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Issue 12, 2018
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Mismatch in cation size causes rapid anion dynamics in solid electrolytes: the role of the Arrhenius pre-factor

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Abstract

Crystalline ion conductors exhibiting fast ion dynamics are of utmost importance for the development of, e.g., sensors or rechargeable batteries. In some layer-structured or nanostructured compounds fluorine ions participate in remarkably fast self-diffusion processes. As has been shown earlier, F ion dynamics in nanocrystalline, defect-rich BaF2 is much higher than that in the coarse-grained counterpart BaF2. The thermally metastable fluoride (Ba,Ca)F2, which can be prepared by joint high-energy ball milling of the binary fluorides, exhibits even better ion transport properties. While long-range ion dynamics has been studied recently, less information is known about local ion hopping processes to which 19F nuclear magnetic resonance (NMR) spin–lattice relaxation is sensitive. The present paper aims at understanding ion dynamics in metastable, nanocrystalline (Ba,Ca)F2 by correlating short-range ion hopping with long-range transport properties. Variable-temperature NMR line shapes clearly indicate fast and slow F spin reservoirs. Surprisingly, from an atomic-scale point of view increased ion dynamics at intermediate values of composition is reflected by increased absolute spin–lattice relaxation rates rather than by a distinct minimum in activation energy. Hence, the pre-factor of the underlying Arrhenius relation, which is determined by the number of mobile spins, the attempt frequency and entropy effects, is identified as the parameter that directly enhances short-range ion dynamics in metastable (Ba,Ca)F2. Concerted ion migration could also play an important role to explain the anomalies seen in NMR spin–lattice relaxation.

Graphical abstract: Mismatch in cation size causes rapid anion dynamics in solid electrolytes: the role of the Arrhenius pre-factor

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Publication details

The article was received on 28 Nov 2017, accepted on 10 Feb 2018 and first published on 21 Feb 2018


Article type: Paper
DOI: 10.1039/C7DT04487A
Citation: Dalton Trans., 2018,47, 4105-4117
  • Open access: Creative Commons BY license
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    Mismatch in cation size causes rapid anion dynamics in solid electrolytes: the role of the Arrhenius pre-factor

    S. Breuer and M. Wilkening, Dalton Trans., 2018, 47, 4105
    DOI: 10.1039/C7DT04487A

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