Structural topology of potassium uranyl chromates: crystal structures of K8[(UO2)(CrO4)4](NO3)2, K5[(UO2)(CrO4)3](NO3)(H2O)3, K4[(UO2)3(CrO4)5](H2O)8 and K2[(UO2)2(CrO4)3(H2O)2](H2O)4

✍ Scribed by Krivovichev, Sergey V.; Burns, Peter C.

• Book ID: 115519505
• Publisher: Oldenbourg Wissenschaftsverlag
• Year: 2003
• Tongue: English
• Weight: 377 KB
• Volume: 218
• Category: Article
• ISSN: 2194-4946
• DOI: 10.1524/zkri.218.11.725.20298

✦ Synopsis

Abstract

Crystals of four potassium uranyl chromates, K~8~(UO~2~)(CrO~4~)~4~~2~ (1), K~5~(UO~2~)(CrO~4~)~3~·(H~2~O)~3~ (2), K~4~(UO~2~)~3~(CrO~4~)~5~~8~ (3) and K~2~(UO~2~)~2~·(CrO~4~)~3~(H~2~O)~2~~4~ (4), have been synthesized by evaporation of aqueous solutions of (UO~2~)(NO~3~)~2~(H~2~O)~6~ and K~2~CrO~4~. The structure of (1) (triclinic, __P__1̅, a = 7.0397(6), b = 9.7341(9), c = 9.7568(9) Å, α = 105.846(2), β = 97.992(2), γ = 93.271(2)°, V = 633.8(1) Å^3^, Z = 1) was solved by direct methods and refined to __R__1 = 0.044 (__wR__2 = 0.098). It is based upon clusters of composition [(UO~2~)(CrO~4~)~4~] that consist of __Ur__O~4~ square bipyramids (Ur: UO~2~
^2+^ uranyl ion) that share corners with four different CrO~4~ tetrahedra. The clusters are arranged in layers parallel to the (100) plane. K^+^ cations and NO~3~
^-^ groups are located between the clusters. The crystal structure of (2) (orthorhombic, __P__2~1~2~1~2~1~, a = 6.1112(11), b = 12.136(2), c = 27.464(4) Å, V = 2036.9(6) Å^3^, Z = 4) was solved by direct methods and refined to __R__1 = 0.047 (__wR__2 = 0.071). it contains chains of composition [(UO~2~)(CrO~4~)~3~] that are parallel to the b axis. The chains consist of __Ur__O~5~ pentagonal bipyramids that share all equatorial corners with different CrO~4~ tetrahedra. Planes containing the equatorial ligands of the __Ur__O~5~ pentagonal bipyramids are approximately parallel to the (100) plane. K^+^ cations, NO~3~ groups and H~2~O groups are between the chains. The crystal structure of (3) (monoclinic, __P__2~1~/c, a = 8.2336(7), b = 18.8042(17), c = 21.2413(18) Å, β = 89.979(2)°, V = 3288.7(5) Å^3^, Z = 4) has been solved by direct methods from a crystal twinned on the (001) plane and refined to __R__1 = 0.060 (__wR__2 = 0.125). The structure is based upon topologically complex sheets of composition [(UO~2~)~3~(CrO~4~)~5~] with __Ur__O~5~ pentagonal bipyramids that share their corners with CrO~4~ tetrahedra. The sheets are parallel to the (010) plane. K^+^ cations and H~2~O groups are located in the interlayer between the sheets. The crystal structure of (4) (monoclinic, __P__2~1~/c, a = 10.7417(5), b = 14.5290(7), c = 14.1387(6) Å, β = 108.135(1)°, V = 2097.0(2) Å^3^, Z = 4) was solved by direct methods and refined to __R__1 = 0.036 (__wR__2 = 0.067). It is based upon sheets of composition [(UO~2~)~2~(CrO~4~)~3~(H~2~O)~2~] that are parallel to the (010) plane. K^+^ cations and additional H~2~O groups are in the interlayer. Nodal representations of uranyl chromate structural units are developed, and a comparison with related structures are presented.