Retention of a Paramagnetic Ground State at Low Temperatures in a Family of Structurally Related U^IV 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 U^IVU^VIO₂(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 U^IVU^VIO₂(PO₄)₂ in order to characterize this unusual magnetic behavior

[Co(H₂O)₆]₃[U₂O₄F₇]₂: A Model System for Understanding the Formation of Dimensionally Reduced Materials

A cobalt containing mixed-anion uranyl oxide-fluoride was synthesized using a mild hydrothermal crystal growth method. The uranyl ions are condensed into a one-dimensional chain motif separated by isolated cobalt hexa-aqua centers. The 1-D uranyl material was characterized by single-crystal and powder X-ray diffraction. The optical, thermal, and magnetic properties were studied and are reported herein. Using the reported composition as a model, a system was derived to predict the dimensionality of hypothetical structures to advance our ability to rationally predict solid-state materials

Understanding the Formation of Salt-Inclusion Phases: An Enhanced Flux Growth Method for the Targeted Synthesis of Salt-Inclusion Cesium Halide Uranyl Silicates

Salt-inclusion compounds (SICs) are known for their structural diversity and their potential applications, including luminescence and radioactive waste storage forms. Currently, the majority of salt-inclusion phases are grown serendipitously and the targeted growth of SICs has met with only moderate success. We report an enhanced flux growth method for the targeted growth of SICs. Specifically, the use of (1) metal halide reagents and (2) reactions with small surface area to volume ratios are found to favor the growth of salt-inclusion compounds over pure oxides and thus enable a more targeted synthetic route for their preparation. The Cs–X–U–Si–O (X = F, Cl) pentanary phase space is used as a model system to demonstrate the generality of this enhanced flux method approach. Single crystals of four new salt-inclusion uranyl silicates, [Cs₃F]­[(UO₂)­(Si₄O₁₀)], [Cs₂Cs₅F]­[(UO₂)₂(Si₆O₁₇)], [Cs₉Cs₆Cl]­[(UO₂)₇(Si₆O₁₇)₂(Si₄O₁₂)], and [Cs₂Cs₅F]­[(UO₂)₃(Si₂O₇)₂], were grown using this enhanced flux growth method. A detailed discussion of the factors that favor salt-inclusion phases during synthesis and why specifically uranyl silicates make excellent frameworks for salt-inclusion phases is given

A Family of A‑Site Cation-Deficient Double-Perovskite-Related Iridates: Ln₉Sr₂Ir₄O₂₄ (Ln = La, Pr, Nd, Sm)

The compositions of the general formula Ln_{11–<i>x</i>}Sr_<i>x</i>Ir₄O₂₄ (Ln = La, Pr, Nd, Sm; 1.37 ≥ <i>x</i> ≥ 2) belonging to a family of A-site cation-deficient double-perovskite-related oxide iridates were grown as highly faceted single crystals from a molten strontium chloride flux. Their structures were determined by single-crystal X-ray diffraction. On the basis of the single-crystal results, additional compositions, Ln₉Sr₂Ir₄O₂₄ (Ln = La, Pr, Nd, Sm), were prepared as polycrystalline powders via solid-state reactions and structurally characterized by Rietveld refinement. The compositions Ln₉Sr₂Ir₄O₂₄ (Ln = La, Pr, Nd, Sm) contain Ir­(V) and Ir­(IV) in a 1:3 ratio with an average iridium oxidation state of 4.25. The single-crystal compositions La_9.15Sr_1.85Ir₄O₂₄ and Pr_9.63Sr_1.37Ir₄O₂₄ contain relatively less Ir­(V), with the average iridium oxidation states being 4.21 and 4.09, respectively. The magnetic properties of Ln₉Sr₂Ir₄O₂₄ (Ln = La, Pr, Nd, Sm) were measured, and complex magnetic behavior was observed in all cases at temperatures below 30 K

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

A series of new U­(IV) and Th­(IV) fluorides, Na₇U₆F₃₁ (<b>1</b>), NaUF₅ (<b>2</b>), NaU₂F₉ (<b>3</b>), KTh₂F₉ (<b>4</b>), NaTh₂F₉ (<b>5</b>), (H₃O)­Th₃F₁₃ (<b>6</b>), and (H₃O)­U₃F₁₃ (<b>7</b>), was obtained using hydrothermal and low-temperature flux methods. Mild hydrothermal reactions with uranyl acetate as a precursor yielded <b>1</b>, <b>7</b>, and the monoclinic polymorph of NaU₂F₉, whereas direct reactions between UF₄ and NaF led to the formation of <b>2</b> and orthorhombic NaU₂F₉ (<b>3</b>). 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 <b>1</b>, <b>2</b>, 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 μ_B

Flux versus Hydrothermal Growth: Polymorphism of A₂(UO₂)Si₂O₆ (A = Rb, Cs)

Single crystals of two new uranyl silicates were grown from mixed alkali fluoride/alkali chloride fluxes. Polymorphism is observed in Cs₂(UO₂)­Si₂O₆, where hydrothermal conditions lead to the α polymorph and flux growth favors the β polymorph. Rb₂(UO₂)­Si₂O₆ crystallizes in a monoclinic distortion of the α-Cs₂(UO₂)­Si₂O₆ structure type with space group <i>I</i>2/<i>a</i> and lattice parameters <i>a</i> = 14.9932(5) Å, <i>b</i> = 14.8032(5) Å, <i>c</i> = 16.2377(9) Å, and β = 90.7220(10)°. β-Cs₂(UO₂)­Si₂O₆ constitutes a new polymorph of Cs₂(UO₂)­Si₂O₆ and crystallizes in the monoclinic space group <i>C</i>2 with lattice parameters <i>a</i> = 12.136(2) Å, <i>b</i> = 10.0426(17) Å, <i>c</i> = 7.7896(13) Å, and β = 95.849(4)°. The two structures are compared, the driving force behind the observed polymorphism is discussed, and the luminescence properties are reported for each compound

