Tracer particle diffusion in crystal-like ordered colloidal suspensions

Dynamic light scattering experiments are reported on crystal-like ordered c olloidal suspensions composed of two types of electrostatically interacting particles having similar size and effective particle charge. The particles differ only in their refractive indices. We obtain the incoherent (self) i ntermediate scattering function of these mixtures by using only a trace con centration of one of the species and matching the refractive index of the s uspending liquid medium to that of the other. Localization of the tracer pa rticles about randomly located lattice sites of the colloidal crystal is ma nifested in the nonergodicity of the incoherent intermediate scattering fun ctions. The latter, and the particle mean-squared displacements obtained fr om them, plateau to finite values at long times. In analogy to neutron scat tering these plateau values of the incoherent intermediate scattering funct ions may be called the elastic incoherent structure factor. (C) 1999 Americ an Institute of Physics. [S0021-9606(99)51602-4].