Fifty years of fragility: A view from the cheap seats

Abstract Recent advances in understanding the origins of glass-forming fragility in terms of fundamental bonding structure in network-forming glasses (NFGs) includes a proposition that the fragility index is uniquely defined by a single mean field parameter, a coarse-grained variant of the average bond density, 〈nCG〉. This finding resonates with much earlier speculations, first voiced by Nemilov in an under-appreciated paper (Nemilov, 1966), that the fragility in oxide glasses is roughly correlated to the average bridging oxygen per network-forming polyhedron. Here, we provide an abbreviated look at the history of the fragility concept and its correlations with other glass properties as well as its association to the bond topology in network-forming glasses. While we acknowledge the precedence of Nemilov's work in this area (Nemilov, 1966, 1992, 2006, 2007), we also clarify important differences between his qualitative correlation and the more quantitative findings of recent work by ourselves and others.