Crystal structures of Rb2U2O7 and Rb8U9O31, a new layered rubidium uranate

Two alkali metal uranates Rb2U2O7 and Rb8U9O31 have been synthesized by solid state reaction at high temperature and their crystal structures determined from single crystal X-ray diffraction data, collected with a three circles Brucker SMART diffractometer equipped by Mo(Kα) radiation and a charge-coupled device (CCD) detector. Their structures were solved using direct methods and Fourier difference techniques and refined by a least-square method on the basis of F2 for all unique reflections, with R1=0.043R1=0.043 for 53 parameters and 746 independent reflections with I⩾2σ(I)I⩾2σ(I) for Rb2U2O7, monoclinic symmetry, space group P21/c  , a=7.323(2)Å, b=8.004(3)Å, c=6.950(2)Å, β=108.81(1)°β=108.81(1)°, ρmes=6.56(3)g/cm3, ρcal=6.54(2)g/cm3, Z=2Z=2 and R1=0.036R1=0.036 for 141 parameters and 2065 independent reflections with I⩾2σ(I)I⩾2σ(I) for Rb8U9O31, orthorhombic, space group Pbna  , a=6.9925(9)Å, b=14.288(2)Å, c=34.062(5)Å, ρmes=6.47(3)g/cm3, ρcal=6.48(2)g/cm3, Z=4Z=4.The Rb2U2O7 structure presents a strong analogy with that of K2U2O7 and can be described by layers of distorted UO2(O4) octahedra built from dimeric units of edge shared octahedra further linked together by opposite corners. In Rb8U9O31 puckered layers are formed by the association of two different uranium polyhedra, pentagonal bipyramids and distorted octahedra. The structure of Rb8U9O31 is built from a regular succession of ∞1[U4O14]4- infinite ribbons similar to those observed in diuranates M2U2O7 (MK, Rb) and infinite three polyhedra wide ribbons ∞1[U5O21]12-, to create an original undulated sheets ∞2[U9O31]8-.For both compounds Rb+ ions occupy the interlayer space and exhibit comparable mobility with conductivity measurements indicating an Arrhenius-type behavior.

Puckered layered structure of Rb8U9O31.Figure optionsDownload as PowerPoint slide