Pressure Study on New Superconducting Germanium Clathrates

Abstract

We investigate the low temperature properties of the recently discovered clathrates Ba 6 Ge 25 and Na 2 Ba 4 Ge 25 by tuning both materials with hydrostatic pressure. At ambient pressure, Ba 6 Ge 25 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}\\approx 0.24\\,\\hbox{K} out of the resulting bad metal ( \\rho_{0}\\approx 1.5\\,\\hbox{m}\\Omega\\;\\hbox{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 p_{C}\\approx 2.8\\,\\hbox{GPa} , where the structural distortion is completely suppressed and the system exhibits metallic behavior. On replacing 1/3 of the Ba atoms with Na (Na 2 Ba 4 Ge 25 ), no structural transformation is observed below room temperature, and the superconducting transition temperature is higher (T_{C}(p = 0) \\approx 1.05\\,\\hbox{K}) than in the undoped case at ambient pressure but decreases slightly with increasing pressure.