Actinide materials, which are the backbone of nuclear energy, are being examined with growing interest due to the pressing need for carbon-free energy production. Prospects for advanced fuels for next-generation reactors demand a solid understanding of the physical properties of actinide materials, including thermodynamics and quantum mechanics. In this issue of MRS Bulletin, six articles give a broad overview of the current state of actinide science, illustrating current advances and emerging areas of interest in this class of materials, and how this fundamental understanding supports nuclear energy.

Microorganisms moderate local chemical conditions and alter forms of metals indirectly or directly to meet their cellular, species, and consortia needs. The diversity of microorganisms and the complexity of biogeochemical systems ensures that bacterially mediated processes yield a wide range of products, which await discovery by material scientists. Few types of materials produced by environmental bacteria have been analyzed by modern chemical and material science methods. Research on actinide biomaterials has focused on the biomineralization of a few chemical forms of uranium, neptunium, and plutonium. The materials produced are molecular complexes, microcrystalline minerals (most commonly oxides and phosphates) within cells and biofilms, and mineral adsorbates. The actinide biomaterials that emerge from this new research area will impact nuclear waste isolation and increase our understanding of environmental and geological metal cycles and may yield new bioremediation methods and industrially useful materials.

The resurgence of nuclear power as a strategy for reducing greenhouse gas (GHG) emissions has, in parallel, revived interest in the environmental impact of actinides. Just as GHG emissions are the main environmental impact of the combustion of fossil fuels, the fate of actinides, consumed and produced by nuclear reactions, determines whether nuclear power is viewed as an environmentally “friendly” source of energy. In this article, we summarize the sources of actinides in the nuclear fuel cycle, how actinides are separated by chemical processing, the development of actinide-bearing materials, and the behavior of actinides in the environment. At each stage, actinides present a unique and complicated behavior because of the 5f electronic configurations.

Actinide solid-state and coordination chemistry has advanced through unexpected results that have further revealed the complex nature of the 5f elements. Nanoscale control of actinide materials is emerging, as shown by the creation of a considerable range of cluster and tubular topologies. Departures from established structural trends for actinyl ions are provided by cation-cation interactions in which an O atom of one actinyl ion is an equatorial ligand of a bipyramid of another actinyl ion. The solid-state structural complexity of actinide materials has been further demonstrated by open framework materials with interesting properties. The U(VI) tetraoxide core has been added to this cation's repertoire of coordination possibilities. The emergence of pentavalent uranium solid-state and coordination chemistry has resulted from the prudent selection of ligands. Finally, analogues of the uranyl ion have challenged our understanding of this normally unreactive functional group.

Recent discoveries of novel electronic states, including relatively high-temperature superconductivity, in the actinides point to exciting prospects for future discoveries at the bottom of the periodic table. A key ingredient in all of the successes discussed here is the role of high-quality synthesis in enabling advances. Results on PuCoGa51, NpPd5Al2, and single crystal uranium are discussed.

In spite of being rare, actinide elements provide the building blocks for many fascinating condensed-matter systems, both from an experimental and theoretical perspective. Experimental observations of actinide materials are difficult because of rarity, toxicity, radioactivity, and even safety and security. Theory, on the other hand, has its own challenges. Complex crystal and electronic structures are often encountered in actinide materials, as well as pronounced electron correlation effects. Consequently, theoretical modeling of actinide materials and their 5f electronic states is very difficult. Here, we review recent theoretical efforts to describe and sometimes predict the behavior of actinide materials and complexes, such as phase stability including density functional theory (DFT), DFT in conjunction with an additional Coulomb repulsion U (DFT+U), and DFT in combination with dynamical mean-field theory (DFT+DMFT).

Advanced spectroscopic techniques provide new and unique tools for unraveling the nature of the electronic structure of actinide materials. Inelastic neutron scattering experiments, which address temporal aspects of lattice and magnetic fluctuations, probe electromagnetic multipole interactions and the coupling between electronic and vibrational degrees of freedom. Nuclear magnetic resonance clearly demonstrates different magnetic ground states at low temperature. Photoemission spectroscopy provides information on the occupied part of the electronic density of states and has been used to investigate the momentum-resolved electronic structure and the topology of the Fermi surface in a variety of actinide compounds. Furthermore, x-ray absorption and electron energy-loss spectroscopy have been used to probe the relativistic nature, occupation number, and degree of localization of 5f electrons across the actinide series. More recently, element- and edge-specific resonant and non-resonant inelastic x-ray scattering experiments have provided the opportunity of measuring elementary electronic excitations with higher resolution than traditional absorption techniques. Here, we will discuss results from these spectroscopic techniques and what they tell us of the electronic and magnetic properties of selected actinide materials.

The method we introduced in 1992 for measuring hardness and elastic modulus by nanoindentation testing has been widely adopted and used in the characterization of mechanical behavior at small scales. Since its original development, the method has undergone numerous refinements and changes brought about by improvements to testing equipment and techniques, as well as advances in our understanding of the mechanics of elastic-plastic contact. In this article, we briefly review the history of the method, comment on its capabilities and limitations, and discuss some of the emerging areas in materials research where it has played, or promises to play, an important role.