Lithium ion sites and their contribution to the ionic conductivity of RLi2O-B2O3 glasses with R ≤ 1.85

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

“…It can be seen that the lithium pyroborate contains only small fractions of sp 3 boron, with N 4 from 10(1)% for the roller-quenched glass, down to 3(1)% for the liquid at high temperatures circa 1200 K. There is also apparent a gradual rise in N 4 for the liquid as it is cooled, consistent with the higher value observed in the ambient glass. The latter is in good agreement with a value of N 4 = 12.5(2.0)% from 11 B NMR, 15,21 based on a small extrapolation from the highest lithium content NMR data at J = 1.85, to the pyroborate composition at J = 2, using the model of Feller et al 22 Our results for the liquid lithium pyroborate are consistent with the lower bound N 4 (T) derived from vibrational spectroscopic observations of the same melt composition, 25 namely 5%-3% over the 973-1273 K temperature interval (cf. the upper bound of 15%-8%, respectively).…”

“…With this simple extension to the isomerization model, we can now make a van't Hoff analysis 9 for the lithium pyroborate liquid data, as shown in Figure 6A. The data are remarkably well linearized, and in this case have been constrained at T g = 554 K 17 by the N 4 value expected from the model of Feller et al, 22 based on 11 B NMR 15,21 measurements on lithium borate glasses. The resulting ΔH = 13(1) kJ mol -1 is smaller than found for lithium and sodium borates with lower alkali content (Figure 6A).…”

“…Firstly, it is worth noting that 11 B nuclear magnetic resonance (NMR) studies have been performed on lithium borate glasses up to 65 mol% Li 2 O, not quite reaching the pyroborate composition (66.7 mol% Li 2 O). 15,21 Nonetheless, a small extrapolation of a model based on these data 22 predicts a value of N 4 = 12.5% at J = 2. Vibrational spectroscopic evidence 23 points toward disproportionation of tetrahedral pyroborate units (BØ 3 O 2-) into orthoborate triangles (BO 3 3-) and tetrahedral metaborate units (BØ 4 -), and these coexist in minority to the majority trigonal planar pyroborate units (BØO 2 2-), although these may also disproportionate, leading to the introduction of metaborate triangles (BØ 2 O -).…”

“…[10][11][12] We also calculate the nonbridging oxygen fraction † , f nbr (J, T), a useful proxy for lithium ion diffusion and conductivity. [13][14][15] Furthermore, the high lithium content of Li 4 B 2 O 5 , combined with modeling of the intraborate unit oxygen-oxygen correlations, 16 allows a detailed insight into the coordination environments of lithium in the liquid and glass.…”

“…These glasses exhibit the classic signatures of the borate anomaly, with maxima in glass transition temperature circa 29 mol% Li 2 O, 12,17,18 mass density circa 41 mol% Li 2 O, 17,19 and tetrahedral boron fraction circa 37 mol% Li 2 O. 14,15,20,21 Many studies focus on glass compositions around the borate anomaly extrema, where the fraction of nonbridging oxygen is small (0 < f nbr ≲ 30%). Since herein we focus initially on lithium pyroborate, which has a large f nbr (approaching 80%), we summarize below what is currently known about the local † An oxygen is defined as nonbridging if it bonds to exactly one boron and is denoted O -, and bridging if it bonds to exactly two boron and denoted Ø.…”