Actinide high-pressure behavior

The first of the five chapters, which, more or less, deal with experimental aspects of the lanthanide/actinide materials, is concerned with the structural behavior of these materials at high pressures (chapter 113 by U. Benedict and W.B. Holzapfel). The authors show that the spatial extent of the f-wave functions around the nucleus plays an important role in govering the formation of various crystal structure as a function of both pressure and atomic number in the elemental metals. Because of the lack of knowledge of the high-pressure behaviors in their compounds, the systematic study and understanding in f-element compounds is much less advanced than for the metals. 

Whereas Ce through Lu are characterized by filling of 4f shell, actinide elements thorium to lawrencium are characterized by filling of 5f shells. The major difference in bonding behavior in the solid state between these two series of elements is that 5f elements have delocalized f electrons whereas 4f electrons are localized in lanthanides and do not readily participate in bonding. This is where pressure plays an important role at room temperature. High pressure can delocalize the 4f shell and it can cause electron transfer between various subbands in the conduction electrons such as sp—and spd f. Participation of f electrons in bonding is accompanied by a volume coUapse in various lanthanides. 

Already at ambient pressure the field of thermal phase transitions is very rich (Smith and Kmetko 1983), with the most pronounced anomalies for Pu and its alloys. Furthermore, the ambient-temperature section of the generalized high-pressure phase diagram for intra-actinide alloys, as previously discussed by Benedict (1987, 1989) and shown in an updated version in fig. 9, has at first glance little resemblance with the much simpler behavior of the lanthanides, discussed before (cf. figs. 3 and 4). Nevertheless, when one uses also here the available EOS data together with tabulated (Waber and Cromer 1965) and partly interpolated values for the 6p-shell radii, to determine the radius ratios r, = at first for ambient pressure and then also... 

The situation with respect to high-pressure Mossbauer experiments is discussed in section 5. General systematics within the most thoroughly studied series of intermetallics are presented in sectii n6. Section 7 briefly highlights the spin-glass behavior of a class of lanthanide and actinide intermetallics. 

Mossbauer spectroscopy is a powerful tool to gain essential information on the difference in electron structure between intermetallic compounds of 4f and 5f elements. In the latter series the work concentrates on Np materials. For this actinide bulk magnetic and transport data are still scarce and even neutron diffraction results are not plentiful. Mossbauer spectroscopy is thus in some sense a forerunner. It has been shown that measurements under high pressures are pivotal for a deeper insight of electronic properties, in particular with the question of 3d-like itinerant versus 4f-like localized electron behavior of the 5f electrons in Np. It becomes more and more apparent that U and Np behave often quite differently mainly because of the somewhat more pronounced tendency of the latter towards f localization. Nevertheless, the formation of 5f bands via hybridization plays a central role also in Np metallic compounds. 

The actinide systems are somewhat different. UCj, for example, does not have a fully saturated lattice when graphite is present, especially near 1765°. To this extent, oxygen can dissolve in the structure, especially at high temperatures. To a lesser extent, ThC2 would behave in the same way. When this behavior is added to the possibility of concentration gradients, the CO pressure ceases to have any simple signihcance. 

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