International team of researchers employs high pressure to probe bonding in actinide metals

Investigations of protactinium and americium metals provide important insights into actinide bonding concepts

The science question

Pressure, like temperature, is an important variable in the chemistry, physics and materials science of materials. It has been found that the f electrons of the actinide elements-the so-called 5f-electron series of elements-are also greatly affected by pressure. At atmospheric pressure, protactinium, uranium, neptunium, and plutonium have itinerant 5f electrons (electrons involved to varying degrees in the metallic bonding), and they exhibit unusual low-symmetry structures and properties. In contrast, the transplutonium metals normally have localized 5f electrons, display symmetrical structures, and their bonding consists of three non-f-electron conduction electrons (i.e., "trivalent metals"). In this regard they mimic the lanthanide metals.

Oak Ridge's Dick Haire discussed research by an international team on the use of high pressure to probe actinide bonding.

One obvious difference between these f-elements across the series is seen in the variation of their atomic volumes at normal pressure, as shown in the figure on page 18. The similarity of the trivalent, localized 4f- and 5f-electrf-electron metals (i.e., protactinium-plutonium). The latter have much smaller atomic volumes in addition to their lower crystal symmetries. The three divalent f-element metals‹europium, ytterbium, and einsteinium‹are special cases, and result in part due to their higher f-electron promotion energies.

With the some of the early lanthanide metals, it is interesting to note that after acquiring 4f-electron character in their bonding by applying high pressures, they adopt some of the same low-symmetry structures exhibited by the protactinium-plutonium metals that have itinerant 5f electrons. This immediately raises the question of whether applying pressure on the transplutonium metals, which have localized 5f electrons, can bring about significant changes in their bonding.

As these actinides have more spatially extended f-electrons than their lanthanide homologs, it should easier to force their 5f electrons to become bonding by applying pressure-that is, they may be more sensitive to the affects of pressure than their lanthanide counterparts.

Atomic volumes of the f-electron elements reflect differences in bonding present for the elements. The smaller volumes for the protactinium-plutonium metals reflect their itinerant 5f-electrons.

Selecting the f-electrons for study

Studies of the actinides can be difficult, given their varying levels of radioactivity and availability. Of interest to researchers is the potential for forcing increased bonding in the actinides by applying pressure. For example, can pressure force divalent einsteinium metal (element 99) to acquire a third bonding electron? Basically, can pressure force one element to mimic the

Two actinides were selected for recent experimental studies to probe these questions. One study involved americium and a more recent study involved protactinium. (Reports have been published in "Physics Review Letters," Vol. 85, 2961; Phys. Rev.B, 63, 214101, (2001) and 67, 134101, 2003.) The behavior of these two metals under pressure is of special interest; the studies looked at the actinide elements that "initiate" and "complete" the "dip" seen in the actinide volume curve in the figure below.

Protactinium has the distinction of being the first actinide metal with a 5f electron (actinium and thorium have d-electrons), and it already has partially itinerant 5f electrons. In contrast, americium is the first element in the series having fully localized 5f electrons at atmospheric pressure, following the four preceding actinides that have itinerant 5f electrons.

These recent studies were conducted to determine if pressure can force additional 5f-electron bonding in protactinium‹making it more like uranium‹and if pressure can force the onset of 5f-electron bonding in americium-making it behave like the protactinium-plutonium metals, or specifically, its near neighbor plutonium.

An international team of scientists from Oak Ridge National Laboratory, the European Institute of Transuranium Elements, the European Synchrotron Radiation Facility and the Vienna University of Technology carried out the studies.

The researchers examined the structural behaviors of the protactinium and americium up to pressures as high as 130 giga pascals (GPa) using different types of diamond anvil cells to generate static pressures on the metals through different transmitting media (silicon oil or liquid nitrogen). Pressure calibrations employed ruby fluorescence, and/or copper or platinum metals via their known equation of states. Modern diffraction techniques employed multiple wavelengths of high-intensity radiation from the synchrotron and charge-coupled device (CCD) detectors.

What was found experimentally?

The research found that the tetragonal structure of protactinium at atmospheric pressure, where itinerant 5f-electron behavior in the bonding already exists, converts to an orthorhombic structure at about 77 GPa that is isostructural with that of alpha-uranium.

