Systematic properties of actinide metals

Systematic properties of actinide metals

Journal of the Less-Common SYSTEMATIC Metals, 121 (1986) 1 - 13 1 PROPERTIES OF ACTINIDE METALS* JOHN W. WARD Division of Materials Science Alamo...

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Journal of the Less-Common


Metals, 121 (1986) 1 - 13



JOHN W. WARD Division of Materials Science Alamos, NM 8 7545 (U.S.A.)

and Technology,

Los Alqmos





The solid state and thermodynamic properties of the 7th~row metals actinium through einsteinium are reviewed, particularly in terms of new advances since the Asilomar meeting. General properties for all the elements are considered, and the individual metals are surveyed for new results as well as for unresolved problems. The thermodynamic properties of the metals thorium through einsteinium are mostly well-established, except for the cohesive energy of protactinium. Here some unmeasured values still remain, particularly the high temperature heat capacities. The relation of 5f-electron bonding to 3d element behaviour is discussed in terms of the electronegativities of the elements, and the homologous shift of the metals americium through einsteinium to correspond with the series neodymium to europium is noted. Plutonium is characterized as an actinide trying hard to become a rare earth, and properties of the “transition region” are briefly discussed. A new’ understanding of f-electron bonding and structure has come from sophisticated calculations showing the effects of internal pressure, spin polarization energy and spin-orbit coupling. Recent high pressure results for the transplutonium metals shed light on these special materials.

1. Introduction The fascinating physicochemical properties of the actinide metals and their alloys and compounds arise in part from the complex solid state and electronic natures of these metals. This is especially true of the early or socalled “light” actinides - those metals up to and including plutonium. A great deal has been written about the effects of f-electron bonding in the early actinide metals protactinium through plutonium; it can be said that much is now understood, but that controversies still exist, particularly with regard to types of magnetism and to real electronic structures. The intention of this paper is to summarize briefly the status of what is well known and not so well known, where controversies might exist and where work needs to be done. Although the summary is very much from an experimentalist’s point-of-view, major attention is paid to the new contributions from theory, particularly since the Asilomar meeting. *Paper presented at Actinides 85, Aix en Provence, September 2 - 6, 1985. 0022-5088/86/$3.50

@ Elsevier Sequoia/Printed in The Netherlands


2. General observations 2.1. Structure and bonding As is welI known by now, the electronic properties of the light actinides can be compared with those of the early 3d transition metals, i.e. those that are highly-electropositive. The 5f (or 3d) electrons have highly-extended wavefunctions are band-like and considered to be involved in metallic bonding. As such, these metals exhibit large activation energies for chemical reaction (e.g. hydriding) and show no magnetism. In contrast to the 3d, the 5f orbitals are “polarized” +/-, like p electrons (and the light actinides form interesting intermetahics with the p-electron metals). At protactinium the 5f electrons appear below the Fermi level. As discussed by Ward et al. [l], the wavefunctions are even more extended than for the 3d electrons, but more diffuse. In addition, the electron densities do not favor cubic structures, and this plus the polar nature of the wavefunction produces exotic low temperature allotropes, as the structure struggles to achieve a minimum energy configuration. However, unusual as this may seem, the energies required for the allotropic transformations are small; for example, the six allotropic transitions in plutonium occur with a total energy change of only 2230 calories, through melting - less than that required for the simple melting of most metals. Conversely the solid state and transport properties (e.g. resistivity, magnetism, bulk modulus, melting point etc.) can be greatly affected. Both neptunium and plutonium melt at only 640 “C, thereby achieving maximum participation in f bonding in the liquid state. Plutonium has the highest viscosity and one of the largest liquid ranges before boiling of any metal (compare the behaviour of gallium, a strong pbonding metal). The addition of more and more f electrons is also accompanied by bandnarrowing; thus, there are often these two competing effects, i.e. more fcharacter but a tendency toward localization. Which effect predominates is often a case of what intermetallic or compound is being formed, which is beyond the scope of this paper, but for the metals the situation is now clear, owing to recent excellent theoretical studies and band calculations. 2.2. Electronic structure The latest information and discussions about recent advances in theoretical studies has been collected and reviewed by Brooks et al. [ 21. Very briefly, the contribution to bonding in the hybridized conduction band is made up of the s, p, d and f electrons; the s and p electrons especially prefer large volumes (i.e. lower “valencies”), and the f electrons prefer smaller volumes. Therein is the battle across the early actinides, as more and more f electrons are added, but with increasingly narrower bandwidths. The calculations are expressed in terms of internal pressures, and thereby the actinides become smaller in regular fashion with more f electrons, as do the transition metals with more d electrons. However, in contrast, the cohesive energies of the early actinides fall regularly, whereas these energies rise rapidly for the transition metals with smaller volumes and more d electrons.


