Liquid fragility maximum in lithium borate glass-forming melts related to the local structure

The structure of liquid lithium pyroborate, Li₄B₂O₅ (J = Li/B = 2), has been measured over a wide temperature range by high-energy X-ray diffraction, and compared to that of its glass and borate liquids of other compositions. The results indicate a gradual increase in tetrahedral boron fraction from 3(1)% to 6(1)% during cooling from T = 1271(15) to 721(8) K, consistent with the larger N₄ = 10(1)% found for the glass, and literature ¹¹B nuclear magnetic resonance measurements. van't Hoff analysis based on a simple boron isomerization reaction BØ₃O^2– ⇌ BØO₂^2– yields ΔH = 13(1) kJ mol^–1 and ΔS = 40(1) J mol^–1 K^–1 for the boron coordination change from 4 to 3, which are, respectively, smaller and larger than found for singly charged isomers for J ≤ 1. With these, we extend our model for N₄(J, T), nonbridging oxygen fraction f_nbr(J, T), configurational heat capacity $C_P^{\mathrm{conf}}(J,T)$, and entropy S^conf(J, T) contributions up to J = 3. A maximum is revealed in $C_P^{\mathrm{conf}}(J,T=T_g)$ at J = 1, and shown semi-quantitatively to lead to a corresponding maximum in fragility contribution, akin to that observed in the total fragilities by temperature-modulated differential scanning calorimetry. Lithium is bound to 4.6(2) oxygen in the pyroborate liquid, with 2.7(1) bonds centered around 1.946(8) Å and 1.9(1) around 2.42(1) Å. In the glass, n_LiO = 5.4(4), the increase being due to an increase in the number of short Li–O bonds.