Single crystal growth and structural characterization of four complex uranium oxides: CaUO4, β-Ca3UO6, K4CaU3O12, and K4SrU3O12

Single crystals of complex uranium oxides, CaUO4, β-Ca3UO6, K4CaU3O12 and K4SrU3O12 were grown from carbonate melts. The crystal structures of the four uranates were determined by single crystal X-ray diffraction. CaUO4 crystallizes in the hexagonal space group R-3m, with lattice parameters a = 6.2570(7) Å and α = 36.04(2)°. The U6+ atom in CaUO4 is 8-coordinate and exhibits hexagonal bipyramidal geometry with six long and two short U–O bonds, typical of a uranyl species. β-Ca3UO6 forms in the monoclinic space group P21/n, with lattice parameters a = 5.728(1) Å, b = 5.956(1) Å, c = 8.298(2) Å, and β = 90.55(3)°, and adopts a distorted double perovskite structure. K4CaU3O12 and K4SrU3O12 crystallize in the cubic space group Im-3m with lattice parameters a = 8.483(1) Å and a = 8.582(1) Å, respectively. In all three perovskite-type oxides, the U(VI) cation is located in an octahedral coordination environment and exhibits typical uranyl geometry with four long and two short U–O bonds.

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► Synthesis of CaUO4, Ca3UO6, and K4MU3O12 single crystals reported.
► K2CO3 employed for flux crystal growth.
► Ordered perovskite family oxides presented.
► Single crystal structures discussed.