What prevents the (s, p) electrons from winning out sooner is the interplay between bonding and the so-called spin polarization energy. Very simply, the question is whether the lowest energy ground state is with f bonding (hybridization = delocalization) with the spins non-poZari.zed, or, with no (or little) f bonding and the spin poZarized, i.e. aligned. The latter case is of course localization, and this occurs suddenly at americium, where the energy for alignment is finally predominant over that for bonding. These recent, sophisticated calculations have produced much insight and valuable information to the experimentalist; for example, excellent values for atomic volumes and/or radii and bulk moduli. The much more complicated calculations required to give final cohesive energies, which involve all the contributions from the various kinds of electrons, configurational energy differences, socalled promotion energies and possible spin-orbit coupling effects, have yet to give values close to those from experiments. Suffice it to say, however, that tremendous strides have been made in the area of self-consistent, fully relativistic band-structure calculations.

2.3. Chemistry and Solid State Chemistry The actinides are certainly considered as highly electropositive in chemical reactions, and that continues, of course, through the series (i.e. past the half-filled shell where localization occurs and rare-earth-like behaviour is expected) in contrast to the transition metals, which become increasingly more electronegative across the series. However, it is not at all clear what the situation is in the low-temperature, highly f-bonded state. For example, the solubility of hydrogen in uranium metal has been carefully studied by Powell [3], and for the a-U phase (and some alloys) the heat of solution is endothermic, i.e. the f-bonded metal is behaving like a more electronegative late transition metal, for example iron. Yet actual hydriding is a vigorous exethermic reaction. In all cases the light actinides have a powerful electronic route available, akin to a valence change in other metals - that of localization-delocalization, to achieve a minimum energy state. The importance and uniqueness of this route cannot be overemphasized. Thus, for example a-PalIs-, forms a stable b-cc. lattice with over 50% H vacancies because the driving force for hydriding causes localization (and therefore magnetism) as a means to accomodate the hydrogens, where metal atom motion at these low temperatures is very difficult. The reverse effect (delocalization) with pressure has been explored at Benedict’s laboratory [4 - 61 in Karlsruhe. High pressures cause all the metals Am - Cf to collapse (americium through several phases) eventually to the o-U structure. The amount of pressure is related to the distance of the f levels from the Fermi level, i.e. pressure causes the f electrons to be pinned at the Fermi level and again become part of the conduction band. This behaviour is of course similar to that found many years ago with cerium, and very recently also with praseodymium. It is also interesting that even very recently


