Investigation of superconducting and normal-state properties of the filled-skutterudite systemPrPt4Ge12−xSbx

We report a study of the superconducting and normal-state properties of the filled-skutterudite system ${\mathrm{PrPt}}₄{\mathrm{Ge}}_{12\ensuremath{-}x}{\mathrm{Sb}}_x$. Polycrystalline samples with Sb concentrations up to $x=5$ were synthesized and investigated by means of x-ray diffraction, electrical resistivity, magnetic susceptibility, and specific heat measurements. We observed a suppression of superconductivity with increasing Sb substitution up to $x=4$, above which no signature of superconductivity was observed down to 140 mK. The Sommerfeld coefficient, $\ensuremath{\gamma}$, of superconducting specimens decreases with increasing $x$ up to $x=3$, suggesting that superconductivity may depend on the density of electronic states in this system. The specific heat for $x=0.5$ exhibits an exponential temperature dependence in the superconducting state, reminiscent of a nodeless superconducting energy gap. We observed evidence for a weak ``rattling'' mode associated with the Pr ions, characterized by an Einstein temperature ${\mathrm{\ensuremath{\Theta}}}_{\mathrm{E}}\ensuremath{\sim}60$ K for $0\ensuremath{\le}x\ensuremath{\le}5$; however, the rattling mode may not play any role in suppressing superconductivity.