Contrast X-ray powder diffraction of solid rare gas nanocrystals in silica gel mesopores

摘要

Nanocrystals of Kr and Xe have been prepared by condensing the rare gas into the mesopores of dry silica gel with specified pore diameters of 22, 40, 60, and 100 A, and their X-ray powder diffractograms are reported. In the small angle range the scattering intensity was reduced by 70-98% since the electron densities of these rare gas solids and of SiO_2 nearly match. This allows one to monitor the degree of pore filling. In the wide angle range some additional scattering intensity appears from the solid rare gases, and this was separated from the underlying silica gel diffractogram. All features of the solids in the pores are broadened considerably with respect to the patterns of the respective extended crystals. The broadening increases with decreasing pore size, the diffractogram of Kr in silica gel 22 being similar to those of amorphous solids. Diffraction patterns of Kr and Xe clusters calculated with appropriate geometries and degrees of disorder reproduce the experimental data. The dimensions of these clusters and the specified mean pore diameters of the respective silica gels match very well. The measurement of the X-ray contrast in the wide range should provide a powerful complementary approach to porosimetry.