(ref. 2) the predictions favored a valence change, i.e. Bk3+ to Bk4+ and so forth. A valence change in fact does occur in “normal” metals, for example Yb2+ to Yb3+ under pressure. 2.4. The late actinides The real nature of the late actinides, americium and beyond, is very interesting. These f electrons are localized and fully magnetic, but still close to the Fermi level. As pointed out previously by Johansson [7] most of the solid state and physical properties of these metals are clearly analogous not to their normal rareearth counterparts directly above, but shifted so that americium corresponds to praseodymium, and finally divalent einsteinium to divalent europium. One can also draw analogies between curium and gadolinium, but here the argument is mainly a thermodynamic one, because the half-filled shell stabilizes the trivalent gas. The resultant vapor pressures and cohesive energies are therefore similar. However, the phase behavior of curium is similar to that for neodymium. The incipient stability of the divalent state is seen in both samarium and californium, where both trivalent and divalent chemistry is observed. Divalent samarium has been shown to coexist with the trivalent state in surface studies by photoemission; it would be interesting to conduct a similar study on califomium, where the tendency toward divalency is even stronger. At einsteinium the fully divalent metal is established, as shown in recent vapor pressure studies by Kleinschmidt et al. [S]. The divalent state then continues, as there is no half-filled shell effect like that at europium. Comparative values for the late actinides and early rare-earths are given in Table 1.

2.5. Thermodynamic properties The cohesive energies of all the actinides Th - Es have been. well measured except for a controversy concerning protactinium, by the Knudsen effusion target and the mass spectrometric method. These data have been extensively reviewed and thermochemical values tabulated for solid, liquid and gas in ref. 1. Most of the data for the lighter actinides is older and well established; a recent high-temperature study on plutonium by Bradberry and Ohse [lo], carrying the pressures to 2219 K, is in excellent agreement with earlier work. A long-standing controversy between empirical predictions and extensive experimental studies for americium has happily been resolved with a newly determined term value for the ground state. For the metals curium and beyond there exist little data for heat capacities especially at high temperatures. However, Ward and Hill [9] have established a correlation relating crystal entropy to metallic radius, atomic weight, magnetic properties and electronic structure. This correlation relies on calculation of entropy values, for unmeasured metals, based on comparison with a closely similar metal for which experimental values are available; the technique has been shown to be accurate to within a few-tenths of an entropy unit for most metals in the periodic table. A prediction of 13.2 cal K-r mol-’ for the crystal entropy of americium was the exact value found in

5 TABLE 1 Comparative values for the actinides Am - I% and lanthanides Pr - Eu Metal pair(s)

Melting point (K)

Boiling point

d.h.e.p., f.c.c. d.h.c.p., f.c.c.

1449 1204

2220 3785

5.97 6.56

13.20 17.67

67.9 85.0

d.h.c.p., f.c.c. d.h.c.p., f.c.c. SJGdd) (h.c.p., b.c.c.)

1618 1289 (1585)

3383 3341 (3539)

6.62a 6.55 (6.56)

17.20a 18.99 (16.24)

92.2 79.3 (95.0)


74.1 ?

Am Pr

Crystal phases



21 K mol-‘)

FkK mol-‘)

02gs Valency =I mol-‘) IIIf ILIV) III( IV) 111(IV) 111(IV) (III)

Bk Pm

d.h.c.p., f.c.c. d.h.c.p., f.c.c.

1323 1350.

2900 2730 (?)

Cf Sm

d.h.c.p., f.c.c. &Sm(d.h.c.p. under pressure)

1173 1345

1745 2064

EL70a 7.06

19.25a 16.61

46.9 49.4

111,II III,11

Es Eu

f.c.c. b.c.c.

1133 1090

1241 1870

6.3ga 6.48

21.38a 19.31

31.8 41.9






( ), other possible valence states. sFrom the entropy correlation of Ward and Hill [ 9 3.

