Fluorite-structured PbF2-based materials are promising solid-state electrolytes, so studying how their structure changes under different conditions or over time is relevant. The synthesis of Pb0.8Sr0.2F2 crystals with hollow box morphology and a fluorite structure was carried out by treating the surface of the aqueous mixed Pb(CH3COO)2/Sr(CH3COO)2 solution with gaseous hydrogen fluoride (HF). X-ray powder diffraction, electron microscopy, Raman spectroscopy, and nuclear magnetic resonance (NMR) techniques were used to characterise the changes that occur in nanocrystals during thermal treatment, electron beam irradiation, and storage for one year. The possibility of the formation of Pb nanoparticles by the action of the electron beam on the Pb0.8Sr0.2F2 nanocrystals was shown. Fluorine diffusion coefficients at 200–300 °C were measured using 19F static field gradient NMR diffusometry.