Absorption and emission spectroscopy of Tm3+ dopants in ultratransparent oxyfluoride glass ceramics indicates that most of the active dopants have been incorporated into the nanocrystals. The size of the nanocrystals has been estimated based on experimental studies of polarized and depolarized low-frequency Raman scattering of the Tm3+-doped glass ceramics in the range 0.5–100 cm−1. Symmetric and quadrupolar acoustic vibrations have been observed and are ascribed to the respective modes of the nanoparticles, which are argued to be β-PbF2 nanocrystals. The size distribution of the nanoparticles has been deduced from the shape of the acoustic bands. The Raman data show that the mean size of the PbF2 nanocrystals increases and that the width of the size distribution decreases with the time and temperature of the heat treatment.