lanthanide and actinide elements ... The crystal structure behaviour of ... For the earlier actinide metals electronic structure calculations â where the 5f orbitals.
Hyperfine Interactions 128 (2000) 41–66
41
Structural and electronic relationships between the lanthanide and actinide elements B¨orje Johansson Condensed Matter Theory Group, Department of Physics, Uppsala University, Box 530, S-751 21 Uppsala, Sweden
The similarity and difference between the solid state properties of the 4f and 5f transition metals are pointed out. The heavier 5f elements show properties which have direct correspondence to the early 4f transition metals, suggesting a localized behaviour of the 5f electrons for those metals. On the other hand, the fact that Pu metal has a 30% lower volume than its neighbour heavier element, Am, suggests a tremendous difference in the properties of the 5f electrons for this element relative to the heavier actinides. This change in behaviour between Pu and Am can be viewed as a Mott transition within the 5f shell as a function of the atomic number Z. On the metallic 5f side of the Mott transition (i.e., early actinides), the elements show most unusual crystal structures, the common feature being their low symmetry. An analogous behaviour for the lanthanides is found in cerium metal under compression, where structures typical for the light actinides have been observed experimentally. A generalized phase diagram for the actinides is shown to contain features comparable to the individual phase diagram of Ce metal. The crystal structure behaviour of the lanthanides and heavier actinides is determined by the number of 5d (or 6d) electrons in the metallic state, since for these elements the f electrons are localized and nonbonding. For the earlier actinide metals electronic structure calculations – where the 5f orbitals are treated as part of the valence bands – account very well for the observed ground state crystal structures. The distorted structures can be understood as Peierls distortions away from the symmetric bcc structure and originate from strongly bonding 5f electrons occupying relatively narrow 5f states. High pressure is an extremely useful experimental tool to demonstrate the interrelationship between the lanthanides and the actinides.
1.
The atomic volume and valence assignments
Already the behaviour of the atomic volumes of the actinide metals reveals unique features not met within any other series of transition elements. In figure 1 we have plotted the atomic volumes for most of the metals in the Periodic Table. It is a wellknown fact that for practically all the lanthanide elements the 4f electrons are localized, or nonbonding. As a consequence most lanthanides have three valence electrons and there is a smooth (approximately linear) change in volume between lanthanum and lutetium, which are both trivalent metals. The exceptions are the anomalously large volumes of europium and ytterbium, which are due to the fact these two elements are divalent metals (i.e., they have only two valence electrons of spd type). J.C. Baltzer AG, Science Publishers
42
B. Johansson / Relationships between the lanthanide and actinide elements
Figure 1. The experimental atomic volumes of the 3d, 4d and 5d transition metals, the lanthanides and the actinides. The low volume data for the earlier lanthanides and the transplutonium elements are estimations of the equilibrium volumes for the case that the f electrons were itinerant.
The number of bonding valence electrons at the beginning of the 5d series may be inferred from a comparison between the volumes of the rare earths and the 5d transition metals. On the right in figure 1, Yb is clearly a divalent lanthanide metal. Similarly, lutetium is a trivalent lanthanide and has three valence electrons. On the left in the same figure, these two elements are seen to be also the first two members of the 5d transition series and the third, hafnium, will accordingly have four valence electrons. In contrast, the assignment of valencies to the actinides should be treated with great caution. The observed atomic volumes suggest that the actinides display two different behaviours, one comparable to the lanthanides and the other comparable to the 3d, 4d and 5d transition elements, with a dramatic crossover between Pu and Am. Such a fundamental change within a transition series appears to be unique and requires a most careful analysis. As a complement to the consideration of the atomic volume as a guidance for the valence state, another type of analysis directs attention to the
