Moessbauer and DTA studies of the crystallisation of borosilicate glasses

Continuous decreases in quadrupole splitting for Fe3+ were observed by isothermal annealing of a series of borosilicate glasses denoted by xR2O, 100-x(B2O3, SiO2), 7Fe2O3, in which R denotes Na, K, Rb, or Cs, at temperatures below the individual glass transition temperatures. The decrease is ascribed to an increased symmetry around Fe3+ caused by the diffusion of alkali metal ions and rearrangement of oxygen atoms. Activation energies concerned with the decrease in the quadrupole splitting were found to increase in the order Cs, Rb, K, Na when the alkali content of the glasses was constant. The increase is ascribed to an increasing electrostatic force between the alkali metal ion and the oxygen atom with decreasing ionic radius of the alkali metal ion. It was also found that the activation energies increased with increasing alkali content, and this is ascribed to an increasing fraction of nonbridging oxygens in the glasses. Decreases in quadrupole splitting and linewidth (FWHM) were also observed by isothermal annealing experiments at temperatures above the glass transition temperature. The decrease is attributed to a rearrangement of network formers such as B, Si, and Fe as well as of the oxygens and the alkali metal ions. (author)