Crystal structure and properties of the uranium chalcogenides α-US2 and α-USe2

The uranium chalcogenides α-US₂ and α-USe₂ crystallize in the tetragonal space group $\text{P4}\text{ncc}$ and not $\text{I4}\text{mcm}$ as previously reported. Their crystal structures have been determined from single-crystal X-ray diffraction data and refined to R = 0.041 and R = 0.034, respectively. Uranium atoms occupy two crystallographic sites: U(1) in (8f) and U(2) in (4c). The fourfold position, U(2), is incompletely filled with three uranium atoms which show a significant uniaxial delocalization. Properties are discussed in relation to this new crystal structure.