The classical diffusion of particles in an inhomogeneous periodic system is studied employing the Fokker-Planck equation. The full width at half-maximum (fwhm) of the quasielastic peak in the dynamic structure factorS(q,ω) is calculated numerically by the matrix continued fraction method up to large values of the momentum transfer covering several Brillouin zones. It is shown that fwhm exhibits strong oscillations with the scattering wave-vector q as it has been observed in β-Ag 2 S by the mean of neutron scattering.