At this pressure, the protactinium's atomic volume is reduced to about 62 percent of that at atmospheric pressure. Accompanying this first-order transformation from the protactinium-I (Pa-I) to protactinium-II (Pa-II) crystal structure is a small relative volume collapse of 0.8 percent. It is postulated that structural transformation with pressure (the Pa-I to Pa-II transformation) reflects an acquisition of additional 5f-electron character in the metallic bonding and alters its basic properties.

With americium under pressure, it was determined that the delocalization of its 5f- electrons occurs in two steps, with the progressive formation of lower-symmetry crystal structures (i.e., the third and fourth structures).

Overall there are three phases changes up to 100 GPa; the first change from double hexagonal close-packed (dhcp) to face-centered cubic (fcc) represents only a second-order phase change. In the first transition involving delocalization (third phase, Am-III), americium acquires the structure of gamma- plutonium‹an orthorhombic structure (space group Fddd)‹at about 10 GPa, this structure is accepted as reflecting itinerant 5f-electron character.

This first-order transformation (Am-II to Am-III) is accompanied by a small but significant relative volume collapse of about 2 percent, which signifies that a change in bonding occurs. It is followed by formation of a fourth phase (Am-IV), a primitive orthorhombic structure (space group, Pnma) at about 18 GPa and exists up to 100 GPa. The latter is closely related to a Cmcm structure, the alpha-uranium structure and the Pa-II structure); this structure is also accepted as reflecting the presence of itinerant 5f electrons in the metallic bonding. Accompanying the formation of Am-IV is a more significant relative volume collapse of about 7 percent.

Thus, pressure has forced the delocalization of the 5f electrons in americium after its atomic volume and interatomic distances are reduced considerably (at 100 GPa the volume is about 46 percent of that at atmospheric pressure). Under pressure, the Am-II phase becomes like plutonium (its near neighbor), and at higher pressures, americium then adopts the alpha-uranium structure.

An important difference between the two metals is that americium starts out having fully delocalized 5f electrons, while the 5f electrons of protactinium already exhibit some 5f-electron itinerancy at atmospheric pressure. Both acquire additional 5f-character in their bonding with pressure. This initial difference is reflected in specific behavior and properties under pressure. Comparisons between the compression and structural behaviors of americium, uranium, and a 50 atomic percent americium-curium alloy with pressure are shown in the figure on page 26.

The more compressible nature and overall volume reduction of americium metal is due to it starting as a fully localized 5f-electron, trivalent metal, whereas uranium and protactinium initially have itinerant 5f electrons. the figure to the left also shows that the slope of the compressibility curves for alpha-uranium, Pa-II, Am -IV, and the 50 atomic percent (Am,Cm)-IV structures (i.e., in the high-pressure regions where they exist) are all very similar, as they should be, given that all have either an alpha-uranium or a closely related structure.

A comparison of the compression behavior of uranium, protactinium, americium, and a 50 atomic percent americium-curium alloy is shown. Bulk modulli and pressure derivatives extracted from the data are also given. (Adapted from the "Journal of Condensed Matter Physics," Vol. 15, S2297, 2003.)

The latest published calculations based on theory for the behavior of americium under pressure were in agreement with this experimental work. In addition, Per Söderlind (LLNL) and Olof Eriksson of the University of Uppsala, Sweden, have also used a calculational approach involving Gibb's free energies to predict a series of structural transformations for protactinium metal under pressure (see "Physics Review B, Vol. 56, 10719, 1997).

Our recent experimental work on protactinium confirmed their prediction that an alpha-uranium structure would be formed under pressure from the body-centered tetragonal form, with the differences between experiment and theory being only at which pressure and at what relative atomic volume the Pa-I to Pa-II transition occurs. However, the story for protactinium remains incomplete, as theory predicts that additional structural transformations will occur at much higher pressures (presently difficult to reach experimentally).

It will be a difficult but interesting challenge to try to reach these ultra high pressures to confirm the theoretical predictions.

Additional Aspects

Several other pieces of important information can be acquired from such experimental pressure studies (i.e., the bulk modulus and its pressure derivative acquired by applying the Birch-Murnaghan equation of state). In a simple picture, the bulk modulus indicates the "stiffness" of a structure's lattice‹the more rigid the structure, the less compressible it is, and the more likely contains a greater degree of bonding. Bulk modulli and pressure derivatives are given in the figure at left for americium, uranium, protactinium, and the americium-curium alloy. Note that the modulus for americium is about 29 GPa (derived from Am-I phase), while those for alpha-uranium and the Pa-I structure are over 100 GPa.

