The Brillouin light scattering technique was used to determine the complex mechanical modulus, which describes the dynamic response of molecular structures, for ${\mathrm{B}}_2$${\mathrm{O}}_3$ and binary alkali borate. The effects of temperature, alkali concentration, oxygen and water vapor partial pressures on the structural developments and on the thermal activation of dissipative processes were examined. The glass transition in these systems is characterized by a discontinuity in the temperature dependence of the elastic component of this modulus. Above ${\mathit{T}}_{\mathit{g}}$, the elastic modulus decreases with a faster rate, the higher the alkali concentration. The complete structural evolution from a room temperature glass to a liquid near the boiling point was found to involve several distinct mechanisms, which become gradually activated with increasing temperature. By using a mechanical relaxation formalism, the activation energies and preexponential time constants describing the mechanical degradation, as well as the molecular rearrangements associated with each mechanism were determined. For a given system, the initial network degradation is characterized by the smallest activation energy. The motion involved in this process is that of boron atoms oscillating between triangular and tetrahedral coordination, upon exchanging one of their oxygen neighbors. During this phase boroxol rings open, without the formation of nonbridging oxygens. At intermediate temperatures the motion of alkali cations between network segments is activated, and at very high temperatures complete network disintegration takes place, leaving ionic species whose motion occurs by complete dissociation from their immediate neighbors.

• Received 12 January 1994

DOI:https://doi.org/10.1103/PhysRevB.50.17