U₈Al₁₉Si₆, A Uranium Aluminide Silicide with a Stuffed Supercell Grown from Aluminum Flux

U₈Al₁₉Si₆ is formed from the reaction of uranium oxide and silicon in aluminum flux. Growth of this material is dependent on the presence of surface hydroxyl groups on alumina in the crucible. The compound forms as large cuboid crystals with a complex new structure that is a cubic stuffed supercell of UAl₃ (AuCu₃ parent structure type). It features a 4-fold expansion of the unit cell axis and the addition of atoms on two extra positions resulting in Pm3n symmetry. The ordered distribution of aluminum and silicon was determined using single crystal neutron diffraction. The magnetic susceptibility shows temperature independent paramagnetic behavior, while heat capacity measurements are consistent with Fermi liquid characteristics at low temperatures where the Sommerfeld constant is enhanced in comparison to that of a conventional metal. These bulk thermodynamic properties indicate that U₈Al₁₉Si₆ is an itinerant electron Pauli paramagnet with moderately enhanced mass charge carrier quasiparticles.