ ions. (C) 2001 American Institute of P hysics.