A novel family of microporous uranyl germanates: Framework topology and complexity of the crystal structures

Abstract

Five novel microporous uranyl germanates, Cs[(UO2)2(HGe2O7)] (1), Co2[(UO2)8(HGe2O7)4](H2O)6 (2), Ba2[(UO2)8(HGe2O7)4](H2O)3 (3), K4[(UO2)8(HGe2O7)4](H2O)2 (4), and Rb4[(UO2)8(HGe2O7)4](H2O)2 (5), have been synthesized under hydrothermal conditions. Single-crystal X-ray analyses indicate that 1 has an orthorhombic structure, space group Cmcm, while compounds 2–5 are isomorphous and have a tetragonal structure, space group I4/mcm. All compounds consist of heteropolyhedral frameworks that are based on chains of UO7 pentagonal bipyramids that share edges with GeO4 tetrahedra. These chains are either assembled into sheets (1) or nanotubules (2–5) to form a microporous framework characterized by the presence of [HGe2O7]5- dimers. The framework of 1 contains a channel with an effective channel dimension of 2.55 × 2.55 Å, and occupied by Cs atoms, whereas the unprecedented framework of 2–5 contains two channels of different dimensionality (1.88 × 1.88 and 3.82 × 5.59 Å of effective channel dimension) extending along the c crystallographic axis. In addition to water molecules, these channels accommodate mono- and divalent cations, with ionic radii ranging from 0.745 Å ([6]Co2+) to 1.61 Å ([8]Rb+). An extended structural comparison is provided between these compounds and similar compounds, as well as a discussion concerning the topology and the complexity of the uranyl germanates with framework-type structures.