The radioactive chemical elements that follow actinium in the Periodic Table form the actinide series. These elements are the backbone of nuclear fission technologies for electricity supply, with important applications in other strategic fields, from water management to space exploration and human health.
As the atomic number increases in the series from 90 to 103, added electrons enter the highly anisotropic 5f shell. This shell is characterized by a radial wavefunction extending relatively far from the nucleus, so that 5f electrons can either form band states or retain a localized behaviour. With 5f-electrons poised at the edge between non-bonding and bonding configurations, actinide elements are prone to lattice instabilities and plutonium, which goes through six allotropic forms in heating to its melting point, is a fascinating example of how complex their structural behaviour can be.
Equally intricate are the electronic properties of actinides. From one side, the intra-atomic electron correlation is strong, for the large electric charge of the nucleus pulls the electrons close to each other, favoring the formation of a magnetic moment. On the other hand, quantum fluctuations of electronic and magnetic degrees of freedom can be so strong that the magnetism melts, promoting emergent properties and new classes of materials behaviour around the points of instability. Other sources of complexity are the hybridization between 5f and conduction-electron states, which may give rise to a variety of different phenomena, from unconventional superconductivity to topologically protected states, and the interplay of spin and unquenched orbital degrees of freedom, which can result in exotic phase transitions driven by hidden order parameters.
Besides fundamental science interests, achieving a deep understanding of actinides is vital to ensuring a safe deployment of civil nuclear technologies. However, only a few facilities are available worldwide where actinide materials can be safely investigated. Among these, a prominent position is occupied by the laboratories operated by the Joint Research Centre (JRC) of the European Commission. In its establishment located near the charming German town of Karlsruhe (one of the sites of the JRC’s Directorate for Nuclear Safety and Security), the JRC operates state-of-the-art instruments for measuring spectroscopic, thermodynamic, magnetic, and electrical transport properties of radioactive materials, together with specialised facilities for the preparation of high-quality samples, from single crystals to organometallic complexes and epitaxial thin films. Available techniques include, among others, magic-angle-spinning nuclear magnetic resonance, photoemission and Mössbauer spectroscopy, SQUID magnetometry, Seebeck-, and Hall-effect probes. By exploring materials properties in a wide range of temperature, pressure, and magnetic field, studies performed at the JRC are helping in bringing the actinide knowledge to a “material-by-design” level.
A considerable effort is also dedicated to the development of radiopharmaceuticals for targeted cancer therapies based on alpha particles emitters, and to the development of radioisotopes power systems for the European space exploration programme.
Safety precautions at Universities in Europe have almost excluded the possibility of working beyond uranium in the periodic table; with JRC’s unique facilities being opened to the academic community on the basis of peer- reviewed proposals, Europe’s researchers can still be at the cutting edge of this vital field.