The infrared spectra of NaGdF4 nanocrystalline powders were investigated. It was shown that the spectra demonstrated a significant dependence on the size of crystallites. In particular, the intensities of vibrational bands of IR spectra decreased proportionally to the inverse of crystallite size. The results are discussed in terms of confined phonon modes. The phonon confinement effect was most pronounced for lower energy vibrations and was observed both for stretching and bending vibrations of Gd-F and (Gd. Na)-F bonds as well as for T'(Na,Gd) vibrations. (c) 2005 Elsevier B.V. All rights reserved.