It can be informative to compare the compression behaviors of the different metals, the pressures at which structural transformations occur, and the overall picture of the relative volume behaviors.

The close relationship between the body-centered tetragonal structure of protactinium (Pa-I) and the orthorhombic Cmcm structure of alpha-uranium can be seen in the figures at right. The main difference between the structures occurs from a buckling of the "chains" in the latter due to the "y" parameter of the 4c sites of Cmcm. When the Pa-I structure changes to the Pa-II structure, the "y" parameter of the Pa-II structure differs by less than 0.02 from that of alpha-uranium.

The Am-IV structure also shows a close similarity to that of alpha-uranium; it is only necessary to shift the position parameter "z" of the Pnma structure 4c sites to zero from "z=0,10". The three structures in the top of the figure at the left show the close relationship that exists between the four phases. The structural sequence for americium metal under pressure is shown in the lower part of that figure. The first two transformation processes basically involve changes in the stacking sequence of planes; and the f"buckling" of the hexagonal planes.

Structures of selected actinides under pressure. The similarity of the protactinium-II and americium-IV structure to that of the alpha-uranium structure is evident. The lower segment shows the structural sequence of americium under pressure to 100 giga pascals (GPa).

Special facets of americium and protactinium

Important changes in the role of the 5f electrons in the actinides can be extracted from perturbations in their atomic volumes and the structures displayed under pressure. As stated above, the work on americium and protactinium metals under pressure have shown that pressures increases the itinerant (bonding) nature of their 5f electrons. The onset in (americium) or increase in (protactinium) delocalized 5f-electron character reduces the atomic volumes of the metals, as well as producing structures of lower symmetry. In contrast, heating plutonium reduces the 5f-electron

Using a simple picture, plutonium becomes more like the localized 5f-electron metals upon heating and americium becomes more like plutonium under pressure. Both changes reflect the changing influence of the 5f electrons in the elements' bonding.

Future experiments

A number of experiments are envisioned to further explore the actinide metals under pressure to better understand the science that is occurring. Certainly, studies under higher pressures are desired to explore the potential for an onset of new interactions that may drive the formation of new structures and alter the bonding.

There is also the need to continue to actinide elements with higher Z values using these improved experimental techniques, and both curium and californium metals are two potential candidates. Their behavior under pressure will be affected by the withdrawal of their 5f electrons from their Fermi edges, due to the increased nuclear charge. The latter should make it more difficult to force the delocalization of their 5f electrons, and should require higher pressures to bring about.

Einsteinium is the highest actinide for which such pressure experiments can be envisioned, given the quantities of actinides expected to be available. The behavior of this metal under pressure would be very interesting to study, but it is also an exceptionally difficult material to study because of the very short half-life and intense radiation of its most available isotope (Es-253).

An attempt to study this metal under pressure was unsuccessful, due to crystal destruction by its self-irradiation. A special interest in einsteinium's behavior under pressure is that, in principle, this divalent metal could be forced by pressure to become a trivalent metal like the americium through californium elements at atmospheric pressure. With additional pressure, it may be possible to force einsteinium's 5f electrons to delocalize, perhaps displaying the behavior observed with americium metal under pressure. One can also easily envision numerous other studies of actinide alloys and/or compounds under pressure.

Conclusion

Studies under pressure can provide important insights into the 5f-electron behaviors and their changing roles. It appears that orthorhombic structure-types (i.e., alpha-uranium or the closely related, Pnma structure-type) are encountered under pressures in the region of 100 Gpa. Whether this type of structure is a dominant high-pressure structure in this region remains to be determined. Theory predicts more symmetrical structures may again be encountered at even higher pressures, due to an increase in dominance of electrostatic interactions and Born-Mayer repulsions.

These studies with americium and protactinium metals will not only help other researchers to better understand the bonding and electronic structure of two elements, but also that of other actinides. They should enable the development of important systematics concerning the actinides. The experimental data should also serve as a platform from which one may evaluate and fine-tune theoretical predictions, thereby enhancing the overall understanding of these complex actinide metals. In addition, such research may contribute to an improved understanding of other elements in the Periodic Table.

This paper was contributed by Richard Haire, CSD, Oak Ridge National Laboratory; Steve Heathman and Monique Idiri, European Commission, JRC, Institute for Transuranium Elements; Tristan Le Bihan, European Synchrotron Radiation Facility; and Andreas Lindbaum, Vienna University of Technology, Institute for Solid State Physics.

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