Elasticity, deformation and fracture of mixed fluoride–phosphate glasses

• Comprehensive data on the mechanical properties of fluoride–phosphate glasses are provided.
• A structural rational is obtained for explaining the breakdown of simplistic models.
• Design-principles are deducted for optically active fluoride–phosphate glasses.

Fluoride–phosphate (FP) glasses have been developed (in terms of glass forming ability) as a more stable alternative to fluoride glasses. Benefitting from the low polarizability of the fluoride anion, high electronic band-gap and low phonon energy, they present one of the most important host species for the inclusion of rare-earth and other optically active dopants into fiber and bulk glass. A major limitation, however, has been the mechanical performance of these glasses and their susceptibility to damage and fracture. Here, we provide comprehensive data on elasticity, plasticity and fracture of a series of mixed fluoride–phosphate glasses with strongly ionic bonding character, spanning the compositional join of strontium metaphosphate and alkaline earth aluminum fluoride.Simplistic models such as that of Makishima and Mackenzie can reproduce the mechanical properties of these glasses within a limited compositional range. This break-down is explained in the context of glass structure, in particular, the role of intermediate VIAl3 + species which act as bridges between the fluoride and the phosphate sub-networks. A simple binominal model is implemented to estimate the atomic fractions of aluminum as well as of magnesium cations, which are linked to two or more phosphate units, and used as an indicator for the connectivity between the anion sub-networks. Such adjustment of the Makishima–Mackenzie model provides an unambiguous rational for the elastic properties, but also for the susceptibility to inelastic deformation and fracture of FP glasses.