Mild hydrothermal crystal growth of new uranium(IV) fluorides, Na3.13Mg1.43U6F30 and Na2.50Mn1.75U6F30: Structures, optical and magnetic properties

• Published in: Journal of solid state chemistry 2016-04, Vol.236 (C), p.83-88
• Main Authors: Yeon, Jeongho, Smith, Mark D., Tapp, Joshua, Möller, Angela, zur Loye, Hans-Conrad
• Format: Article
• Language: English
• Subjects: 3-D framework; Crystal growth; In situ reduction; Magnetism; Uranium(IV)
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/j.jssc.2015.04.036

Two new uranium(IV) fluorides, Na3.13Mg1.43U6F30 (1) and Na2.50Mn1.75U6F30 (2), were synthesized through an in situ mild hydrothermal route, and were structurally characterized by single crystal X-ray diffraction. The compounds exhibit complex crystal structures composed of corner- or edge-shared UF9 and MF6 (M=Mg, Mn) polyhedra, forming hexagonal channels in the three-dimensional framework, in which ordered or disordered divalent metal and sodium atoms reside. The large hexagonal voids contain the nearly regular M(II)F6 octahedra and sodium ions, whereas the small hexagonal cavities include M(II) and sodium ions on a mixed-occupied site. Magnetic susceptibility measurements yielded effective magnetic moments of 8.36 and 11.6µB for 1 and 2, respectively, confirming the presence and oxidation states of U(IV) and Mn(II). The large negative Weiss constants indicate the spin gap between a triplet and a singlet state in the U(IV). Magnetization data as a function of applied fields revealed that 2 exhibits paramagnetic behavior due to the nonmagnetic singlet ground state of U(IV) at low temperature. UV–vis diffuse reflectance and X-ray photoelectron spectroscopy data were also analyzed. Two new quaternary U(IV) fluorides, Na3.13Mg1.43U6F30 and Na2.50Mn1.75U6F30, were crystallized via an in situ reduction step of U(VI) to U(IV) under mild hydrothermal conditions. The compounds show complex crystal structures based on the 3-D building block of U6F30. Magnetic property measurements revealed that the U(IV) exhibits a nonmagnetic singlet ground state at low temperature with a spin gap. [Display omitted] •Na3.13Mg1.43U6F30 and Na2.50Mn1.75U6F30 have been synthesized and characterized.•The U(IV) fluorides exhibit complex three-dimensional crystal structures.•The optical properties were investigated.•Magnetic susceptibility data were collected.