  A detailed investigation of phonon excitations and charge carrier dynamics in
single crystals of Fe_{1-x}Cu_{x}Cr_{2}S_{4} (x = 0, 0.2, 0.4, 0.5) has been
performed by using infrared spectroscopy. In FeCr_{2}S_{4} the phonon
eigenmodes are strongly affected by the onset of magnetic order. Despite
enhanced screening effects, a continuous evolution of the phonon excitations
can be observed in the doped compounds with x = 0.2 (metallic) and x = 0.4, 0.5
(bad metals), but the effect of magnetic ordering on the phonons is strongly
reduced compared to x = 0. The Drude-like charge-carrier contribution to the
optical conductivity in the doped samples indicates that the colossal
magneto-resistance effect results from the suppression of spin-disorder
scattering.
