Similarly, when more sophisticated experimental tools were made available for studies at high pressure, it became increasingly clear that there is no dramatic change in the electron occupation of the 4f orbital across the volume collapse region 3, 4. However, it was later demonstrated that the energy difference between these two metal valence states in cerium is large 2, of the order of 2 eV, completely invalidating this conventional valence change picture for the α-γ transition. This promotion model provides a satisfactory explanation of the drastic volume collapse, since four valence electrons bind considerably stronger than just three. The idea was that the localized 4f electron in trivalent cerium becomes promoted to the conduction band. An early explanation for its most remarkable α-γ volume collapse was that this transition originates from a valence change of cerium 1 from a trivalent rare-earth like metal with one localized 4f electron, to a tetravalent metallic state, comparable to titanium or zirconium. This agreement makes it possible to suggest that an appropriate crossroad position for cerium in The Periodic Table.Ĭerium is one of the most fascinating elements 1 in the periodic table and continues to attract a lot of attention. A study of this fundamental characteristic of the cerium volume collapse is made in present paper and we show that the localized ⇌ delocalized 4f electron picture provides an adequate description of this unique behavior. Therefore the isostructural aspect of the α-γ transition has now to be seriously addressed in the theoretical modeling, something which has been very much neglected. Most recently, detailed experiments have been able to remove this worrisome uncertainty. However, over the years it has been repeatedly questioned whether the cerium collapse really is isostructural. Two main models of the electronic aspect of this transformation have been proposed one where the 4f electron undergoes a change from being localized into an itinerant metallic state and one where the focus is on the interaction between the 4f electron and the conduction electrons, often referred to as the Kondo volume collapse model. In particular a lot of attention has been directed towards its high pressure behavior, where an isostructural volume collapse (γ phase → α phase) has been observed. The properties of the cerium metal have intrigued physicists and chemists for many decades.