Synthesis, structure, and optical properties of CsU2(PO4)3

CsU2(PO4)3 was synthesized in highest yield by the reaction in a fused-silica tube of U, P, and Se in a CsCl flux at 1273 K. It crystallizes with four formula units in space group P21/n of the monoclinic system in a new structure type. The structure of CsU2(PO4)3 is composed of U and Cs atoms coordinated by PO43− units in distorted octahedral arrangements. Each U atom corner shares with six PO43− units. Each Cs atom face shares with one, edge shares with two, and corner shares with three PO43− units. The structure shares some features with the sodium zirconium phosphate structure type. X-ray powder diffraction results demonstrate that the present CsU2(PO4)3 compound crystallizes in a structure different from the previously reported β′- and γ-CsU2(PO4)3 compounds. CsU2(PO4)3 is highly pleochroic, as demonstrated by single-crystal optical absorption measurements.