Nanocrystalline CaF2 particles obtained by high-energy ball milling

Structural changes in mechanically treated CaF2 powders have been studied by X-ray diffraction, transmission electron microscopy, thermal analysis, 19F and 1H MAS Nuclear Magnetic Resonance and Electron Paramagnetic Resonance methods. Applying the same methods, the results could be compared with those of mechanochemically synthesized CaF2 samples, prepared for the first time in this study by high-energy ball milling. The applied methods indicate that the mechanically treated samples become, under the applied conditions, nanocrystalline. Unexpectedly, the mechanochemically synthesized samples show the same effects, i.e., nanocrystalline samples were formed. In contrast to many oxide compounds, a weak amorphization takes place only after a strong mechanical impact, and essentially in grain boundaries of spherical particles. Observed effects after the application of mechanical impact like broadening of XRD reflections, broadening of 19F resonances, or the decreasing ability for Hradical dot trapping at ambient temperature, are mainly due to decreasing particle sizes as well as very small structural changes in the bulk of the particles. Surprisingly, the spin relaxation of both nuclear spins (19F) and electron spins (Hradical dot) appears to be the most sensitive tool for the investigation of mechanically and chemically induced changes indicating the increasing surface to bulk ratio with increasing mechanical impact.