The Rigidity of Vitreous Networks: An Alternative Viewpoint

An alternative, and much more intuitive, approach is proposed to the rigidity of (relatively) strain‐free oxide glass networks, based on effectively rigid basic structural units and the true degrees of freedom that allow the formation of such networks; viz . bond torsion angles plus the bond angle at the bridging oxygen atoms. These ideas are extended to borate networks that include rigid superstructural units with no internal degrees of freedom in the form of variable bond and torsion angles, and it is shown that, for an isostatic network, the average (super)structural unit connectivity is equal to 4. The role of network rigidity in determining glass formation is discussed, together with the effects of steric hindrance, and a comparison with conventional constraints theory is presented for vitreous SiO ₂ and B ₂ O ₃ . Finally, it is argued that the so‐called intermediate phase is merely an extended rigidity transition range, due to the chemical nano‐heterogeneity that characterizes the structure of glasses having more than one component, and that, in the case of Ge‐Se glasses, the bonding in the interfacial regions between the Se and GeSe ₂ regions exhibits significant metalloid character.