ASAXS study of CaF_2 nanoparticles embedded in a silicate glass matrix

摘要

The formation and growth of nanosized CaF_2 crystallites by heat treatment of an oxyfluoride glass of composition 7.65Na_2O-7.69K_2O-10.58CaO-12.5CaF_2-5.77Al_2O_3-55.8SiO_2 (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 CaF_2 crystallites are surrounded by a shell of lower electron density. This depletion shell of growing thickness hinders and finally limits the growth of CaF_2 crystallites. Moreover, in samples that were annealed for 10 h and more, additional very small heterogeneities (1.6 nm diameter) were found.