later precise heat capacity studies. The importance of the crystal entropy s”,,, lies in the fact that this value is the base number for generating freeenergy functions for the condensed phases. Precise vapor pressure measurements then give heats and entropies of vaporization to the temperatures of measurement, and, using the heat capacity of the known metal as a guide, a self-consistent high temperature heat capacity curve can be generated. From this and the measured values, the free energy functions and all the thermodynamic values, solid, liquid and gas can be calculated. The most recent results are also reviewed and discussed in ref. 1. An update from the 1975 conference in Baden-Baden is given in Figs. 1 and 2, extending the known data through einsteinium. Figure 1 shows the relation of metallic radius to crystal entropy, and Fig. 2 clarifies why there are large deviations for the rare earths and actinides, because of the large magnetic entropy changes. For gz9s the regularity of the “non-magnetic” baseline is clearly seen, except for a deviation at curium, which has many low-lying energy levels. The regular magnetism of the actinide gases is also evident. It is doubly fortunate for the thermodynamic data for the actinides that excellent spectroscopic work has been done on the gases and that vapor pressure studies with very pure samples have been possible. A summary plot of the cohesive energies is given in Fig. 3. Also impressive are the data available for heats of solution, mostly from Fuger’s laboratory in Liege, for all the metals from thorium through californium. These data are also correlated in ref. 1 together with complete thermochemical values to 3000 K.














To W





















n 3




’ I

’ I

’ I
















V Es “b- w,d”curve



Fig. 2. Comparison of gaseous entropies for the lanthanides and actinides at 298 K and 1400 K. d”, divalent curve for the lanthanides; d’, trivalent gases; IJ, actinides. Open symbols denote full entropies, closed symbols denote the non-magnetic portion of the entropy for each element.

Fig. 1. Comparison plots of metallic radii (lower) and crystal entropies (upper) for the metallic elements,



"Non-mop" baseline



49 -

9 53 8 2 52 ffl

g 54



- 120


-loo $4,1



f 1 8 -.z






U Pa
















Fig. 3. Comparative plot of the cohesive energies of the actinide metals, kilojoules per mole (left ordinate) or kilocalories per mole (right ordinate). The two values at protactinium represent the discrepancy still unresolved at that element.

3. The metals The ~tentio~ here is to denote briefly new me~u~rnen~, measurements that need repeating and values that are yet to be determined. Naturally this gives a very incomplete picture as any venture into intermetallics or compounds would immediately reveal a large literature where a similar analysis would be made. 3.1. Actinium This is an extremely disagreeable metal to work with because of the intense radioactivity growing in rapidly from the daughters. For this reason and the simple scarcity of the material, little has been done. The metallic radius is nearly the same as that for lanthanum, and calculations show that actinium should be very lanthanum like, with no f character present. As such it is not even an “actinide”, in the sense that most chemists and physicists think of - namely having f electrons present. Nevertheless, here is a blank in the Periodic Table, and there is always a strong desire to fill such blanks. Two experiments that must eventually be done are the measurement of the vapor pressure (cohesive energy) and tests for superconducti~ty. Both these experiments are actually in the planning stage with Los Alamos-Oak Ridge cooperation.

3.2. Thorium Tho~um has been studied to a fare-thee-well since the beginning of the atomic age. Even so, recent high-purity sample technology has allowed measurements to be improved and extended. Homologs are titanium, zirconium


and hafnium, and the resistivity is typical of a transition metal. The thermodynamic properties are also well catalogued, with good heat capacity measurements to the liquid. Nevertheless, there are a few surprises. The first is the unexpe~t~ly large metallic radius - ahnost that for a typical trivalent metal. As shown in photoemission and Bremsstrahlung isochromat spectroscopy (BIS) by Baer and Lang [ 111, the massive empty f band hovers just above the Fermi level, perhaps with a tail leading into the conduction band; this might explain the unexpectedly large electronic specific heat term.

3.3. Protactinium At protactinium we find the first “real” actinide, with a broad-band f electron in the conduction band. The resistivity still looks like that of a transition metal, but with the (easily ionized) f electron gone, the chemistry becomes suddenly very tantalum-like, i.e. pentavalent. Because of the scarcity and refractory nature there have been few good experiments with protactinium. Superconductivity and low temperature heat capacity {to 18 K) have been measured, as well as magnetic susceptibility; these results are discussed in ref. 1. No high temperature heat capacity measurements have been made, which is unfortunate for two reasons: these are needed for an accurate correlation of the vapor pressure measurements to the cohesive energy and thermodynamics, and there is apparently a phase transition, possibly b.c.t. to f.c.c., somewhere between 1200 and 1500 K. Since the oxide formed above these temperat~es is PaOz (the predominant species in the vapor), there is the implication that there is a substantial change in properties, perhaps to tetravalency, at high temperatures. As noted above, there is still a major discrepancy between two vapor pressure studies which must be resolved by additional work; possible reasons for the disagreement are discussed in ref. 1. The heat capacity to room temperature has been measured at Harwell, and these data should soon be available. There are as yet no photoemission data for this element, which should also be interesting.

