The β-polymorph of uranium phosphide selenide

β-UPSe was synthesized from the reaction of U2Se3, P and Se in a CsCl flux in a fused-silica tube. It crystallizes with four formula units in the tetragonal 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-antiprismatic 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 tetragonal space group P4/nmm in the PbFCl structure type.

-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 tetragonal 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 squareantiprismatic 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 -UPSe, which crystallizes with two formula units in the tetragonal space group P4/nmm in the PbFCl structure type.
Funding for this research was kindly provided by the US Department of Energy, Basic Energy Sciences, Chemical Sciences, Biosciences, and Geosciences Division and Division of Materials Science and Engineering grant ER-15522. SEM analyses were conducted in the Electron Probe Instrumentation Center (EPIC) at the Northwestern University Atomic and Nanoscale Characterization Experimental (NUANCE) Center, supported by the NSF-NSEC, the NSF-MRSEC, the Keck Foundation, the State of Illinois, and Northwestern University. Single-crystal data were collected at the IMSERC X-ray Facility at Northwestern University, supported by the International Institute of Nanotechnology (IIN).
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: WM2560).
i75 Acta Crystallographica Section E
We have synthesized β-UPSe from a CsCl flux, in contrast to previous reported synthetic methods. This compound crystallizes with four formula units in the tetragonal space group I4/mmm (Figure 1) in the UGeTe structure type. The asymmetric unit comprises atoms U1 (site symmetry 4mm), Se1 (4mm), and P1 (mmm.) (Figure 2). U is coordinated in a monocapped square-antiprismatic arrangement, where the square face is formed by P atoms and the other five vertices are Se atoms. These moieties face-share along the four triangular faces formed by two Se atoms and one P atom with adjacent moieties. They also face-share along the square faces formed by P atoms. Finally, each of the four edges on the cap are shared with another moiety so that the U atoms are staggered when viewed down [001]. Viewed on the side of the basal plane, each atom type lies on its own plane.
The structure is related to that of α-UPSe (Zygmunt et al., 1974a;Hulliger, 1968). The only difference is that instead of face-sharing along the square faces formed by the P atoms, the moieties edge-share with four other moieties, such that the U atoms are staggered in a checkerboard pattern when viewed down [001].
Reagents were loaded in an argon-filled glove box, and the tube was flame-sealed under 10 -4 Torr vacuum. The tube was placed in a computer-controlled furnace and heated to 1273 K in 96 h, held there for 4 h, cooled to 1223 K in 12 h, held there for 96 h, and then cooled to 298 K in 350 h. The solidified flux was washed off with water. Qualitative EDS analysis using a Hitachi S-3400 SEM showed the presence of U, P, and Se, with no detectable Cs or Cl content in the crystals.
The refinement results in the placement of a half-occupied P1 atom in the 8j position. The resultant disorder at +x,1/2,0 (P1a) and -x,1/2,0 (P1b) leads to impossible P1a-P1b distances of 0.52 Å and 2.42 Å. The P1 atoms are arranged in a square net 2.813 Å on a side. The distribution of P1a squares and P1b squares within a structure is random, as no supercell reflections were observed. Fig. 1. Structure of β-UPSe. U atoms are black, P atoms are brown, and Se atoms are orange.   (13)