RAMAN-SCATTERING AND MEDIUM-RANGE ORDER IN TERNARY BORATE GLASSES

Low frequency Raman scattering associated with sound velocity measurements on ternary borate glasses (B2O3-0.6Li2O-yLinX with n = 1, X = F, Cl, Br, I or n = 2, X = SO4) are reported. The ''boson peak'' is analyzed on the basis of the model proposed by Martin and Brenig. The sound velocity decreases while the structural correlation length (SCL) increases with the lithium salt content and with the anionic radii. As the addition of lithium halide does not change significantly the ratio between the BO3 and BO4 units, these results indicate that the two basic units are redistributed into the matrix to form larger closed borate entities with BO4 such as diborate rings. This contributes to the expansion and the softening of the B-O network by diminution of its connectivity to incorporate the anionic species in interstitial sites. The small variations of the SCL in the case of sulfoborate glasses, seems to indicate that sulfate ions act as a modifier. The existence in the network, of new sulfoborate units in which the sulfate ions are connected with BO4 tetrahedra is expected.