Synthesis, Crystal Structure, and Comparison of Two New Uranyl Vanadate Layered Compounds: M6(UO2)5(VO4) 2O5 with M=Na, K

Two new alkali uranyl vanadates, M6(UO2)5(VO4)2O5 with M=Na, K, have been synthesized by solid state reactions and their structures determined from single-crystal X-ray diffraction data. They crystallize in the monoclinic system with space group P21/c. Unit cell b, c, and BETAparameters are almost identical in both compounds while the a parameter in the Na compound is doubled: Na––a=12.584(1) ?, b=24.360(2) ?, c=7.050(1) ?, BETA=100.61(1)°, V=2124.2 ?3, Z=4, Dmes= 5.60(2) g/cm3, Dcal=5.63 g/cm3; K––a=6.856(1) ?, b=24.797(3) ?, c=7.135(1) ?, BETA=98.79(8)°, V=1198.8 ?3, Z=2, Dmes=5.25(2) g/cm3, Dcal=5.25 g/cm3. A full-matrix least-squares refinement yielded, respectively, R=0.039, Rw=0.043 and R=0.037; Rw=0.046 for 3293 and 2096 independent reflections with I>3SSIGMA collected on a Philips PW 1100 diffractometer (MoKALPHA radiation). Both structures are characterized by [(UO2)5(VO4)2O5x]6n-n corrugated layers parallel to the (100) plane. The layers are built up from VO4 tetrahedra, UO7 pentagonal bipyramids, and UO6 distorted octahedra. The UO7 pentagonal bipyramids are associated by sharing opposite equatorial edges to form zig-zag infinite chains (UO5)n parallel to the c axis. These chains are linked together on one side by VO4 tetrahedra and on other side by UO6 octahedra and UO7 corner sharing. Both structures differ simply by the symmetry of the layers. The unit cell contains one centrosymmetric layer in the K compound, whereas in the two-layer unit cell of the Na compound two noncentrosymmetric consecutive layers are related by an inversion center. The layers appear to be held together by the alkali ions. The mobility of the M+ ions within the interlayer space in M6(UO2)5(VO4)2O5 and carnotite analog compounds is compared.