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

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 fromT= 1271(15) to 721(8) K, consistent with the largerN4 = 10(1)% found for the glass, and literature11B nuclear magnetic resonance measurements. van't Hoff analysis based on a simple boron isomerization reaction BØ3O2⇌ BØO22–yields ΔH= 13(1) kJ mol–1and ΔS= 40(1) J mol–1 K–1for the boron coordination change from 4 to 3, which are, respectively, smaller and larger than found for singly charged isomers forJ ≤ 1. With these, we extend our model forN4(J,T), nonbridging oxygen fractionfnbr(J,T), configurational heat capacity , and entropySconf(J,T) contributions up toJ= 3. A maximum is revealed in atJ= 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,nLiO= 5.4(4), the increase being due to an increase in the number of short Li–O bonds.

 
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Award ID(s):
1746230 1950337
NSF-PAR ID:
10383742
Author(s) / Creator(s):
 ;  ;  ;  ;  ;  
Publisher / Repository:
Wiley-Blackwell
Date Published:
Journal Name:
International Journal of Applied Glass Science
Volume:
14
Issue:
1
ISSN:
2041-1286
Page Range / eLocation ID:
p. 52-68
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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