Phonon mean free path of silicate glasses: a useful parameter to distinguish between framework and nonframework cations

Assuming that heat is transported by lattice vibrations (phonons) in silicate glasses, their thermal conductivity is correlated with the product of sound velocity, volumetric heat capacity, and phonon mean free path (MFP). The sound velocity and heat capacity have been studied extensively, but the origin of the composition-induced variation in the MFP remains unclear. The present study investigated MFP in M2/nO–SiO2 (Mn+: Li+, Na+, Ca2+, Sr2+, or Pb2+) glasses with a variation of M2/nO content. The MFP of the silica glass decreased with the addition of M2/nO. The effect of the type of metallic cation on the MFP was minimal for the selected alkali and alkaline-earth silicate glasses. By contrast, the MFP of lead silicate glasses was higher than those of alkali or alkaline-earth silicate glasses when the metallic cation contents were comparable. Previous studies have demonstrated that alkali and alkaline-earth cations act as nonframework species that break the silicate network structure, whereas lead cations have inconclusive structural roles. Our data indicate that lead cations partly act as framework cations and that phonons tend to be scattered near nonframework cations in silicate glasses. Thus, the phonon MFP is a useful parameter for determining the structural role of metallic cations in silicate glass via phonon propagation.