The dynamic structure factor of a series of sodium aluminophosphate melts has been measured using photon correlation spectroscopy. These melts consist of oxide networks that undergo a uniform increase in the lattice connectivity (per structural unit) from 〈n〉 = 2 to 〈n〉 ≈ 2.8 with the addition of alumina. Characteristics of the main viscoelastic relaxation (its shape parameter β and fragility m) were evaluated and the fragility is seen to decrease in a manner identical with that reported previously for both chalcogenide and sodium phosphate melts. To explain this common dependence on connectivity, we turn to the bond lattice model and simulations that indicate a direct proportionality between fragility and the excess entropy gained in the removal of a lattice constraint. We propose that the variation in the fragility with lattice connectivity might be decomposed into two regimes. In the first regime at 〈n〉 <2.4, the linear decrease in m with 〈n〉 is argued to reflect a decreasing configurational entropy associated with the formation of covalent crosslinks between polymer-like chains that vanishes in the vicinity of the rigidity threshold. In the second regime at 〈n〉 > 2.4, we speculate that the variation in m with 〈n〉 results from a weaker, vibrational contribution to the entropy.
All Science Journal Classification (ASJC) codes
- Ceramics and Composites
- Materials Chemistry