The newly‐discovered clathrates Ba6Ge25and its relatives consist of a rigidG ermanium framework, in which Barium or other metallic atoms are trapped. These ‘guest’ atoms can ‘rattle’ freely in the oversized cages at high temperature. In Ba6Ge25, some of Ba atoms aref rozen into the split positions randomly below 200 K. This structural phase transition results in a two‐step resistivity jump from metallic behavior to semiconductor‐like behavior characterized by a mean free path of 3Å. Surprisingly, a BCS‐like superconducting transition occursa tTc≈ 0.24 K in such a ‘bad metal’. By applying hydrostatic pressure, the structural phase transition is suppressed butTcincreases drastically.Tcreaches a maximum value of 3.85K at a critical pressurepc≈ 2.8 GPa, where the structural distortion is completely suppressed and the system exhibits metallic behavior over the whole temperature range. Higher pressures lead to a slight decrease ofTc. On replacing 1/3 of the Ba atoms with Na, no structural phase transition iso bserved and the compound displays a similar behavior to Ba6Ge25abovepc, withTc≈ 1.05 K at ambient pressure. The relationship between the superconductivity and the structural phase transition is briefly discussed. © 2002 American Institute of Physics