Atsushi Ikeda-Ohno

The solid-solution aqueous-solution (SSAS) system often involves dissolution/precipitation and redox reactions simultaneously. Comprehensive understanding of the SSAS system requires mechanistic insights into the dissolution mechanism of the solid solution based on reliable characterisation of the solid phases. Given this background, this study investigates the dissolution behaviour of zirconium–cerium oxide solid solution ((Zr,Ce)O₂/(Ce,Zr)O₂), which contains the redox-active metal Ce (Ce(III/IV)) and is of particular importance in the nuclear industry. The solid phases of the solid solution were comprehensively characterised by powder X-ray diffraction (PXRD) with Rietveld analysis and X-ray absorption spectroscopy (XAS) with factor analysis, indicating that the solid solution was primarily composed of tetragonal-(Zr,Ce)O₂ and cubic-(Ce,Zr)O₂. Water immersion of the solid solution leads to the dissolution of the solid phase at the solid–liquid interface. The addition of a reductant to the system reduces Ce(IV) in the solid solution to -(III) at the surface, promoting the dissolution of Ce from the solid-solution phase. The release of Ce from the solid solution also enriches the Zr content in the remaining solid-solution phase at the surface, which makes it more insoluble. This results in the formation of a protective layer at the solid surface, retarding further dissolution of the solid-solution phase.

Dissipative structures often appear as an unstable counterpart of ordered structures owing to fluctuations that do not form a homogeneous phase. Even a multiphase mixture may simultaneously undergo one chemical reaction near equilibrium and another one that is far from equilibrium. Here, we observed in real time crystal seed formation and simultaneous nanocrystal aggregation proceeding from Ce^IV complexes to CeO₂ nanoparticles in an acidic aqueous solution, and investigated the resultant hierarchical nanoarchitecture. The formed particles exhibited two very different size ranges, resulting in further pattern formation with opalescence. The hierarchically assembled structures in solutions were CeO₂ colloids, viz. primary core clusters (1–3 nm) of crystalline ceria and secondary clusters (20–30 nm) assembled through surface ions. Such self-assembly is widespread in multi-component complex fluids, paradoxically moderating hierarchical reactions. Stability and instability are not only critical but also complementary for co-optimisation around the nearby free energy landscape prior to bifurcation.

Despite a wide variety of its practical applications, handiness, and cost-effectiveness, the advance of lamp-based heating device is obstructed by one technical difficulty in measuring the temperature on a heated material. This difficulty originates in the combination of polychromatic light source and a radiation thermometer that determines temperature from radiation (i.e. light). A new system developed in this study overcomes this intrinsic difficulty by measuring exclusively the radiation from the heated material, allowing us to perform the direct and in-situ measurement of temperature in a light-based heating device (an arc image furnace). Test measurements demonstrated the reliability of temperature measurement using the developed system as well as its promising potential for the determination of emissivity at high temperature particularly in the infrared region.

The structure of a new type of polyoxo cluster complex that contains thirteen uranium atoms, {U₁₃}, is reported. The complex crystallized from methanol containing tetravalent uranium (U^IV) with a basic organic ligand, and was characterized as dichloridooctacosa-μ₂-methanolato-octakis(methanolato)octa-μ₄-oxido-tridecauranium, [U₁₃(CH₃O)_35.7Cl_2.3O₈] or [U₁₃(μ₄-O_oxo)₈Cl _x (MeO)_38–x ] (x = 2.3, MeO = methoxide) (I), by single-crystal X-ray diffraction. The characterized {U₁₃} polyoxo cluster complex (I) possesses a single cubic uranium polyhedron at the centre of the cluster core. To the best of our knowledge, this is the very first example of a polyoxo actinide complex that bears a single cubic polyhedron in its structure. The cubic polyhedron in I is well comparable in shape with those in bulk UO₂. The U—O bonds in the cubic polyhedron of I are, however, significantly shorter than those not only in bulk UO₂ but also in another analogue in the {U₃₈} cluster. This shortening of U—O bonds, together with BVS calculations and the overall negative charge (2−) of I, suggests that the central uranium atom in I, which forms the single cubic coordination polyhedron, is presumably oxidized to the pentavalent state (U^V) from the original tetravalent state (U^IV). Complex I is, hence, the first example of a polyoxo cluster possessing a single cubic coordination polyhedron of U^V.

Tetradentate N₂O₂-type Schiff base complexes with tetravalent 4f- and 5f-block metals, [M(salpn)₂] (H₂salpn = N,N′-disalicylidene-1,3-diaminopropane; M = Ce, Th, U, Np, and Pu), were characterised both in the solid state and in solution.

The molecular characterization based on multi‐technique approach has led to major highlights on revealing the coordination of americium with citric acid. The structure of the different complexes at pH 1 and 3 are described. These characterizations are made possible by the comparison of the americium‐citric acid system with the americium‐tricarballylic acid (one analogue of the citric acid without the alpha‐hydroxo group). The structural analyses (Vis spectrophotometry, NMR, EXAFS, TRLFS and capillary electrophoresis) were carried out after the establishment of the speciation distribution diagrams, allowing to take into account the species repartition for structural analysis data treatment. For the 1:1 complex the hydroxo group is counter intuitively deprotonated and coordinated to the Am(III) at pH 1 as well as two carboxylate functions, whereas at pH 3 the hydroxo is not coordinated and stays protonated allowing the three carboxylate functions to coordinate the metallic cation. Therefore, the hydroxo group affects the Am complexation differently depending on the pHs: by inductive effect at pH 3, and by direct coordination at pH 1.