Size effect on infrared spectra of La0.25Ca0.75 MnO3 nanoparticles: Evidences of melting charge ordering

A softening of bending mode and a hardening of stretching mode on cooling are observed in infrared spectra of La(0.25)Ca(0.75)MnO(3 )nanoparticles with particle sizes (D) of 40-1000 nm, indicating part of melting charge ordering in the samples. The particle size variations of the inverse of effective number of carriers n(eff)* and optical gap show their maxima around D = 350 nm at T = 6 K and then decrease as D reduces from 350 nm to 40 nm. The signals of the decreasing localization of charge carriers provide the possible evidences for the suppression of charge ordering with the decreasing particle size. These observations are explained based on the model of disordered surface and bulk-like core.