Humboldt University, Physics Department, Berlin, Germany
Department of Physics and Astronomy, Uppsala University, Box 516, S-751 20 Uppsala, Sweden
Earlier address: Applied Materials Physics, Dep. of Materials Science and Engineering Royal Institute of Technology, SE — 100 44 Stockholm, Sweden
In March this year it will be 150 years ago when the Russian chemist Dmitri Mendeleev innovated the Periodic Table for the Elements (1869). (In 1905 Mendeleev became a member of The Royal Swedish Academy of Sciences). Shortly after Mendeleev´s achievement the German chemist Lothar Meyer published his version of the Periodic Table. This 150 year Periodic Table anniversary will attract a lot of attention world-wide.
The next major progress in the development of the Periodic Table was made in the 1940ies, when Glenn Seaborg, working within the Manhattan Project, failed to observe oxidation states above 4 for the element curium. This failure prompted him to formulate the “actinide hypothesis”. Instead of a 6d transition series, the elements beyond actinium (Ac) form a block of 5f elements, directly corresponding to the 4f elements, the rare earths or lanthanides. Thus, at the heavy end of the Elements one finds a second f transition series, the actinides. Therefore, two blocks of f Elements, the 4f and 5f transition series, became established.
In the condensed (metallic) phase the 5f elements, actinides, show many properties which have direct correspondence to the 4f transition metals, the lanthanides. For example the solid state properties of americium and the heavier actinide elements are such that they appear to be almost twins with the lanthanide metals. The reason for this is that for these elements the 5f electrons form localized moments, just like the 4f electrons do for the lanthanides.
However, the behaviour of the lighter actinides, Th-Pu, appear at first to be unique in the Periodic Table, since here the 5f electrons are delocalized — metallic — and actively contribute to the atomic bonding in the condensed phase. Interestingly, later development has demonstrated a remarkable similarity between the solid state properties of compressed Ce and the earlier actinide metals. Here the volume collapse accompanying the pressure induced isostructural γ — α transition in Ce is considered as a Mott transition, namely, from localized to delocalized (metallic) 4f states. An analogous volume collapse behavior can also be identified for the actinide series, where the sudden spectacular volume increase between Pu and Am can be viewed upon as a Mott transition within the 5f shell as a function of the atomic number Z.
On the itinerant side of the Mott transition, the earlier actinides (Pa—Pu) show low symmetry structures at ambient conditions; while across the border, the heavier elements (Am—Cf) assume the dhcp structure, an atomic arrangement typical for the trivalent lanthanide elements with localized 4f magnetic moments.
The strange and unexpected appearance of the δ -phase (fcc crystal structure) in the phase diagram of Pu is another consequence of the borderline behavior of the 5f electrons between a localized or an itinerant appearance.