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)

Abstract

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 UO 7 uranyl pentagonal bipyramids, and Cr occurs in edge-sharing dimers of CrO 5 square pyramids. The two sets of dimers share edges, forming sheets of composition [(UO 2 ) 2 Cr 2 O 8 ] 2-that are topologically identical to uranyl divanadate sheets, [(UO 2 ) 2 V 2 O 8 ] 2-, that occur in compounds of the carnotite group. K 2 [(UO 2 ) 2 Cr 2 O 8 ] (KUCr), Rb 2 [(UO 2 ) 2 Cr 2 O 8 ] (RbUCr), and Cs 2 [(UO 2 ) 2 Cr 2 O 8 ] (CsUCr) are isostructural with K 2 [(UO 2 ) 2 V 2 O 8 ], and Mg[(UO 2 ) 2 Cr 2 O 8 ](H 2 O) 4 (MgUCr) is isostructural with Ni[(UO 2 ) 2 V 2 O 8 ]-(H 2 O) 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 d 1 cation chromium(V). The KUCr moments undergo a sharp transition from paramagnetic to antiferromagnetic behavior at T N ) 18.5 K. XAFS data for the U L 3 edge and XANES of the Cr K edge of KUCr yield coordinations and oxidation states consistent with the crystal structure analysis.

Synthesis, characterization, and crystal structures of uranyl compounds containing mixed chromium oxidation states

“…The sum of bond valences incident upon the Cr(1) site ranges from 4.74 to 4.75 valence units over the three compounds, consistent with the site being occupied by Cr(V). This coordination geometry is identical to the square pyramids found in uranyl compounds having the carnotite-type structure and containing Cr(V) [6]. The compounds in this study also contain four symmetrically distinct Cr(VI) sites, each of which is tetrahedrally coordinated by O atoms with bond lengths ranging from 1.575(9) to 1.682(7)Å, with an average length of 1.64(3)Å and average bond valence of 6.07 (6).…”

“…This coordination geometry is identical to the square pyramids found in uranyl compounds having the carnotite-type structure and containing Cr(V) [6]. The compounds in this study also contain four symmetrically distinct Cr(VI) sites, each of which is tetrahedrally coordinated by O atoms with bond lengths ranging from 1.575(9) to 1.682(7)Å, with an average length of 1.64(3)Å and average bond valence of 6.07 (6). Again, emphasizing the four-member ring of equatorial-vertexsharing uranyl pentagonal bipyramids (Fig.…”

“…In these compounds, Cr(VI) is reduced to Cr(V) in the structure in response to the substitution of trivalent cations for divalent cations, and the atomic packing only permits the Cr(V) cations to occupy tetrahedral sites. In contrast, compounds 1-3, as well as Cr(V) compounds we reported earlier [6], grew from aqueous solutions. This suggests that a square pyramidal configuration of ligands about Cr(V) may occur in aqueous solution, and perhaps linkage of such a bipyramid with uranyl polyhedra hinders the disproportionation to Cr(III) and Cr(VI).…”

“…However, Cr(V) has been stabilized in several organic complexes, where it is typically coordinated by five O atoms in a square pyramidal arrangement [2][3][4][5]. Several years ago, we reported the synthesis of four uranyl Cr(V) oxides with carnotitetype sheets, which to date are the only known examples of inorganic compounds containing Cr(V) in square pyramidal coordination environments [6]. In these structures, two such pyramids share an edge, and Cr(V) bond lengths range from 1.57 to 1.90Å.…”

Crystal Chemistry and Structures of (Ca,U) Titanate Pyrochlores

“…BVS have been used by a number of researchers along with structure refinements to quantify the uranium oxidation states at specific crystallographic sites within synthetic compounds and minerals, including those containing mixed‐valence uranium 46–54 . For example, BVS calculations were carried out for U(UO 2 )(PO 4 ) 2 using refined bond lengths 47 .…”