Crystal structure and spectral characteristics of double sodium neptunium(IV) sulfate

Single crystals of Na10Np2(SO4)(9).4H(2)O were grown and the structure of t his compound was studied (rhombic unit cell, a = 27.259(5), b = 6.897(1), c = 17.721(4) Angstrom, space group Pbcn, Z = 4; R-1 = 0.0943 from 4564 refl ections, wR(2) = 0.1723 from 7750 independent reflections). The crystal str ucture of double Na Np sulfate consists of doubled anionic chains [Np-2(SO4 )(9)(H2O)(2)](n)(10n-) directed along [010], Na+ cations, and molecules of coordinated water. The central Np(IV) atom has coordination number (CN) 9. It is surrounded by eight oxygen atoms belonging to five sulfate ions and o ne water molecule. The Np4+ polyhedron cannot be described as a regular pol yhedron typical for CN 9. The length of the Np-O bonds varies from 2.267 to 2.488 Angstrom, (average 2.400 Angstrom). Four of the five independent SO4 groups are bidentate (in this case, two groups are bridging, combining Np atoms into infinite doubled chains), and one SO4 group is monodentate. Five crystallographically independent Na+ cations are arranged between the anio nic chains and combine them to three-dimensional skeleton. One independent water molecule enters into the Np coordination sphere, and the other one is the common vertex for two Na polyhedra. The electronic absorption spectrum of Na10NP2(SO4)(9). 4H(2)O registered in an NaCl matrix showed that the lo ng-wave characteristic band reflecting coordination surrounding of the cent ral atom is split into two components (953.0 and 973.0 nm) with different i ntensities.