Boehmite (γ-AlOOH) and gibbsite (α-Al(OH) 3 ) are important archetype (oxy)hydroxides of aluminum in nature that also play diverse roles across a plethora of industrial applications. Developing the ability to understand and predict the properties and characteristics of these materials, on the basis of their natural growth or synthesis pathways, is an important 1 fundamental science enterprise with wide ranging impacts. The present study describes bulk and surface characteristics of these novel materials in comprehensive detail, using a collectively-sophisticated set of experimental capabilities, including a range of conventional laboratory solids analyses and national user facility analyses such as synchrotron X-ray absorption and scattering spectroscopies, as well as small angle neutron scattering. Their thermal stability is investigated using in situ temperature-dependent Raman spectroscopy. These pure and effectively defect-free materials are ideal for synthesis of advanced alumina products.

Combination of uranium, peroxide, and mono- (Na, K) or divalent (Mg, Ca, Sr) cations under alkaline aqueous conditions results in the rapid formation of anionic uranyl triperoxide monomers (UTs), (UO(O)), exhibiting unique Raman signatures. Electronic structure calculations were decisive for the interpretation of the spectra and assignment of unexpected signals associated with vibrations of the uranyl and peroxide ions. Assignments were verified by O isotopic labeling of the uranyl ions supporting the computational-based interpretation of the experimentally observed peaks and the assignment of a novel asymmetric vibration of the peroxide ligands,(O).

Solid UO dissolution and uranium speciation in aqueous solutions that promote formation of uranyl peroxide macroanions was examined, with a focus on the role of alkali metals. UO powders were dissolved in solutions containing XOH (X = Li, Na, K) and 30% HO. Inductively coupled plasma optical emission spectrometry (ICP-OES) measurements of solutions revealed linear trends of uranium versus alkali concentration in solutions resulting from oxidative dissolution of UO, with X:U molar ratios of 1.0, showing that alkali availability determines the U concentrations in solution. The maximum U concentration in solution was 4.20 × 10 parts per million (ppm), which is comparable to concentrations attained by dissolving UO in boiling nitric acid, and was achieved by lithium hydroxide promoted dissolution. Raman spectroscopy and electrospray ionization mass spectrometry (ESI-MS) of solutions indicate that dissolution is accompanied by the formation of various uranyl peroxide cluster species, the identity of which is alkali concentration dependent, revealing remarkably complex speciation at high concentrations of base.

Uranium concentrations as high as 2.94 × 10 parts per million (1.82 mol of U/1 kg of HO) occur in water containing nanoscale uranyl cage clusters. The anionic cage clusters, with diameters of 1.5-2.5 nm, are charge-balanced by encapsulated cations, as well as cations within their electrical double layer in solution. The concentration of uranium in these systems is impacted by the countercations (K, Li, Na), and molecular dynamics simulations have predicted their distributions in selected cases. Formation of uranyl cages prevents hydrolysis reactions that would result in formation of insoluble uranyl solids under alkaline conditions, and these spherical clusters reach concentrations that require close packing in solution.

The first neutron diffraction study of a single crystal containing uranyl peroxide nanoclusters is reported for pyrophosphate-functionalized Na44K6[(UO2)24(O2)24(P2O7)12][IO3]2·140H2O (1). Relative to earlier X-ray studies, neutron diffraction provides superior information concerning the positions of H atoms and lighter counterions. Hydrogen positions have been assigned and reveal an extensive network of H-bonds; notably, most O atoms present in the anionic cluster accept H-bonds from surrounding H2O molecules, and none of the surface-bound O atoms are protonated. The D4h symmetry of the cage is consistent with the presence of six encapsulated K cations, which appear to stabilize the lower symmetry variant of this cluster. (31)P NMR measurements demonstrate retention of this symmetry in solution, while in situ (31)P NMR studies suggest an acid-catalyzed mechanism for the assembly of 1 across a wide range of pH values.