Bond counting strategies in an oxygen centric perspective on the structure of oxide glasses

Bond counting strategies provide a simple but robust methodology for examining the connectivity of the network forming motifs in oxide glasses. This utility is demonstrated for several case examples in which diffraction is combined with solid-state nuclear magnetic resonance and other methods to probe the glass structure. The approach is used to show that four-coordinated Zn2+ and Mg2+ ions act as network modifiers in alkali bearing metasilicate glasses. Analytical models are then derived for the structure of mono- or di-valent cation containing aluminosilicate glasses in which the fraction of six-coordinated Al3+ ions is negligibly small and the other aluminium species are regarded as network formers. The results lead to an assessment of the network modifying versus charge compensating roles of the Zn2+ and Mg2+ ions. Next, the effect of high-pressure on the network connectivity of aluminosilicates is investigated, leading to a quantification of the fraction of triply bonded oxygen atoms (or oxygen triclusters) in fully polymerised structures. A network forming role is then found for six-coordinated Nb5+ ions in the binary Nb2O5–NaPO3 system. Finally, it is noted that the oxygen packing fraction acts as a marker for structural change in network-forming oxides under high-pressure conditions. The findings illustrate the power of the bond-counting methodology in elucidating the structure of amorphous oxides and show the rich diversity in framework structures that originates on progressing from a Zachariasen viewpoint of oxide networks.