From ∞1[(UO 2) 2O(MoO 4) 4] 6− to ∞1[(UO 2) 2(MoO 4) 3(MoO 5)] 6− infinite chains in A6U 2Mo 4O 21 ( A=Na, K, Rb, Cs) compounds: Synthesis and crystal structure of Cs 6[(UO 2) 2(MoO 4) 3(MoO 5)

Abstract

A new caesium uranyl molybdate belonging to the M 6U 2Mo 4O 21 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 A 6[(UO 2) 2O(MoO 4) 4], the caesium compound Cs 6U 2Mo 4O 21 should be written Cs 6[(UO 2) 2(MoO 4) 3(MoO 5)] with molybdenum atoms in tetrahedral and square pyramidal environments. Cs 6[(UO 2) 2(MoO 4) 3(MoO 5)] 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) Å 3, Z=4, ρ mes=5.02(2) g/cm 3 and ρ cal=5.08(3) g/cm 3. A full-matrix least-squares refinement on the basis of F 2 yielded R 1=0.0464 and w R 2=0.0950 for 596 parameters with 6964 independent reflections with I≥2 σ( 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[(UO 2) 2(MoO 4) 3(MoO 5)] 6− parallels chains built from U 2O 13 dimeric units, MoO 4 tetrahedra and MoO 5 square pyramids, whereas, Na, K and Rb compounds are characterized by ∞ 1[(UO 2) 2O(MoO 4) 4] 6− parallel chains formulated simply of U 2O 13 units and MoO 4 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).