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

Abstract

Abstract Crystals of four potassium uranyl chromates, K 8 [(UO 2 )(CrO 4 ) 4 ](NO 3 ) 2 ( 1 ), K 5 [(UO 2 )(CrO 4 ) 3 ](NO 3 )·(H 2 O) 3 ( 2 ), K 4 [(UO 2 ) 3 (CrO 4 ) 5 ](H 2 O) 8 ( 3 ) and K 2 [(UO 2 ) 2 ·(CrO 4 ) 3 (H 2 O) 2 ](H 2 O) 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.