Syntheses, Structures, Magnetic Properties, and X-ray Absorption Spectra of Carnotite-type Uranyl Chromium(V) Oxides: A[(UO2)2Cr2O8] (H2O)n (A = K2, Rb2, Cs2, Mg; n = 0, 4)

Four uranyl chromium(V) oxides were synthesized by mild hydrothermal methods and their structures were solved using single-crystal X-ray diffraction data. In these structures, U occurs in edge-sharing dimers of UO7 uranyl pentagonal bipyramids, and Cr occurs in edge-sharing dimers of CrO5 square pyramids. The two sets of dimers share edges, forming sheets of composition [(UO2)2Cr2O8]2- that are topologically identical to uranyl divanadate sheets, [(UO2)2V2O8]2-, that occur in compounds of the carnotite group. K2[(UO2)2Cr2O8] (KUCr), Rb2[(UO2)2Cr2O8] (RbUCr), and Cs2[(UO2) 2Cr2O8] (CsUCr) are isostructural with K2[(UO2)2V2O8], and Mg[(UO2)2Cr2O8](H2O)4 (MgUCr) is isostructural with Ni [(UO2)2V2O8](H2O)4. The stoichiometries of these compounds imply that Cr is present as chromium(V), which is supported by the results of bond valence analyses. EPR spectra and magnetic susceptibility data for KUCr are consistent with the presence of the d1 cation chromium(V). The KUCr moments undergo a sharp transition from paramagnetic to antiferromagnetic behavior at TN = 18.5 K. XAFS data for the U L3 edge and XANES of the Cr K edge of KUCr yield coordinations and oxidation states consistent with the crystal structure analysis. The presence of a pre-edge peak due solely to Cr5+ sounds a cautionary note for XANES determination of the chromate(VI) content of environmental samples. Crystallographic data for KUCr, RbUCr, CsUCr: monoclinic space group P21/c (No. 14), with a = 6.5483(8) (angstrom), b = 8.3548(10) (angstrom), c = 10.4200(13) (angstrom), (beta)= 105.040(3)°; a = 6.8677(6) (angstrom), b = 8.3599(7) (angstrom), c = 10.4625(8) (angstrom), (beta)= 106.004 (2)°; and a = 7.2643(15) (angstrom), b = 8.3803(17) (angstrom), c = 10.5100(22) (angstrom), (beta)= 106.399(5)°, respectively. Crystal data for MgUCr: orthorhombic space group Pnma (No. 62), with a = 10.5825(19) (angstrom), b = 15.1337(27) (angstrom), c = 8.1425(14) (angstrom).