THERMAL TRANSPORT IN SILICATE-GLASSES .1. LOCALIZED PHONONS

Measurements of the thermal diffusivities between 80 and 500 K for fiv e silicate glasses of composition 0.15(M2O)0.05(ZnO)0.05(BaO)0.05(Eu2O 3)0.70(SiO2), where M is Li, Na, K, Rb, or Cs are presented. In each c ase the thermal diffusivity asymptotically approaches a linear tempera ture dependence at high temperatures. The data are analyzed using a mo del that contains extended phonons and, above a mobility edge, localiz ed phonons that participate in the transport through thermally activat ed 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 t o the thermal diffusivities are studied as functions of the mobility e dge across this isostructural series of glasses.