Synthesis, crystal structure, and comparison of two new uranyl vanadate layered compounds: M-6(UO2)(5)(VO4)(2)O-5 with M = Na, K

Two new alkali uranyl vanadates, M-6(UO2)(5)(VO4)(2)O-5 with M = Na, K, hav e been synthesized by solid state reactions and their structures determined from single-crystal X-ray diffraction data. They crystallize in the monocl inic system with space group P2(1)/c. Unit cell b, c, and beta parameters a re almost identical in both compounds while the a parameter in the Na compo und is doubled: Na-a = 12.584(1) Angstrom, b = 24.360(2) Angstrom, c = 7.05 0(1) Angstrom, beta = 100.61(1)degrees, V = 2124.2 Angstrom (3), Z = 4, D-m es = 5.60(2) g/cm(3), D-cal = 5.63 g/cm(3); K-a = 6.856(1) Angstrom, b = 24 .797(3) Angstrom, c = 7.135(1) Angstrom, beta = 98.79(8)degrees, V = 1198.8 Angstrom (3), Z = 2, D-mes = 5.25(2) g/cm(3), D-cal = 5.25 g/cm(3). A full -matrix least-squares refinement yielded, respectively, R = 0.039, R-w = 0. 043 and R = 0.037; R-w = 0.046 for 3293 and 2096 independent reflections wi th I > 3 sigma collected on a Philips PW 1100 diffractometer (MoK alpha rad iation). Both structures are characterized by [(UO2)(5)(VO4)(2)O(5)x](n)(6n -) corrugated layers parallel to the (100) plane. The layers are built up f rom VO4 tetrahedra, UO7 pentagonal bipyramids, and UO6 distorted octahedra. The UO7 pentagonal bipyramids are associated by sharing opposite equatoria l edges to form zig-zag infinite chains (UO5)(n) parallel to the c axis. Th ese chains are linked together on one side by VO4 tetrahedra and on other s ide by UO6 octahedra and UO7 corner sharing. Both structures differ simply by the symmetry of the layers. The unit cell contains one centrosymmetric l ayer in the K compound, whereas in the two-layer unit cell of the Na compou nd two noncentrosymmetric consecutive layers are related by an inversion ce nter. The layers appear to be held together by the alkali ions. The mobilit y of the M+ ions within the interlayer space in M-6(UO2)(5)(VO4)(2)O-5 and carnotite analog compounds is compared. (C) 2000 Academic Press.