ASAXS study of CaF₂ nanoparticles embedded in a silicate glass matrix

The formation and growth of nanosized CaF₂ crystallites by heat treatment of an oxyfluoride glass of composition 7.65Na₂O-7.69K₂O-10.58CaO-12.5CaF₂-5.77Al₂O₃-55.8SiO₂ (wt%) was investigated using anomalous small-angle X-ray scattering (ASAXS). A recently developed vacuum version of the hybrid pixel detector Pilatus 1M was used for the ASAXS measurements below the Ca K-edge of 4038 eV down to 3800 eV. ASAXS investigation allows the determination of structural parameters such as size and size distribution of nanoparticles and characterizes the spatial distribution of the resonant element, Ca. The method reveals quantitatively that the growing CaF₂ crystallites are surrounded by a shell of lower electron density. This depletion shell of growing thickness hinders and finally limits the growth of CaF₂ crystallites. Moreover, in samples that were annealed for 10 h and more, additional very small heterogeneities (1.6 nm diameter) were found.