Normal and superconducting state properties of the (Pr$_{1-x}$Nd$_x$)Os$_4$Sb$_{12}$ system

Our previous experimental studies of the (Pr$_{1-x}$Nd$_x$)Os$_4$Sb$_{12}$ system revealed that: (1) superconductivity and ferromagnetism from both end compounds were suppressed almost monotonically toward $x = 0.55$, (2) retention of the antiferroquadrupolar order phase in (Pr$_{1-x}$Nd$_x$)Os$_4$Sb$_{12}$ to higher values of $x$ than in Pr(Os$_{1-x}$Ru$_{x}$)$_4$Sb$_{12}$, (3) two possible CEF energy level schemes in NdOs$_4$Sb$_{12}$, in which the ground state is either the $\Gamma_{6}$ doublet or $\Gamma_{8}^{(2)}$ quartet. Recent ultrasonic measurements in the NdOs$_4$Sb$_{12}$ sample ($x=1$) revealed softening of the C$_{44}$ mode, which indicated that the CEF ground state in this compound is more likely to be the $\Gamma_{8}^{(2)}$ quartet. The lattice parameter in (Pr$_{1-x}$Nd$_x$)Os$_4$Sb$_{12}$ seems to increase slightly from PrOs$_4$Sb$_{12}$ ($x = 0$) toward NdOs$_4$Sb$_{12}$ ($x = 1$). The T-x and H-x phase diagrams related to superconductivity, ferromagnetism, antiferroquadrupolar order, and the CEF energy level scheme for the (Pr$_{1-x}$Nd$_x$)Os$_4$Sb$_{12}$ system for $0 \le x \le 1$ will be discussed.