Visco-elastic relaxations in alkali borates and alkali silicates

The Brillouin light scattering technique was used to study short time relaxations in glass forming oxide melts. The line shape analysis of Brillouin spectra yields a complex mechanical modulus. The temperature dependence of this modulus provides insight into the structural disintegration above the glass transition, as well as the thermally activated mechanisms which facilitate the momentum transport in the process of viscous dissipation. A series of binary and ternary alkali borates and alkali silicates have been investigated. The results show fundamental differences in the way alkali cations affect the network structure in borates as compared to silicates. While these cations move relatively freely within the rigid silicate network, their motions are strongly coupled to the network relaxation in borates. The activation energies for cation motion are in good agreement with those found by other methods.