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Abstract Insights into structure‐conductivity mechanisms are investigated for a series of six (dinitrile)2LiPF6 molecular crystals with varied alkyl chain lengths, N≡C─(CH2)n─C≡N, n = 2, 3, 4, 5, 6, and 2Me‐glutaronitrile. The molecular crystals have separate Li+ and channels, with the Li+ions weakly coordinated by four ─C≡N groups. The following correlations are observed: i) shorter Li+⋯ Li+ hopping distances (5.72–8.08 Å) increase ionic conductivity (3.1 × 10−4–0.15 × 10−4 S cm−1 at 25 °C) for all (dinitrile)2LiPF6; ii) when there are unrestricted anion channels, the lithium ion transference number increases ( = 0.39–0.62) as the void volume (565–250 Å3) and Li+⋯ Li+ hopping distance (7.15–5.72 Å) decrease, since a greater fraction of the charge is contributed by the Li+ions; this correlates with n= 2, 4, 5, 6; iii) the exceptions are Gln (n = 3) and 2Me‐Gln, where there are restricted channels for anion migration, and in this case: iv) conductivity decreases (0.57–0.15 × 10−4 S cm−1 at 25 °C), since contributions to the conductivity from anion migration decrease, but v) increases (0.64–0.7) since a greater fraction of the charge is carried by the Li+ ions.
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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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- PAR ID:
- 10383741
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- International Journal of Applied Glass Science
- Volume:
- 14
- Issue:
- 1
- ISSN:
- 2041-1286
- Format(s):
- Medium: X Size: p. 52-68
- Size(s):
- p. 52-68
- Sponsoring Org:
- National Science Foundation
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