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

Abstract

The structure of liquid lithium pyroborate, Li 4 B 2 O 5 (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 N 4 = 10(1)% found for the glass, and literature 11 B nuclear magnetic resonance measurements. van't Hoff analysis based on a simple boron isomerization reaction BØ 3 O 2-⇌ BØO 2 2yields 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 N 4 (J, T), nonbridging oxygen fraction f nbr (J, T), configurational heat capacity 𝐶 conf P (𝐽, 𝑇), and entropy S conf (J, T) contributions up to J = 3. A maximum is revealed in 𝐶 conf P (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.

Key melt properties for controlled synthesis of glass beads by aerodynamic levitation coupled to laser heating

“…Although numerous studies also reported the ADL synthesis and characterization of silicate or borate glasses and melts (for instance, [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47] ) the strong volatility of some chemical components [48][49][50] (e.g., B 2 O 3 and alkali oxides) under laser irradiation still represents a major drawback of ADL-driven materials discovery, limiting its applicability within these chemical domains. Borate and silicate glasses can be easily prepared by conventional melt-quenching, but ADL was recently shown to provide a superior approach to elucidate transition metal saturation and homogenous nucleation in aluminosilicate glasses and glass-ceramics, [51][52][53] again taking advantage of the high melting temperatures, fast cooling rates and suppression of heterogeneous surface nucleation.…”