Direct evidence of rigidity loss and self-organisation in silicate glasses

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The Brillouin elastic free energy change DF between thermally annealed and quenched (Na₂O)ₓ(SiO₂)₁₋ₓ glasses is found to decrease linearly at x > 0.23 (floppy phase), and to nearly vanish at x < 0.18 (stressed- rigid phase). The observed D F(x) variation closely parallels the mean-field floppy mode fraction f(x) in random networks, and fixes the two (floppy, stressed-rigid) elastic phases. In calorimetric measurements, the non-reversing enthalpy near Tg is found to be large at x < 0.18 and at x > 0.23, but to nearly vanish in the 0.18 < x < 0.23 range, suggesting existence of an intermediate phase between the floppy and stressed-rigid phases.