科研之友
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摘要
To obtain a proper understanding of the 5f elements, the actinides, it is useful to compare their behavior with the 4f transition elements, the lanthanides. It is especially rewarding to capitalize on the remarkable similarity between the solid-state properties of compressed Ce and the actinide metals. The intensively studied alpha-gamma transition in Ce is considered to be a Mott transition, namely, the 4f electron changes its behavior from being localized to become delocalized (itinerant/metallic). This change also means that the 4f electron transforms from a non-bonding to a bonding configuration which, in turn, gives rise to a volume collapse. This collapse is isostructural in character, which contributes to the immense interest in this phase transition. An analogous and remarkable change in bonding (cohesive) properties is also found within the actinide series, where the sudden volume increase from Pu to Am (50%) can be viewed as a Mott transition within the 5f shell as a function of atomic number Z. The elements on the metallic side of the 5f Mott transition, i.e. the earlier actinides (Pa-Pu), show low symmetry structures at ambient conditions, while the heavier elements (from Am and beyond) adopt structures typical for the lighter trivalent lanthanide elements with localized 4f electrons. An important consequence of the localized and trivalent behavior in Am is a non-magnetic 5f(6) (J = L + S = 0) configuration for the f electrons. This led to the prediction of superconductivity in americium and subsequently to its experimental verification.
