Focus Area 1 Development of Novel Nuclear Waste Forms
Focus Area 1 Lead by Susan Latturner
The Latturner group has extensive experience in the synthesis of boride and carbide phases from Ln/T-flux mixtures. The binary phase diagrams of early lanthanides (Ln = La, Ce, Pr, Nd) and late first row transition metals (T = Fe, Co, Ni, Cu) all contain eutectic points--for instance, the 75 mol% La / 25 mol% Co eutectic, which melts at 532°C. These low-melting mixtures are excellent solvents for carbon and many other elements. Flux reactions in La/Ni melts have produced complex borides, carbides, and borocarbides including LaRu2Al2B, La21Fe8Sn7C12, and La33Fe14B25C34.
We are exploring reactions of B, C, and both B and C in other Ln/T eutectics to explore how the identity of the lanthanide affects the product formed. Reactions in Ce/Co eutectic have yielded a new borocarbide, Ce10Co2.75B11.5C10, which features borocarbide chains linked to cobalt squares; we are currently investigating the magnetic properties of this phase.
Figure 1: Structure of Ce10Co2.75B11.5C10 viewed down the a-axis. Large grey spheres and small black spheres represent cerium and carbon respectively. Cobalt and boron sites are shown in blue and green
Similar to the Ln/T systems, the binary phase diagrams of uranium with late first row transition metals also feature eutectics, such as the 66 mol% U / 34 mol% Fe mixture that melts at around 725 °C. Reactions such as U/T/C and U/T/B (T = Mn, Fe, Co, Ni) will be explored to synthesize multinary uranium borides, carbides, and borocarbides. The structures, electronic properties, and magnetic behavior of the product phases will be characterized, to compare them to products of corresponding Ln/T/(B or C) reactions. The variable oxidation state of uranium may lead to unique structures and unusual electronic behavior. Additional reactants will be added (for instance, reactions of U/Fe/Sn/C) to target more complex intermetallic carbides and borides.
Transition metal pnictides and chalcogenides have received intense scrutiny during the past decade. In 2006, LaFePO was the first example of the now extensive family of Fe-based superconductors. This set of materials is only the second group of high-temperature superconductors and their discovery has opened a new period of research in crystalline materials. Of particular importance is that while these materials span a wide breadth of chemical combinations, they share a fundamental structural building block based on the tetragonal layered BaAl4 structure. Also notable is that electronic correlations associated with the Fe d-electrons play a crucial role in their behavior: quantum criticality associated with the suppression of iron magnetism to zero temperature is closely associated with the appearance of their superconductivity and anomalous metallic behavior. Motivated by successes studying Fe-based pnictides and chalcogenides, it is appealing to consider what might happen if f-electrons were to be introduced into such an environment. It is already well known that on their own, f-electron intermetallics exhibit a rich variety of correlated electron physics ranging from unconventional superconductivity in PuCoGa5, URu2Si2, and UBe13 to Mott insulator behavior in UO2. These phenomena are governed by strong electronic correlations and the interplay between the numerous degrees of freedom and are frequently associated with the dual nature of f-electrons (localized vs. itinerant). It is natural to expect that combining these properties with those of non-5f Fe-based pnictides and chalcogenides will produce remarkable behaviors. Earlier work suggests that there are structural prototypes that support both actinide elements and iron within the BaAl4 structural framework. Two previously reported examples are UFe2As2 (space group I4/mmm) and UFeAs2 (space group P4/nmm). For both of these materials, synthesis was accomplished by direct reaction of the elements and phase pure material was not obtained. We are exploring reactions of U/Fe binary fluxes with As to identify conditions under which single crystal specimens can be formed.
We will also produce non-phase pure polycrystal specimens, from which we will extract single phase crystallites using the focused ion beam milling technique. Following production of these materials, we will widen our study to include transuranic-containing examples.
There are two anions of particular concern inherent to the Cold War nuclear weapons complex legacy and future advanced nuclear fuel cycles; these are chromate CrO42– and peryechnitate TcO4–. The former is toxic from a chemical standpoint, and the latter is radioactive. Both are readily transported in the environment, and both are problematic during the vitrification of nuclear waste. Chromate forms spinels within the glass, weakening the integrity of the waste form, and pertechnetate easily leaches from the glass. Clearly, we need anion exchange materials that can remove these species from solution.
Materials with extended structures are typically based on an anionic network where the charge is balanced by cations that fill the space between the anionic portions of the structure. This general description applies to a vast array of functional materials. There is, however, a rare alternative to this concept, and that is a solid with a cationic extended structure, whose charge is balanced by unbound anions. Until recently, materials of this kind were largely represented by the hydrotalcite clays. These clays, which occur with many different metal ions, possess metal hydroxide slabs with interlayer anions that can be easily exchanged, making them extremely important for a variety of environmental applications.
We have recently undertaken the study of the preparation, structure elucidation, and physicochemical property measurements of actinide borates, which is motivated in part by a desire to understand the crystallized portions of vitrified nuclear waste. During the course of these studies a highly unusual thorium borate was discovered, [ThB5O6(OH)6][BO(OH)2] (SelectTech). The structure of SelectTech is remarkable in all regards, adopting a cubic framework composed of twelve-coordinate Th4+ surrounded by BO3 and BO4 anions. The BO4 anions chelate the thorium centers, and the BO3 groups occupy single vertices. The thorium atoms resides on a threefold inversion site yielding an almost regular icosahedron. This coordination number is known from classical anions such as [Th(NO3)6]2– , and is accomplished by combining the large size of the Th4+ cation with the small size and chelating nature of the borate anions. The borate anions are polymerized and form B10O24 clusters with threefold symmetry that bridge between the thorium centers, and the hydroxide bridge between borate groups can be inferred from bond distances and bond-valence considerations. The key feature of SelectTech are the channels that extend along . A view of the structure of this material is shown in Figure 1.
Figure 1 : Structure of anionexchange material, [ThB5O6(OH)6][BO(OH)2] (SelectTech), that selectively traps TcO4‒ from the lowactivity waste stream of the melter-recycler in vitrification process
X-ray diffraction studies reveal the presence of a highly disordered entity within the channels. Anion exchange experiments were conducted with a variety of common anions, beginning with halides. These studies, which combined ICP-MS, EDS, and single crystal and powder X-ray diffraction, revealed that not only does anion exchange take place, but that the structure remains intact throughout the exchange. More impressive is that fact that single crystals retain their integrity throughout the exchange, although with these small anions, disorder in the channels remains a crystallographic problem. ICP-MS data indicate extremely minor dissolution of the SelectTech during anion exchange resulting in ppb levels of thorium in solution. The critical anion exchange experiments are replacing the extraframework borate anions with TcO4– and CrO42–. Owing to the intense nature of the charge transfer bands of pertechnetate, we had to use relatively dilute solutions to follow its removal from solution using UV-vis spectroscopy. These studies from as-synthesized intact crystals of SelectTech show rapid uptake of TcO4– from solution with 97% being removed. There is still much work to be done with SelectTech. The as-synthesized material with borate in the channels is obviously not the most ideal form for exchange with pertechnetate or chromate. Pre-exchanged materials, such as SelectTech-Cl with chloride in the channels will likely show more rapid and complete exchange.
We need to explore the full range of exchanged materials, and optimize these for pertechnetate removal. The replacement of thorium with another tetravalent metal will not be trivial, and will require considerable synthetic efforts.