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Abstract

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.