A comparison of the normal and superconducting state properties of Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$ and Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$*

The evolution of unconventional superconductivity and the high field ordered phase (HFOP), the latter of which has been identified with antiferroquadrupolar order, in PrOs$_4$Sb$_{12}$ has been investigated in two pseudoternary systems, Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$ and Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$. In the Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$ system, the superconducting transition temperature $T_{c}$ is suppressed nearly linearly with $x$ to a minimum at $x = 0.6$ from both end member compounds, the upper critical field $H_{c2}$ has an approximately linear dependence on $T$ for $x > 0.4$, and the features related to the HFOP in the electrical resistivity disappear for $x \ge 0.1 $. On the other hand, in the Pr$_{1-x} $Nd$_x$Os$_4$Sb$_{12}$ system, $T_{c}$ and the Curie temperature $\Theta_{C}$ are suppressed monotonically toward $x = 0.55$ from $x=0$ and $x=1$, respectively, no linear $T$ dependence of $H_{c2}$ is observed, the HFOP persists up to at least $x \sim 0.45$, and there is an indication of the coexistence of superconductivity and ferromagnetism for $x \sim 0.45$ according to specific heat measurements. In both systems, $H_{c2}(T)$ is limited by the orbital motion of the electrons and the decrease of $T_{c}$ with $x$ from $x=0$ is nearly the same.