Justin B. Felder

Synthetic strategies for new vanadium oxyfluorides containing novel building blocks: structures of V(IV) and V(V) containing Sr4V3O5F13, Pb7V4O8F18, Pb2VO2F5, and Pb2VOF6

Four new vanadium oxyfluorides (VOFs), Sr4V3O5F13 (1), Pb7V4O8F18 (2), Pb2VO2F5 (3), and Pb2VOF6 (4), have been synthesized under mild hydrothermal conditions. The choice of starting reagents, AF2 or A(CH3CO2)2·xH2O (A = Pb, alkaline earth), determined the oxidation states of vanadium in the final products. The reaction of V2O5 with AF2 leads, consistently, to the formation of V(V) compounds, while the use of A(CH3CO2)2·xH2O results in V(IV) containing compounds, suggesting that the acetate species behaves as an effective mild reducing agent. The crystal structures, characterized by single crystal X-ray diffraction, revealed that the compounds exhibit various anionic VOF building blocks, including dimeric and trinuclear units, as well as one-dimensional chains. All compounds contain fluorine atoms that are not bonded to the vanadium atoms, which are located between two-dimensional layers consisting of corner- or edge-shared FA3 or FA4 polyhedra that separate the vanadium containing building blocks. The magnetic susceptibility data for 4 were measured as a function of temperature, yielding an effective magnetic moment of 1.72 μB that confirms the presence of V(IV). UV-vis reflectance and thermal properties were also characterized.

Synthetic Strategies for the Synthesis of Ternary Uranium(IV) and Thorium(IV) Fluorides

A series of new U(IV) and Th(IV) fluorides, Na7U6F31 (1), NaUF5 (2), NaU2F9 (3), KTh2F9 (4), NaTh2F9 (5), (H3O)Th3F13 (6), and (H3O)U3F13 (7), was obtained using hydrothermal and low-temperature flux methods. Mild hydrothermal reactions with uranyl acetate as a precursor yielded 1, 7, and the monoclinic polymorph of NaU2F9, whereas direct reactions between UF4 and NaF led to the formation of 2 and orthorhombic NaU2F9 (3). This highlights an unexpected difference in reaction products when different starting uranium sources are used. All seven compounds were characterized by single-crystal X-ray diffraction, and their structures are compared on the basis of cation topology, revealing a close topological resemblance between fluorides on the basis of the layers observed in NaUF5(H2O). Phase-pure samples of 1, 2, and both polymorphs of NaU2F9 were obtained, and their spectroscopic and magnetic properties were measured. The UV-vis data are dominated by the presence of U4+ cations and agree well with the electronic transitions. Effective magnetic moments of the studied compounds were found to range from 3.08 to 3.59 μB.

Retention of a Paramagnetic Ground State at Low Temperatures in a Family of Structurally Related UIV Phosphates

A new uranium fluoride phosphate, UFPO4, was synthesized via a mild hydrothermal route and characterized optically, thermally, and magnetically. Two thermal transformation products, U2O(PO4)2 and UIVUVIO2(PO4)2, were discovered to be structurally related, and were subsequently synthesized for bulk property measurements. All three materials failed to follow Curie-Weiss behavior at low temperatures, attributed to the nearly ubiquitous singlet ground state of U(IV), transitioning into a Curie-Weiss paramagnetic regime at high temperatures. Neutron diffraction experiments were performed on UFPO4 and UIVUVIO2(PO4)2 in order to characterize this unusual magnetic behavior.

Application of a mild hydrothermal method to the synthesis of mixed transition-metal(II)/uranium(IV) fluorides

Single crystals of five transition metal uranium fluorides were obtained via the use of a mild hydrothermal route. Uranyl acetate was used as both the uranium source and the reducing agent for an in situ reduction of U(VI) to U(IV). The synthesized materials are present as both two- and three-dimensional structures and contain uranium in 9-fold coordination environments. Magnetic susceptibility measurements indicate that the reported materials remain paramagnetic down to 2 K, with no evidence for the existence of long-range magnetic ordering. Thermogravimetric analysis studies of the reported materials are also presented.

Breaking a Paradigm: Observation of Magnetic Order in the Purple U(IV) Phosphite: U(HPO3)2

The hydrothermal crystal growth of a new uranium phosphite, U(HPO3)2, is reported. This material was found to exhibit optical and magnetic properties not commonly observed in U(IV) containing materials, specifically purple coloration and low temperature magnetism. We discuss the synthesis, structure determination, and characterization of the title compound and comment on its optical, thermal, and magnetic properties. The magnetic behavior is consistent with frustrated antiferromagnetism that arises from the hexagonal honeycomb lattice of U(IV) ions. This material gives rare evidence for U(IV) ions participating in magnetic order.

Supercritical synthesis and topological analysis of K5U5O17(OH)

High quality single crystals of the monoclinic phase K5U5O17(OH) were synthesized via a hydrothermal method utilizing the supercritical regime of water. The structure of K5U5O17(OH) is isotypic with the Na phase, Na5U5O17(OH), however crystallizes in a slightly different space group due to size effects. K5U5O17(OH) has the same sheet-anion structure as the Na analog, whose topology is analyzed here for the first time. K5U5O17(OH) shows fluorescence typical for uranyl species.