A novel cage for actinides: A6W4Al43 (A = U and Pu)

Abstract

We report on synthesis and characterization of the compounds A6W4Al43 (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 and 3.4080 , 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 /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 A6W4Al43 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.