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

Highlights: • The softening of bending mode and the hardening of stretching mode are observed. • The sample with size of 350 nm shows the highest degree of charge localization. • The degree of charge localization decreases as particle size reduces. • The more disordered particle surface may lead to the stronger melting of charge ordering. -- Abstract: A softening of bending mode and a hardening of stretching mode on cooling are observed in infrared spectra of La_0.25Ca_0.75MnO₃ 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 $n_{eff}^{\ast }$ 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.