3.4. Uranium There is of course a huge literature for uranium metal, and most properties (resistivity, magnetism etc.) are well understood in terms of increasing f character in the conduction band. As noted earlier, the complex a-U structure leads to some unusual chemistry and physics, such as the endothermic solubility of hydrogen. More studies in the “complex tetragonal” /3-U phase would be welcome. Here there are 30 atoms per cell, and the average M-M distance remains 3.05 A, i.e. f bonding is nearly the same as for the (Yphase. However, the electrical resistivity falls for the /3 phase (much more so for the 7 phase). The review of Brodsky et al. [12] is recommended for an extended analysis of the electrical and magnetic properties of uranium and also of other actinides.


Heat capacities have been well measured from the point of superconductivity through the cy phase to about 940 K. Above this temperature there is considerable disagreement between a number of experiments and more conclusive values are needed. The liquid values appear again to be reasonable and consistent.

3.5. Neptunium Here again is a situation where metal scarcity and lack of industrial application has resulted in a paucity of measurements. Well determined are the crystal structures, phases and temperatures of transition. A calculated maximum in the f electron contribution to metallic bonding coincides with the smallest M-M distance in the actinides, and also (with plutonium) the lowest melting point. The M-M distance only reaches 3.05 A in the y phase (b.c.c.), and one could consider neptunium to be the ultimate “happy actinide”, free of all traces of transition metal behavior and not yet affected by the rare-earth series to come. The metal is, of course, non-magnetic, and a pronounced departure from normal resistivity behavior is seen, with a large negative d2p/dT2 term. New results close to publication are X-ray and UV photoelectron spectroscopy (XPS/UPS) measurements on some very high-purity double electrorefined metal prepared at Los Alamos, as well as high-pressure diamond anvil studies and a reexamination of the Np+H system. Still needed are high-temperature (more than 480 K) heat capacity measurements. The vapor pressure has been well measured, but U-Pu averages had to be used to form the free-energy functions. 3.6. Plutonium Plutonium is the classic “transition region” actinide - hence the six allotropic phases and multitude of puzzling properties. This metal is of course the initial reason for this series of g-year conferences, still called PZutonium-1970 but changed at Baden-Baden in 1975 to encompass all the actinides, because of the broader and broader activities in the other metals. If neptunium was the ultimate “happy actinide”, then plutonium is perhaps the most “unhappy actinide”, or the actinide which would most like to become rare-earth-like. As shown in Fig. 4, a pressure of only about 20 kbar will convert the behavior of plutonium to neptunium-like behavior. However, at normal pressure, higher temperatures (i.e. increased entropy) produce a variety of new and less f-bonded structures. There are five 5f electrons in plutonium, but the f band has become quite narrow (3.1 eV), even in the (Yphase. As pointed out earlier, the energies required to effect these transformations are very small, and in fact recent as-yet unpublished work at Los Alamos shows that only slight rearrangements in the various phases of the structures are necessary to go rather easily from the (IIphase all the way to the E phase. A summary of properties of plutonium, presented first in the 1970 Santa Fe conference by Morgan [13] is shown in Fig. 5. Here most of



PU too D


1 20

1 I I 40 50 30 PRESSURE (kbar)

I 60

I 70


Fig. 4. The phases of plutonium as a function of applied pressure.



