structure type as a cage for Uranium and Plutonium*

Single crystals of A₆W₄Al₄₃ (A=U/Pu) were synthesized for the first time using a molten Al flux. This structure features large lattice constants (a≈10.97, c≈17.71Å) and cage-like W-Al polyhedra that surround the A ions. XRD suggests that, unlike for some other cage-like structures, the A ions do not exhibit soft lattice modes. Magnetic susceptibility measurements for U₆W₄Al₄₃ show Curie-Weiss behavior, where fits to the data yield an effective magnetic moment near 2.0 µ_B/U. At low-T the magnetic susceptibility deviates from the Curie-Weiss law and saturates to a constant value, as seen for other spin fluctuating materials. There is evidence from the electrical transport and heat capacity for the formation of a heavy Fermi liquid ground state due to Kondo lattice behavior. In contrast, Pu₆W₄Al₄₃ exhibits nearly T-independent Pauli paramagnetism, suggesting delocalization of the 5f electrons. We will discuss these results in the context of the Hill plot, where both compounds are found near the inter-actinide distance that separates delocalized and localized f-electron behavior.