STRUCTURE TOPOLOGY AND HYDROGEN BONDING IN MARTHOZITE, Cu2+[(UO2)3(SeO3)2O2](H2O)8, A COMPARISON WITH GUILLEMINITE, Ba[(UO2)3(SeO3)2O2](H2O)3

Abstract

The crystal structure of marthozite, Cu 2+ [(UO 2 ) 3 (SeO 3 ) 2 O 2 ] (H 2 O) 8 , a 6.9879(4), b 16.4537(10), c 17.2229(10) A, V 1980.2(3) A 3 , Pbn 2 1 , Z = 4, D calc = 4.37 g/cm 3 , has been solved by direct methods and refined to an R index of 5.7% for 4364 observed (| F o | > 4σ F ) reflections collected with a four-circle diffractometer fitted with Mo K α X-radiation and a CCD detector. There are three unique U sites, each occupied by U 6+ with two short U –O uranyl bonds (1.76–1.82 A) and coordination numbers of [8], [7] and [7], respectively. There are two unique Se sites, each occupied by Se 4+ and coordinated by three O atoms ( Se –O ≈ 1.70 A), forming a triangular pyramid with Se at the apex, indicative of stereoactive lone-pair behavior in Se 4+ . There is one Cu site, occupied by Cu 2+ in octahedral coordination by four (H 2 O) groups ( Cu –H 2 O ≈ 2.0 A) and two O atoms ( Cu –O ≈ 2.4 A). The structural unit is a sheet of composition [(UO 2 ) 3 (SeO 3 ) 2 O 2 ], comprised of chains of edge-sharing (Uϕ n ) polyhedra extending along [100] that are cross-linked in the [001] direction by (SeO 3 ) groups; this sheet is topologically identical to the structural unit in guilleminite, Ba [(UO 2 ) 3 (SeO 3 ) 2 O 2 ] (H 2 O) 3 . Adjacent sheets are linked through interstitial Cu 2+ cations via Cu 2+ –O apical bonds and via H bonds that involve both (H 2 O) groups bonded to Cu 2+ and interstitial (H 2 O) groups not bonded to any cation. The principal differences between marthozite and guilleminite involve the interlayer species. In guilleminite, the Ba atom is centrally positioned with respect to the chain of (UO n ) polyhedra, whereas in marthozite, the Cu 2+ atom is located off the pseudo-mirror plane of the chain of (UO n ) polyhedra. This differential positioning of the interstitial cations results in an intersheet separation in marthozite that is nearly 1 A greater than that in guilleminite, despite the fact that Ba (in guilleminite) is much larger than Cu 2+ (in marthozite). The H-bond arrangement in marthozite is very different from that in guilleminite. In marthozite, there are eight unique interlayer (H 2 O) groups; four of these (H 2 O) groups bond directly to Cu 2+ and four are held in the structure solely by H bonds. In guilleminite, there are two unique interlayer (H 2 O) groups, both of which bond to the interlayer Ba atoms.