We report optical investigations of the electronic properties of the clath- rate compound Ba₆Ge₂₅ in which at room temperature Ba atoms ``rattle'' in Ge-network cavities. When lowering the temperature across T_S\simeq 200 K a lock-in of the Ba-atoms to split-site positions in the cages is observed. The low energy Drude type of reflectivity is characterized by a low charge carrier density which smoothly varies with temperature. However, the Drude relaxation time of the charge carriers is found to be almost temperature independent, especially when cooling below T_S where, according to thermopower data, the effective mass is enhanced. This behavior could indicate a formation of polaronic quasi particles below T_S which is also supported by previous measurements of magnetic susceptibility of Ba₆Ge₂₅.

PACS numbers: 71.27.+a, 72.15.--v, 78.30.--j