Glass transition and fragility of iron phosphate glasses.

Changes in the heat capacity from the glass state ( C _pg) to the supercooled liquid state ( C _pl), Δ C _p= C _pl− C _pg, the ratio of heat capacities, C _pl/ C _pg , width of the glass transition temperature, Δ T _g, and activation enthalpy for structural relaxation, Δ H ∗ , in the glass transition region for the xNa ₂O–(20− x)R ₂O–32Fe ₂O ₃–48P ₂O ₅ (R=K and Cs) mol% glasses, where x =0, 5, 10, 15, 20, have been investigated using differential scanning calorimetry. Δ C _p and C _pl/ C _pg are in the range of 57–68 J/mol/K and 1.41–1.51, respectively, for the xNa ₂O–(20− x)R ₂O–32Fe ₂O ₃–48P ₂O ₅ (R=K and Cs) mol% glasses, which suggests that the alkali-containing iron phosphate glasses, similar to the alkali-free binary iron phosphate glass, also belong in the category of fragile liquids. Adding 20 mol% of alkali oxides to the 40Fe ₂O ₃–60P ₂O ₅ (mol%) composition does not change the glass transition behavior or melt fragility very much, but does increase the glass transition temperature, T _g. No mixed-alkali effect is observed on any glass transition properties for the iron phosphate glass.