Three New Sodium Neptunyl(V) Selenate Hydrates: Structures, Raman Spectroscopy, and Magnetism

Green crystals of Na­(NpO2)­(SeO4)­(H2O) (1), Na3(NpO2)­(SeO4)2(H2O) (2),
and Na3(NpO2)­(SeO4)2(H2O)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(NpO2)­(SO4)­(H2O) (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 H2O 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 [(NpO2)­(SeO4)2(H2O)]3– chains connected
by Na+ cations. Each NpO2+ cation
is coordinated by four monodentate SeO42– anions and one H2O molecule to form a pentagonal bipyramid.
The structure of 3 is constructed by one-dimensional
[(NpO2)­(SeO4)2]3– chains separated by Na+ cations and H2O molecules.
These chains have two configurations resulting in two disordered orientations
of the Se(2)­O42– tetrahedra. Each NpO2+ cation is coordinated by one bidentate Se(1)­O42– and three monodentate Se(2)­O42– 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) μB per Np. Well above
the ordering temperature, the susceptibility follows Curie–Weiss
behavior, with an average effective moment of 3.65(10) μB per Np and a Weiss constant of 14(1) K. Correlations between
lattice dimensionality and magnetic behavior are discussed.