Low-temperature ordered phase of CaU(PO4)2: synthesis and crystal structure

The crystal structure of the new compound CaU(P04)2 was determined from X-ray single-crysta! data and refined to R(F) = 0.025 and Rw(F) = 0.023 for 993 independent observed reflections. The unit cell is ortho­ rhombic, a = 13.926(4) A, b = 6.958(3) A and c = 6.136(2) A, space group Pnma. The structure is Ca, U ordered and in some aspects similar to the xenotime and anhydrite structure. The coordination polyhedra for the large meta! ions are described as semi regular trigonododecahedra. Key-words : crysta! structure, synthesis, CaU(P04)2, xenotime, monazite-type.