THERMAL TRANSPORT IN SILICATE-GLASSES .2. EXTENDED PHONONS

In this paper we present measurements of the thermal diffusivities of five glasses between 100 and 500 K. These glasses have composition 0.1 5(Na2O)0.15(MO)0.05(Eu2O3)0.65(SiO2), where M is a divalent network mo difier Mg, Ca, Sr, Ba, or Zn. The thermal transport is represented by a two-carrier model of conventional phonon-gas transport by extended p honons and thermally activated hopping of phonons whose eigenvectors a re localized in the disordered network of the glass. The later contrib ution is required to explain the increase in the thermal diffusivity a t temperatures above room temperature. The extended phonons produce a transport that is a decreasing function of temperature between 100 and 250 K in these glasses. Using the boson peak from Raman scattering as a measure of the phonon mobility edge, agreement with the temperature and frequency dependence. of the two-localized-one-extended-phonon an harmonic process studied by Jagannathan et al. is obtained for both th e localized and the extended phonons. The model is also applied to fus ed silica to account for results between 100 and 1100 K, with the modi fication that the limiting process for the extended states does not ex plicitly depend on temperature.