SHORT AND MEDIUM-RANGE ORDER IN TERNARY BORATE GLASSES

Low frequency Raman scattering experiments are described in ternary li thium berate glasses (B2O3-0.6 Li2O-yLi(n)X with n = 1, X = F, Cl, Br and I; or n = 2, X = SO4). The 'boson peak' is analyzed in terms of a correlation length of phonon propagation and of a phonon-photon coupli ng. The concentration and the chemical composition dependance of the s ound velocity, as well those of the structural correlation length dedu ced from the Raman spectra are discussed. Using a linear dependance fo r the coupling parameter, the experimental density of vibrational stat es is obtained and compared with those calculated in binary lithium be rate glasses. Two different structural modifications depending on the nature of the anions of the 'doping salt' (halide or sulfate ions) are proposed to describe the glass network conformation.