Thermal transport in silicate glasses. I. Localized phonons

Measurements of the thermal diffusivities between 80 and 500 K for five silicate glasses of composition 0.15(${\mathit{M}}_2$O)0.05(ZnO)0.05(BaO)0.05(${\mathrm{Eu}}_2$ ${\mathrm{O}}_3$)0.70(${\mathrm{SiO}}_2$), where M is Li, Na, K, Rb, or Cs are presented. In each case the thermal diffusivity asymptotically approaches a linear temperature dependence at high temperatures. The data are analyzed using a model that contains extended phonons and, above a mobility edge, localized phonons that participate in the transport through thermally activated hopping. The lowest-frequency Raman peak is used as a quantitative measure of the mobility edge. The thermally averaged relaxation times that characterize the localized-mode and extended-mode contributions to the thermal diffusivities are studied as functions of the mobility edge across this isostructural series of glasses.