ULTRA-SMALL-ANGLE X-RAY-SCATTERING PROFILE OF COLLOIDAL SILICA CRYSTAL OF 4-FOLD SYMMETRY

Several orders of Bragg diffraction were observed by ultra-small-angle X-ray scattering (USAXS) for colloidal silica particles with the average radius being 560 Angstrom and with narrow size distribution (standard deviation being 8%) in a salt-free aqueous dispersion in a capillary. Diffraction peaks were displayed at such small angles as 149 x n '' with n being an integer. The same profile was observed when the capillary was rotated around its axis by (90 x m)degrees (m: integer). Furthermore, a different profile was found at (45 + 90 x m)degrees with the first peak at 109 ''. Thus the USAXS profiles indicate 4-fold symmetry of a colloidal crystal in the dispersion. It was concluded that a body-centered-cubic lattice was maintained with the [001] direction being vertically upward and parallel to the capillary axis and with the lattice constant a being 3000 Angstrom. The closest interparticle distance obtained from the value of a was 2600 Angstrom, while the average distance from the overall concentration was 2900 Angstrom, indicating the non-space-filling nature of the crystal.