Fig. 5. Comparative superposition of the physical, electrical and magnetic properties of the phases of plutonium.

the changes seem to make sense, e.g. the electrical resistivity drops, the metal becomes softer etc., but the magnetic susceptibility, especially for the 6 phase, is still not well understood. The 6 phase itself is fascinating and the subject of considerable recent interest. Here is a simple f.c.c. structure, but still non-magnetic, and with a lattice parameter still too small to be considered appropriate for a rare earth. XPS/UPS studies are very difficult for this phase because oxidation occurs so easily at 400 “C. However, work done to date would indicate a density-of-states more like that of americium (see below), with the f level no longer pinned at the Fermi level. Yet heat capacity measurements give an electronic specific heat coefficient r(pure) = 53 mJ K-* mol-’ (1981), a number almost three times larger than for CGPU,which already has the largest coefficient known for any metal. Thus the number paradoxically for the 6 phase implies much more electron density at the Fermi level, in contrast to the photoemission data, and this phase is a candidate for incipient heavy fermion character, a new and very exciting subject.


Heat capacities, cohesive energy, vapor pressure and thermodynamic properties are well established for plutonium, into the liquid. As noted earlier, the return to f bonding at higher temperatures results in a liquid with unusual properties. 3.7. Americium As noted earlier, the behavior of americium is now well understood in terms of the effect of spin-pairing energy, and the thermodynamics is also now in good order. Heat capacity to 300 K has been carefully measured, and the high temperature values estimated by the method of Ward and Hill [ 91. Of major interest is the new XPS/UPS study by Naegele et al. [141, showing well-defined multiplet f peaks well removed from the Fermi level. A great deal more is now better understood regarding the photoelectron spectra of the actinides, particularly in terms of the degrees and kinds of screening and the appearance of satellites. Naegele et al. [15] have recently reviewed the present status of 5 f photoelectron spectroscopy. Another interesting result is the resolution of the elusive d.h.c.p. to f.c.c. transition by Shushakov et al. [ 161, who determined a value of 1042 K using very pure material. In the new high pressure work by Benedict et al. [5], which has already been alluded to, the final collapse to f bonding was found at 23.5 GPa. The electronic properties of americium (resistivity, magnetic susceptibility, electronic specific heat etc.) are still not so well understood and more work in this area would be welcomed. 3.8. Curium Data for this particularly unpleasant element are surprisingly complete, with resistivity, magnetic moment and vapor pressure studies in good order. Intense radioactivity precludes heat capacity measurements, but the method of Ward and Hill [ 91 has supplied both a good crystal entropy and a selfconsistent high temperature heat capacity curve. Otherwise, there appear to to be no great surprises with curium, except for the collapse to f bonding at high pressure (43 GPa) discussed earlier.

3.9. Berkelium

For berkelium also, the heat capacity-vapor pressure-entropy correlation has produced a reasonable result, and thermodynamic properties sre considered to be in good order. The magnetism is also that of a well-localised f* configuration. To date, the largest samples of pure metal have totalled only a few milligrams, so no transport studies (e.g. resistivity) are possible. Of some interest is the high-temperature “expanded” f.c.c. structure (also for curium) as compared with the f.c.c. structure found under high pressure. Berkelium collapses to the o-U structure at 25 GPa, concomittant with the initial distance in energy of the 5f level(s) (about 2 eV) below the Fermi level. Other studies (e.g. photoemission) also await larger samples.


3.10. Californium This element continues to attract interest because the properties begin to bridge the transition level toward divalency, as does samarium in the rare earths (also thulium later on). Again, an “expanded” high temperature f.c.c. phase was found, as well another f.c.c. phase at high pressure; the collapse to the a-U structure requires 48 GPa. As noted above, photoemission studies would be interesting, given finally a large enough sample. Resistivity studies and even heat capacity measurements should theoretically be possible. Thermodynamically the element has been well characterized, particularly by the technique of Ward and Hill [9]. The vapor pressure is midway between that for samarium and europium.

