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Fragile and strong glass melts : A possible structural explanation

The fragile or strong character of glass forming melts is a frequently discussed subject in the last years, constituting a new manner of evidencing the existing structural differences in this kind of liquids. The propriety used, as a rule, to illustrate these differences is the viscosity variation with temperature. Strong melts follow an Arrhenius type curve and for the fragile ones a T-V-F type equation describe more satisfactory the process. Especially for oxide glass forming melts the most accepted concept is that the strong liquids have a network type structure with an uniform coordination number and high energy chemical bonds while the fragile ones have a fragmented structure with complex coordination and lower energy chemical bonds. One observes that the structural models elaborated in our laboratory reflect and explain these properties. For the fragile melts with a macro-anionic structure, the important role of dipole-dipole bonds is highlighted. Using some published data the existence of a correlation between the fragility and the basicity was evidenced. The basicity values show that the strong melts have a higher tendency to undercool in vitreous state than the fragile ones which have lower glass forming properties.