The determination of the crystal structure of tetrapotassium uranyl tricarbonate by powder X‐ray diffraction method

A uranyl compound, K₄UO₂(CO₃)₃ has been characterized by powder X‐ray diffraction method. M.W. = 606.46, monoclinic, C2/c (No. 15), a = 1.0240(7), b = 0.9198(4), c = 1.2222(12) nm, β = 95.12 (4)°, V = 1.1466(5) nm³, Z = 4, D = 3.468 g/cm³, D_c = 3.513 g/cm, λ(Cu Ka₁) = 0.1540598nm, T = 298K. The structure was solved by heavy atom method and Fourier synthesis, and refined by full‐matrix least‐squares method to R = 0.1185 for 275 reflections. The uranium (VI) atom is in an eight‐coordinate distorted hexagonal‐bipyramidal environment with creasy fan shape. The linear uranyl group approaches to perpendicular to the equatorial plane in which three carbonate groups are chelated. U(VI) has two linear oxygen atoms closer to it (U]—0 = 0.1767 (5) nm) than six other neighbours (U—O ranging from 0.2516 to 0.2568 nm). The distances between carbon atoms and uncoordinated oxygen atoms are 0.122 (1) and 0.123(1) nm, which are distinctly different from those between carbon and coordinated oxygen atoms (mean 0.134(6) nm). This fact reveals the non‐equivalence of one oxygen atom to the other two in each carbonate. In K₄UO₂(CO₃)₃, the O—O distance for the adjacent carbonate groups is 0.2794(4)nm approaching to the sum of Van der Waals radii of two oxygen atoms. The K—O distances vary between 0.2667 and 0.3131 nm, and each anion is immediately surrounded by six potassium ions, only four of which can be considered to belong to the same structural formula unit, and they are symmetrically located above and below the equatorial plane.