CRYSTAL CHEMISTRY OF URANYL MOLYBDATES. X. THE CRYSTAL STRUCTURE OF AG10[(UO2 )8O8(MO5O20)]

Abstract

Crystals of a new silver uranyl molybdate, Ag-10[(UO2)(8)O-8(Mo5O20)] have been synthesized by high-temperature solid-state reaction. The structure [monoclinic, C2/c, a 24.672(2), b 23.401(2), c 6.7932(4) Angstrom, beta 94-985(2)degrees, V 3907.3(4) Angstrom(3), Z = 4] was solved by direct methods and refined to R1 = 0.049 (wR2 = 0.101) on the basis of 4317 unique observed reflections using a crystal twinned on (001). The twinning causes overlap of the reflections with I = 3n. The structure is based upon complex double sheets of UrO(5) pentagonal bipyramids and Mo polyhedra. The double sheets are parallel to (100) and consist of two [(UO2)(2)O-2(MoO5)] sheets of UrO(5) bipyramids and MoO6 octahedra. Within the [(UO2)(2)O-2(MoO5)] sheets, UrO(5) bipyramids share edges to produce complex chains parallel to the c axis and linked via pairs of edge-sharing MoO6 octahedra. The two [(UO2)(2)O-2(MoO5)] sheets are linked into double [(UO2)(8)O-8(Mo5O20)] sheets via the Mo(3)O-4 tetrahedron located between the sheets. The MoO4 tetrahedra share two corners with two MoO6 octahedra of the sheet above and two corners with MoO6 octahedra from the sheet below, so that Mo5O20 pentanuclear clusters are formed. The uranyl molybdate sheets found in the structure have not been previously observed in uranyl minerals and inorganic compounds. The Ag(2), Ag(3), Ag(6), Ag(7), and Ag(8) atoms are located within the double sheets, whereas the Ag(1), Ag(4), and Ag(5) are in between the double sheets.