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

Hydroflux Crystal Growth of Platinum Group Metal Hydroxides: Sr₆NaPd₂(OH)₁₇, Li₂Pt(OH)₆, Na₂Pt(OH)₆, Sr₂Pt(OH)₈, and Ba₂Pt(OH)₈

Crystals of five complex metal hydroxides containing platinum group metals were grown by a novel low-temperature hydroflux technique, a hybrid approach between the aqueous hydrothermal and the molten hydroxide flux techniques. Sr₆NaPd₂(OH)₁₇ (<b>1</b>) crystallizes in orthorhombic space group <i>Pbcn</i> with lattice parameters <i>a</i> = 19.577(4) Å, <i>b</i> = 13.521(3) Å, and <i>c</i> = 6.885(1) Å. This compound has a three-dimensional framework structure with Sr­(OH)_<i>n</i> polyhedra, Na­(OH)₆ octahedra, and Pd­(OH)₄ square planes. Isostructural phases Li₂Pt­(OH)₆ (<b>2</b>) and Na₂Pt­(OH)₆ (<b>3</b>) crystallize in trigonal space group <i>P</i>-3 with lattice parameters of <i>a</i> = 5.3406(8) Å and <i>c</i> = 4.5684(9) Å and <i>a</i> = 5.7984(8) Å and <i>c</i> = 4.6755(9) Å, respectively. Structures of these materials consist of layers of A­(OH)₆ (A = Li (<b>2</b>), Na (<b>3</b>)) and Pt­(OH)₆ octahedra. Sr₂Pt­(OH)₈ (<b>4</b>) crystallizes in monoclinic space group <i>P</i>2₁/<i>c</i> with lattice parameters <i>a</i> = 5.9717(6) Å, <i>b</i> = 10.997(1) Å, <i>c</i> = 6.0158(6) Å, and β = 113.155(2)°, while Ba₂Pt­(OH)₈ (<b>5</b>) crystallizes in orthorhombic space group <i>Pbca</i> with lattice parameters <i>a</i> = 8.574(2) Å, <i>b</i> = 8.673(2) Å, and <i>c</i> = 10.276(2) Å. Both of these compounds have three-dimensional structures composed of Pt­(OH)₆ octahedra surrounded by either Sr­(OH)₈ or Ba­(OH)₉ polyhedra. Decomposition of these materials into condensed metal oxides, which is of importance to possible catalytic applications, was monitored via thermogravimetric analysis. For example, Na₂Pt­(OH)₆ (<b>3</b>) converts cleanly via dehydration into α-Na₂PtO₃

Design and Crystal Growth of Magnetic Double Perovskite Iridates: <i>Ln</i>₂MIrO₆ (<i>Ln</i> = La, Pr, Nd, Sm-Gd; M = Mg, Ni)

A series of monoclinic distorted double perovskites of the general formula <i>Ln</i>₂MIrO₆ (<i>Ln</i> = La, Pr, Nd, Sm–Gd; M = Mg, Ni) were grown as highly faceted single crystals from a potassium hydroxide flux. The structural distortions and the magnetic interactions in A₂BB′O₆ double perovskites can be “designed” via a judicious choice of A, B, and B′ cation sizes and by selecting magnetic or nonmagnetic ions to occupy the A, B, and/or B′ sites. A study of the relationship between the number of magnetic ions, the degree of monoclinic distortion, and the resulting magnetic interactions was conducted. Magnetic susceptibility and field dependent magnetization measurements were performed for all synthesized compounds. It was determined that smaller A-site lanthanide cations cause more pronounced monoclinic distortions, resulting in smaller M–O–Ir (M = Mg, Ni) bond angles that correlate with higher magnetic ordering temperatures. The magnetic susceptibility and field dependent magnetization data were both consistent with canted antiferromagnetism for most titled compositions, indicating a possible trend of increased spin canting, and thus increased ferromagnetic-like interactions, as a function of smaller lanthanide A site cation size

Flux Crystal Growth and Optical Properties of Two Uranium-Containing Silicates: <i>A</i>₂USiO₆ (<i>A</i> = Cs, Rb)

Single crystals of two uranium silicates, Cs₂USiO₆ and Rb₂USiO₆, have been grown from molten fluoride fluxes and structurally characterized by single-crystal X-ray diffraction. Cs₂USiO₆ crystallizes in the body-centered orthorhombic space group, <i>Immm</i>, with <i>a</i> = 8.5812(4) Å, <i>b</i> = 13.0011(6) Å, and <i>c</i> = 13.8811(7) Å. The size of Rb is slightly too small to fit into this structural framework without effecting slight structural changes that result in a 6-fold superstructure. Sharp satellite peaks were observed in the single-crystal X-ray diffraction data, indicating the existing of a superstructure. The crystals were examined by electron diffraction, the results of which suggest that the structure can be thought of as the <i>Immm</i> isotype (<i>a</i> = 8.4916(6) Å, <i>b</i> = 12.6678(9) Å, and <i>c</i> = 13.5077(9) Å) on average, with an approximately 6-fold superstructure along the <i>c</i> axis. The materials were further characterized by UV–vis reflectance spectroscopy