ANOMALOUS BEHAVIOR OF THE REFRACTIVE-INDEX DURING THE ANNEALING OF DENSIFIED, AMORPHOUS SIO2

The results of measurements of the refractive index and density in samples of amorphous silica, initially densified plastically up to 16% then isochronally annealed at 600 and 800-degrees-C, are discussed. It is found that the isothermal annealing of the density is monotonic and that it does not exhibit the stretched exponential form frequently observed in the relaxation of complex systems. It is suggested that this may be due to structural rearrangements during the annealing process which do not follow the same path as those occurring during the densification process. From the refractive index variation with plastic densification, it is found that the molar polarizability decreases monotonically with increasing density but slower than the case of elastic densification in either amorphous or crystalline SiO2. The difference is attributed to the role of dihedral angle rotations present during elastic densification. The isothermal annealing behaviour of the molar polarization in samples densified plastically by 3% and by 16% is anomalous. One possible explanation of the anomaly may be an interchange of plastic and elastic densification during the annealing process.