The β-polymorph of uranium phosphide selenide

β-UPSe was synthesized from the reaction of U(2)Se(3), P and Se in a CsCl flux in a fused-silica tube. It crystallizes with four formula units in the tetra-gonal space group I4/mmm in the UGeTe structure type. The asymmetric unit comprises one U (site symmetry 4mm), one Se (4mm), and one P (mmm.) atom. The U atom is coordinated in a monocapped square-anti-prismatic arrangement, where the square face is formed by P atoms and the other five vertices are Se atoms. The P site is disordered about a mirror plane, showing half-ocupancy for each of the two resulting P atoms. The title structure is related to that of α-UPSe, which crystallizes with two formula units in the tetra-gonal space group P4/nmm in the PbFCl structure type.