Pressure study on new superconducting germanium clathrates

We investigate the low temperature properties of the recently discovered clathrates Ba6Ge25 and Na2Ba4Ge25 by tuning both materials with hydrostatic pressure. At ambient pressure, Ba6Ge25 undergoes a two-step structural phase transition between 230 K and 180 K from metallic behavior to a high-resistivity state. A superconducting transition occurs at T-C approximate to 0.24 K out of the resulting bad metal (rho(0) approximate to 1.5 mOmega cm). With increasing pressure, the structural phase transition is shifted to lower temperature but T-C increases drastically. T-C reaches a maximum value of 3.85 K at the critical pressure rho(C) approximate to 2.8 GPa, where the structural distortion is completely suppressed and the system exhibits metallic behavior. On replacing 1/3 of the Ba atoms with Na (Na2Ba4Ge25), no structural transformation is observed below room temperature, and the superconducting transition temperature is higher (T-C(p = 0) approximate to 1.05 K) than in the undoped case at ambient pressure but decreases slightly with increasing pressure.