Synthesis and properties of A${}_x$V${}_2$Al${}_{20}$ (A = Th, U, Np, Pu) ternary actinide aluminides

Polycrystalline samples of A${}_x$V${}_2$Al${}_{20}$ (A = Ce, Th, U, Np, Pu; 0.7 =< x =< 1.0) actinide intermetallics were synthesized using the arc-melting method. Crystal structure studies were performed by means of powder x-ray diffraction and the Rietveld refinement method. All studied compounds crystallize in the CeCr${}_2$Al${}_{20}$-type structure (space group Fd-3m, no. 227) with the actinide or Ce atom located in the oversized cage formed by Al atoms. Comparison of the crystallographic results with the reported data for LnV${}_2$Al${}_{20}$ (Ln = lanthanoids) counterparts reveals distinctly different behavior of the lanthanide- and actinide-bearing compounds. This difference is suggested to be caused by fairly localized character of the 4f electrons, whereas itinerant character of the 5f electrons is likely seen for U- and Np-containing phases. Magnetic susceptibility and specific heat measurements did not reveal any magnetic ordering in U${}_{0.8}$V${}_2$Al${}_{20}$, Np${}_{0.8}$V${}_2$Al${}_{20}$ and Pu${}_{0.8}$V${}_2$Al${}_{20}$ down to 2.5 K. A small anomaly in low-temperature specific heat of Ce${}_{0.8}$V${}_2$Al${}_{20}$, U${}_{0.8}$V${}_2$Al${}_{20}$, and Np${}_{0.8}$V${}_2$Al${}_{20}$ is observed, likely arising from a low-energy Einstein mode.