Investigation of superconducting and normal-state properties of the filled-skutterudite system PrPt${}_4$Ge${}_{12-x}$Sb${}_x$

We report a study of the superconducting and normal-state properties of the filled-skutterudite system PrPt${}_4$Ge${}_{12-x}$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, $\gamma$, 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 ${\mathrm{\Theta }}_{\mathrm{E}}\sim$ 60 K for 0 $\le x\le$ 5; however, the rattling mode may not play any role in suppressing superconductivity.