Structure of colloidal crystals in sedimenting mixed dispersions of latex and silica particles.

We report bcc-fcc transitions of colloidal crystals in mixed aqueous dispersions of polystyrene-based latex particles (diameter: D=55.8 nm) and silica particles (diameter: D=170 nm). In the single systems, the silica particles formed bcc crystals and the latex particles did not crystallize. In the binary mixtures of these particles, colloidal crystals with fcc structures were found by the analysis of Kikuchi-Kossel diffraction images. Especially, the samples at low latex fractions started out as bcc structures, and then changed to fcc structures. Due to gravitational sedimentation, the lattice constant increased as the height from the bottom of the dispersion became larger. Furthermore, the lattice constant became smaller at a given silica fraction as the latex fraction increased.