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.
Highlights
The irregular movement of small anions and cations in crystalline and amorphous solids is a natural phenomenon that plays a key role in many devices as well as technological and geological processes
The present work is aimed at describing the interrelations between local disorder and short-range ion dynamics using a suitable model substance that may be of interest as a fast ion conducting solid electrolyte.[4,18,19,20,21,22,23]
As has been shown by earlier studies on nanocrystalline (Ba,Ca)F2 as well as other fluorides and oxides prepared by high-energy ball milling the final average crystallite size reached after many hours of mechanical treatment is in the order of 10 to 17 nm for the mixed samples.[36]
Summary
The irregular movement of small anions and cations in crystalline and amorphous solids is a natural phenomenon that plays a key role in many devices as well as technological and geological processes. I.e., a strongly heterogeneous potential landscape, has been made responsible for the increase in conductivity of Ba0.5Ca0.5F2,28 short-range ion dynamics have so far not been probed Shedding light on these hopping processes is expected to help identify the driving forces that control the increase in ionic diffusivity seen. All measurements were carried out on a Bruker Avance III spectrometer that was connected to a shimmed cryomagnet with a nominal magnetic field of 7 T This field corresponds to a 19F NMR Larmor frequency of ω0/2π = 282.0 MHz. A home-built NMR probe, which is suitable to record NMR signals under static conditions in a temperature range from 193 K to 533 K, was used to record the SLR rates and the 19F NMR spectra. Spectra were referenced to crystalline CaF2, which shows, as a secondary reference, an isotropic chemical shift δiso of 58 ppm; C6F6 served as primary reference
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