Three new sodium neptunyl(V) selenate hydrates: structures, Raman spectroscopy, and magnetism

Green crystals of Na(NpO(2))(SeO(4))(H(2)O) (1), Na(3)(NpO(2))(SeO(4))(2)(H(2)O) (2), and Na(3)(NpO(2))(SeO(4))(2)(H(2)O)(2) (3) have been prepared by a hydrothermal method for 1 or evaporation from aqueous solutions for 2 and 3. The structures of these compounds have been characterized by single-crystal X-ray diffraction. Compound 1 is isostructural with Na(NpO(2))(SO(4))(H(2)O) (4). The structure of 1 consists of ribbons of neptunyl(V) pentagonal bipyramids, which are decorated and further connected by selenate tetrahedra to form a three-dimensional framework. The resulting open channels are filled by Na(+) cations and H(2)O molecules. Within the ribbon, each neptunyl polyhedron shares corners with each other solely through cation-cation interactions (CCIs). The structure of 2 adopts one-dimensional [(NpO(2))(SeO(4))(2)(H(2)O)](3-) chains connected by Na(+) cations. Each NpO(2)(+) cation is coordinated by four monodentate SeO(4)(2-) anions and one H(2)O molecule to form a pentagonal bipyramid. The structure of 3 is constructed by one-dimensional [(NpO(2))(SeO(4))(2)](3-) chains separated by Na(+) cations and H(2)O molecules. These chains have two configurations resulting in two disordered orientations of the Se(2)O(4)(2-) tetrahedra. Each NpO(2)(+) cation is coordinated by one bidentate Se(1)O(4)(2-) and three monodentate Se(2)O(4)(2-) anions to form a pentagonal bipyramid. Raman spectra of 1, 2, and 4 were collected on powder samples. For 1 and 4, the neptunyl symmetric stretch modes (670, 676, 730, and 739 cm(-1)) shift significantly toward lower frequencies compared to that in 2 (773 cm(-1)), and there are several asymmetric neptunyl stretch bands in the region of 760-820 cm(-1). Magnetic measurements obtained from crushed crystals of 1 are consistent with a ferromagnetic ordering of the neptunyl(V) spins at 6.5(2) K, with an average low temperature saturation moment of 2.2(1) μ(B) per Np. Well above the ordering temperature, the susceptibility follows Curie-Weiss behavior, with an average effective moment of 3.65(10) μ(B) per Np and a Weiss constant of 14(1) K. Correlations between lattice dimensionality and magnetic behavior are discussed.