Direct Observation of Defect Dynamics in Nanocrystalline CaF₂: Results from ¹⁹F MAS NMR Spectroscopy

Abstract

The structure and F --ion dynamics in nanocrystalline CaF 2 is studied using 19 F fast magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy in the temperature range 22°C e T e 240°C. The 19 F MAS NMR spectra of nanocrystalline CaF 2 (diameter ∼ 25 nm) shows the presence of ∼0.55% of the F -ions in interstitial sites that are found to be highly mobile at elevated temperatures and undergo rapid site exchange with regular fluorine sites in the lattice via hopping. It is shown that an increased concentration of these defects formed in nanocrystalline CaF 2 may result in an enhancement in ionic conductivity by nearly 3 orders of magnitude compared to that of bulk CaF 2 . SECTION Nanoparticles and NanostructuresT he high anionic conductivity of crystalline ionic fluorides at elevated temperatures has attracted significant attention over the past several decades, and these materials have been extensively studied as model systems to understand defect mobility, diffusion, and sublattice melting in ionic solids.

Surface defect‐enhanced conductivity of calcium fluoride for electrochemical applications

“…The obtained ionic conductivity of synthesized polycrystalline solid CaF 2 electrolyte had been obtained to be 1.9 · 10 −5 S·cm −1 at room temperature, which is several orders of magnitude higher than that of pure and ball‐milled CaF 2 ‐reported values (2 · 10 −7 to 1.6 · 10 −11 S·cm −1 ) . The apparent ion mobilization activation energy of the electrolyte was estimated to be 0.35 eV, which had been reported to be related to the surface formation of interstitial fluoride sites with suggested formation energies of around 0.25 eV. Such activation energy makes the material more attractive as an ion conductor for fluoride shuttle applications.…”

“…Also, the introduction of defects and unit‐cell distortions, geometry frustration at grain boundaries of different crystallites, is reflected in an increase of line broadening . An additional small peak at −132 ppm can be assigned to mobile interstitial defect structures with large concentration at the surface with formation energies around 0.25 eV.…”

“…Spectra were referenced to NaF (aqueous solution). Relevant investigations on CaF 2 nanoparticles for comparison had been reported . Electron paramagnetic resonance (EPR) experiments were carried out at 77 K sample temperature on an X‐band system with output power of 100 mW and 100 kHz magnetic field modulation.…”

Structural Evolution During Precipitation of Alkaline‐Earth Fluoride Nanocrystals in Oxyfluoride Glasses: A Multinuclear Nuclear Magnetic Resonance Spectroscopic Study

“…Previous 19 F MAS NMR studies have shown that interstitial fluorine sites may be present in nanocrystalline CaF 2 at concentration levels of ∼0.5% and are characterized by 19 F chemical shift that is located significantly upfield compared with the 19 F chemical shift of regular F–Ca(4) lattice sites 25 . Therefore, the small but sharp peaks at −100 and −113 ppm in the 19 F NMR spectrum of the SrF 2 ‐oxyfluoride glass–ceramics (Fig.…”

“…concentration levels of B0.5% and are characterized by 19 F chemical shift that is located significantly upfield compared with the 19 F chemical shift of regular F-Ca(4) lattice sites. 25 Therefore, the small but sharp peaks at À100 and À113 ppm in the 19 F NMR spectrum of the SrF 2 -oxyfluoride glass-ceramics ( Fig. 5) can possibly be attributed to interstitial fluorine sites inside the SrF 2 nanocrystals.…”