From ∞1[(UO2)2O(MoO4)4]6− to ∞1[(UO2)2(MoO4)3(MoO5)]6− infinite chains in A6U2Mo4O21 (A=Na, K, Rb, Cs) compounds: Synthesis and crystal structure of Cs6[(UO2)2(MoO4)3(MoO5)]

A new caesium uranyl molybdate belonging to the M6U2Mo4O21 family has been synthesized by solid-state reaction and its structure determined from single-crystal X-ray diffraction data. Contrary to the other alkali uranyl molybdates of this family (A=Na, K, Rb) where molybdenum atoms adopt only tetrahedral coordination and which can be formulated A6[(UO2)2O(MoO4)4], the caesium compound Cs6U2Mo4O21 should be written Cs6[(UO2)2(MoO4)3(MoO5)] with molybdenum atoms in tetrahedral and square pyramidal environments. Cs6[(UO2)2(MoO4)3(MoO5)] crystallizes in the triclinic symmetry with space group P1̄ and a=10.4275(14) Å, b=15.075(2) Å, c=17.806(2) Å, α=70.72(1)°, β=80.38(1)° and γ=86.39(1)°, V=2604.7(6) Å3, Z=4, ρmes=5.02(2) g/cm3 and ρcal=5.08(3) g/cm3. A full-matrix least-squares refinement on the basis of F2 yielded R1=0.0464 and wR2=0.0950 for 596 parameters with 6964 independent reflections with I≥2σ(I) collected on a BRUKER AXS diffractometer with Mo(Kα) radiation and a CCD detector. The crystal structure of Cs compound is characterized by ∞1[(UO2)2(MoO4)3(MoO5)]6− parallels chains built from U2O13 dimeric units, MoO4 tetrahedra and MoO5 square pyramids, whereas, Na, K and Rb compounds are characterized by ∞1[(UO2)2O(MoO4)4]6− parallel chains formulated simply of U2O13 units and MoO4 tetrahedra.Infrared spectroscopy measurements using powdered samples synthesized by solid-state reaction, confirm the structural results. The thermal stability and the electrical conductivity are also studied. The four compounds decompose at low temperature (between 540 and 610 °C).

The staking of ∞1[(UO2)2(MoO4)3(MoO5)]6− infinite uranyl molybdate ribbons in the Cs6[(UO2)2(MoO4)3(MoO5)] structure.Figure optionsDownload as PowerPoint slideHighlights
► Cs6U2Mo4O2 a new compound with bidimensional crystal structure, characterized by infinite uranyl molybdate chains.
► Crystal structure similar to these of the compounds containing Na, K, Rb.
► Molybdenum atoms surrounded by five oxygen atoms to form an original and strongly distorted MoO5 environment.
► The chains arrangement illustrates the key role of the alkaline ionic radius, in the crystal structure distortion for Cs compound.