Combined X-ray diffraction and solid-state ¹⁹F magic angle spinning NMR analysis of lattice defects in nanocrystalline CaF₂

Nanocrystalline CaF₂ powder specimens were produced both by co-precipitation of CaCl₂ and NH₄F and by ball milling of a coarse powder. The specimen homogeneity and a detailed picture of the lattice defects can be assessed by the simultaneous analysis of the powder diffraction pattern and of the solid-state ¹⁹F magic angle spinning NMR T₁ relaxometry data. While diffraction line profiles provide information on domain size distribution and the content of dislocations, T₁ relaxometry is more sensitive to inhomogeneity of the powder (large defect-free grains versus defective small ones). After extensive milling it is possible to obtain fluorite domains of comparable size to the chemically synthesized CaF₂ (circa 10–12 nm), but with a marked difference in the lattice defect types and content. It is then proved that surface defects (related to domain size), line defects (dislocations) and point (Frenkel) defects have a quite different effect on the powder pattern as well as on the T₁ spin-lattice relaxation time.