Mixed Alkaline-Earth Effect in the Metastable Anion Conductor Ba_1–xCa_xF₂ (0 ≤ x ≤ 1): Correlating Long-Range Ion Transport with Local Structures Revealed by Ultrafast ¹⁹F MAS NMR

Abstract

Fast ion conductors are urgently needed in many research areas of materials science. Advanced preparation strategies take advantage of an interplay of structural disorder, nanosize effects, and metastability. Getting access to detailed insights into the microstructure of such solids is crucial to identify the origins of fast ion conduction. High-resolution and high-sensitive spectroscopic techniques are well-suited to meet this challenge. Here, ion transport properties of a highly conducting, metastable fluoride with two isovalent cations were interrelated with the microscopic, atomic-scale structure probed by ultrafast 19 F magic angle spinning (MAS) nuclear magnetic resonance (NMR). Nanocrystalline samples of Ba 1Àx Ca x F 2 (0 e x e 1) were prepared according to a mechanochemical route from BaF 2 and CaF 2 . The resulting DC ion conductivity, when plotted as a function of x, passes through a well-developed maximum, which is located at x m = 0.5, while the associated activation energy E a shows a minimum at x m . As revealed by 19 F MAS NMR, five magnetically inequivalent F sites are present in the cation-mixed fluorides. These sites are characterized by a distinct number of Ba and Ca cations in the first coordination shell: [Ba] n [Ca] 4Àn (0 e n e 4). The mixed sites with n = 1,2,3 dominate the NMR spectra at intermediate values of x. Presumably, the mixed cation sublattice, causing the metastability of the compounds, influences both the formation energy of, for example, F interstitials, as well as the migration energy leading to the fast ion conduction observed.

Fluorine Translational Anion Dynamics in Nanocrystalline Ceramics: SrF2-YF3 Solid Solutions

“…High-energy ball milling also increases the density of defects in the interior of the nanograins [9,24]. Starting with binary fluorides, such as nanocrystalline BaF 2 , whose F anion conductivity exceeds that of BaF 2 monocrystals by some orders of magnitude [11], iso-and aliovalent substitution of the metal cations drastically improves ion transport [10,25]. As activation energy shows a minimum, the same holds for Ba 1−x Ca x F 2 [10].…”

“…Considering nanocrystalline ionic conductors [1][2][3][4], a range of studies report on enhanced anion and cation dynamics [5,6]. Structural disorder and defects [7][8][9], lattice mismatch [10][11][12] as well as size effects [13][14][15][16][17], which results in extended space charge regions, are used to explain the properties of nanocrystalline compounds. Especially for nano-engineered systems, which were prepared by bottom-up procedures, such as gas condensation or epitaxial methods [18,19], space charge regions lead to non-trivial effects that may enhance ion transport.…”

F anion dynamics in cation-mixed nanocrystalline LaF3: SrF2

“…As seen for other systems, see Ref. [12] for a brief review on fluorides, we expect lattice strain and distortions [13][14][15], caused by the difference in ionic radii, to promote ionic transport.…”

“…The activation energy of tysonite-type La 0:9 Ba 0:1 F 2:9 , which has been studied in detail in [10,37], is comparable to that of the Sr analogue. Just for comparison, ion transport in La 0:9 Ba 0:1 F 2:9 [31] turned out to be clearly higher than that in metastable and nanocrystalline Ba 0:5 Ca 0:5 F 2 [32], which crystallizes with cubic symmetry and which has been studied earlier by some of us [15,32]. For the sake of clarity, for most of the samples only the values referring to P1 have been included in Fig.…”

“…The effect of abraded material from vial sets and milling balls has only a negligible effect if other factors, such as cation mixing, governs ion dynamics [14]. As has been documented for other nanocrystalline systems, the main change in ionic conductivity is caused by structural disorder, lattice strain introduced and the mixed cation effect [15][16][17]. The mismatch in size of the cations sensitively changes the potential landscape the mobile F anions are exposed to.…”