Structural transformations, elastic moduli and thermal expansion of permanently compacted B2O3 glasses

We have measured the elastic moduli and the thermal expansion in permanently compacted B2O3 glasses whose structural transformations have been determined by Raman spectroscopy. Growing glass densification, obtained by melt-quenching under pressures in the GPa range, is mainly associated to the decrease of the population of boroxol rings (B3O6), the glassy molecular units formed by connected BO3 planar triangles. Increasing density gives rise to a substantial hardening of the elastic continuum, while reducing the thermal expansion of these glasses. Evaluation of the average thermal Grüneisen parameter γth evidences an increase which has been explained in terms of bending vibrations gradually depressed in glassy networks having a growing packing fraction.