Phonon confinement effect in calcium fluoride nanoparticles

Nanocrystalline CaF2 were obtained by ball-milled synthetic fluorite powders. Raman spectra of nanocrystals with average sizes of 10-25 nm are reported and the correlation between the band shape of the only allowed Raman mode at 322 cm-1 and the crystals dimensions is discussed. While in such systems local temperature and pressure effects play a minor role, variations in the shape of the Raman band can be correctly reproduced by a model based on the phonon confinement that effectively takes into account the anisotropy of the phonon dispersion curve.