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