NaCl structure in a mixture of silica and gold sols

The formation of an NaCl structure in a mixture of a monodisperse silica sol with particle diameter of 5300 A and a gold sol with average particle size of 800 A is described. The mixture was left still for a month, and the particles settled to form a dense layer at the bottom, where phase separation appeared. In the ordered phase only gold particles were observed microscopically as bright spots at their individual lattice points, being arranged in hexagonal or in square pattern. From the distance between the particles in the square pattern and hexagonal pattern, the lattice type for gold particles was concluded to be fcc. Further, it was known that four gold particles forming a square hold one silica particle in its center. On these grounds, the entire lattice was concluded to be of the NaCl-type.