Expansion of the Rich Structures and Magnetic Properties of Neptunium Selenites: Soft Ferromagnetism in Np(SeO₃)₂

Two new neptunium selenites with different oxidation states of the metal centers, Np^IV(SeO₃)₂ and Np^VIO₂(SeO₃), have been synthesized under mild hydrothermal conditions at 200 °C from the reactions of NpO₂ and SeO₂. Np­(SeO₃)₂ crystallizes as brown prisms (space group P2₁/n, a = 7.0089(5) Å, b = 10.5827(8) Å, c = 7.3316(5) Å, β = 106.953(1)°); whereas NpO₂(SeO₃) crystals are garnet-colored with an acicular habit (space group P2₁/m, a = 4.2501(3) Å, b = 9.2223(7) Å, c = 5.3840(4) Å, β = 90.043(2)°). Single-crystal X-ray diffraction studies reveal that the structure of Np­(SeO₃)₂ features a three-dimensional (3D) framework consisting of edge-sharing NpO₈ units that form chains that are linked via SeO₃ units to create a 3D framework. NpO₂(SeO₃) possesses a lamellar structure in which each layer is composed of NpO₈ hexagonal bipyramids bridged via SeO₃^2– anions. Bond-valence sum calculations and UV-vis-NIR absorption spectra support the assignment of tetravalent and hexavalent states of neptunium in Np­(SeO₃)₂ and NpO₂(SeO₃), respectively. Magnetic susceptibility data for Np­(SeO₃)₂ deviates substantially from typical Curie–Weiss behavior, which can be explained by large temperature-independent paramagnetic (TIP) effects. The Np^IV selenite shows weak ferromagnetic ordering at 3.1(1) K with no detectable hysteresis, suggesting soft ferromagnetic behavior.