This perspective focuses on the synthesis, characterization, and modeling of three classes of hierarchical materials with potential for sequestering radionuclides: nanoparticles, porous frameworks, and crystalline salt inclusion phases. The scientific impact of hierarchical structures and the development of the underlying crystal chemistry is discussed as laying the groundwork for the design, local structure control, and synthesis of new forms of matter with tailored properties. This requires development of the necessary scientific understanding of such complex structures through integrated synthesis, characterization, and modeling studies that can allow their purposeful creation and properties. The ultimate practical aim is to provide the means to create novel structure types that can simultaneously sequester multiple radionuclides. The result will lead to the creation of safe and efficient, long lasting waste forms for fission products and transuranic elements that are the products of nuclear materials processing waste streams. The generation of the scientific basis for working toward that goal is presented.

a b s t r a c tAn increased focus on magnetic measurements of oriented single crystals, thin films, and magnetically dilute systems has led to a demand for the measurement of weak magnetic moments. This level of sensitivity and precision can be achieved on SQUID magnetometers by decreasing the size of the detection coils. However, the smaller detection coils can amplify two errors in the magnitude of the measured moment, the sample shape and radial offset effects, which were small and typically unaccounted for on previous magnetometers. We report a simple method to determine the radial offset of a sample by taking advantage of the two basic scan modes, DC and lock-in, typically used on magnetometers. This technique allows for the correction of the sample shape and radial offset effects in order to obtain the true moment of a sample. To show the efficacy of this technique, we report the magnetic properties of Ln 2 O 3 (Ln ¼Gd, Dy, Er).

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.

Superatoms are nanometer-sized molecules or particles that form ordered lattices, mimicking their atomic counterparts. Hierarchical assembly of superatoms gives rise to emergent properties in lattices of quantum dots, p-block clusters, and fullerenes. Here, we introduce a family of uranium-oxysulfate cluster anions whose hierarchical assembly in water is controlled by two parameters: acidity and the lanthanide or transition-metal countercation. In acid, larger LnIII (Ln = La–Ho) link hexamer (U6) oxoclusters into body-centered cubic frameworks, while smaller LnIII (Ln = Er–Lu and Y) promote linking of 14 U6 clusters into hollow superclusters (U84 superatoms). U84 assembles into superlattices including cubic-closest packed, body-centered cubic, and interpenetrating networks, bridged by interstitial countercations and U6 clusters. Divalent transition metals (TM = MnII and ZnII) charge-balance and promote the fusion of 10 U6 and 10 U monomers into a wheel-shaped cluster (U70). Dissolution of U70 in organic media reveals (by small-angle X-ray scattering) that differing supramolecular assemblies are accessed, controlled by TMII-linking of U70 clusters. Magnetic measurements of these assemblies reveal Curie–Weiss behavior at high temperatures, without pairing of the 5f2-electrons down to 2 K.

A series of monoclinic distorted double perovskites of the general formula Ln 2MIrO6 (Ln = 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.