A novel cage for actinides: A ₆W₄Al₄₃ (A = U and Pu)

We report on synthesis and characterization of the compounds A ₆W₄Al₄₃ (A = U and Pu), that form in the hexagonal Ho₆Mo₄Al₄₃ caged-structure family. The A ions reside within W/Al cages where the A-A nearest neighbors form dimers between adjacent W/Al cages, with U-U and Pu-Pu distances of 3.3892 [Formula: see text] and 3.4080 [Formula: see text], respectively. While the W/Al networks provide environments similar to those of other cage-like materials (e.g. filled skutterudites), the atomic displacement parameters from single crystal x-ray diffraction measurements show that the A-ions do not exhibit rattling behavior. We find that there is site interchange disorder on one of the W/Al sites. Magnetic susceptibility measurements show that U₆W₄Al₄₃ displays anisotropic Curie-Weiss behavior where it fits to the data yield an effective magnetic moment near 2.0 [Formula: see text]/U. At low temperatures the magnetic susceptibility deviates from the Curie-Weiss temperature dependence and eventually saturates to a constant value. In contrast, Pu₆W₄Al₄₃ displays nearly temperature independent Pauli paramagnetism for all temperatures, as would be expected if the 5f -electrons are delocalized. The electrical resistivity for U₆W₄Al₄₃ increases slightly with the decreasing temperature, suggesting that it is dominated by f -electronic hybridization effects and disorder scattering that originates from the W/Al site interchange. Specific heat measurements for U₆W₄Al₄₃ further reveal an enhanced electronic Sommerfeld coefficient that is consistent with a moderately enhanced charge carrier effective mass. Together these measurements expose these materials as hosts for unstable f -electron magnetism, where the novel cage-like structures control the phenomena through the spacing between the A ions. Through this combination of mild magnetism, the low cost elements of the Al-W cages, and chemical tunability that has been shown for related materials in the same structure, the A ₆W₄Al₄₃ compounds emerge as promising nuclear waste-forms for transuranics, while the wider family of materials makes an appealing environment for studying f -electron physics in a novel structure.