High-pressure studies on a new superconducting clathrate:: Ba6Ge25

The effect of pressure on the low-temperature states of the newly discovered clathrate Ba6Ge25 is investigated by means of measurements of the electrical resistivity. At ambient pressure, Ba6Ge25 undergoes a two-step structural phase transition between 230 and 180 K from metallic behaviour to a high-resistivity state characterized by a mean free path of about 3 A. Interestingly, a Bardeen-Cooper-Schrieffer-like (BCS-like) superconducting transition occurs at T-C approximate to 0.24 K from the resulting 'bad metal'. With increasing pressure, the structural phase transition is depressed but T-C increases drastically. TC reaches a maximum value of 3.85 K at the critical pressure p(C) approximate to 2.8 GPa, where the structural distortion is completely suppressed and the system exhibits metallic behaviour. Higher pressures lead to a slight decrease of T-C.