3.11. Einsteinium Finally, einsteinium is at the moment the last metal producible in sufficient quantities (micrograms) and with a long enough half-life (20.47 days) to make bulk measurements. The metal is clearly divalent, as shown by Kleinschmidt et al. [8], with a vapor pressure like strontium; data were taken over the range 180 - 529 “C and the calculated boiling point is only 968 “C. There have been as yet no electrical magnetic measurements on the metal, but a magnetic measurement should be possible (studies on the trivalent oxide and fluoride have already given the expected classical moment).

3.12. Fermium, mendelevium, nobelium, lawrencium With the exception of as-yet submicrogram amounts of fermium, no bulk quantities of the metals following einsteinium can be produced, because of the very short half-lives. However, their properties can be easily predicted, with the divalent state extremely stable up to lawrencium. At lawrencium the f14 configuration dictates stable trivalent condensed and gaseous states, and the metal should behave like lutetium.

4. Conclusions By the time of the next conference in 1990, many of the missing data points and experiments that need to be remeasured will have been done. We can expect new and even more exact theoretical calculations extending into intermetallics and compounds, and fields such as heavy fermion systems should be well developed, perhaps leading to other exciting areas of physics. The transition region between itinerant and localized behavior is already under intense study, and this area should be further extended, particularly through studies of intermetallic compounds. Actinide physics and chemistry continues to provide a special window into the most complex electronic properties of the entire periodic table, and we can expect also some more surprises.


References 1 J. W. Ward, P. D. Kleinschmidt and D. E. Peterson, Thermochemical properties of the actinide elements and selected actinide-noble metal intermetallics. In A. Freeman, G. Lander and C. Keller (eds.), Handbooks on the Physics and Chemistry of the Actinides, Vol. 4, North-Holland, Amsterdam, in the press. 2 M. S. S. Brooks, B. Johansson and H. L. Skriver, Electronic structure and bulk ground state properties of the actinides. In A. Freeman, G. Lander and C. Keller teds.), Handbook on the Chemistry and Physics of the Actinides, Vol. 1, NorthHolland, Amsterdam, 1985, Chapter 3,,p. 153. 3 G. L. Powell, J. Phys. Chem., 83 (1979) 605. 4 U. Benedict, J. R. Peterson, R. G. Haire and C. Dufour, J. Phys. F., 14 (1984) L43. 5 U. Benedict, R. G. Haire, J. R. Peterson and J. P. It%, J. Phys. F., 15 (1985) L29. 6 U. Benedict, J. P. Itie, C. Dufour, S. Dabos and J. C. Spirlet, Delocaliiation of 5f electrons in americium metal under pressure: recent results and comparison with other actinides. In N. Edelstein, J. D. Navratil and W. W. Schultz (eds.), Americium and Curium Chemistry and Technology, Reidel, Dordrecht, in the press. 7 B. Johansson, Phys. Rev. B, 11 (1975) 2836. 8 P. D. Kleinschmidt, J. W. Ward, R. G. Haire and G. M. Matlack, J. Chem. Phys., 81 (1984) 473. 9 J. W. Ward and H. H. Hill, in W. Miiller and H. Blank (eds.), Heavy Metal Properties, North-Holland, Amsterdam, 1976, p. 65. 10 M. H. Bradbury and R. W. Ohse, J. Chem. Phys., 70 (1979) 2310. 11 T. Baer and J. K. Lang, Phys. Rev. B, 21 (1980) 2066. 12 M. B. Brodsky, A. J. Arko, A. R. Harvey and W. J. Nellis, in A. J. Freeman and J. B. Darby, Jr., (eds.), The Actinides: Electronic Structure and Related Properties, Vol. 2, Acadmic Press, New York, 1974, Chapter 5, p. 185. 13 J. R. Morgan, in W. N. Miner (ed.), Plutonium 1970 and Other Actinides, Nucl. Metall. Ser. (AZME), 17 (1970) 669. 14 J. R. Naegele, L. Manes, J. C. Sphlet and W. Miiller, Phys. Rev. Lett., 52 (1984) 1834. 15 J. R. Naegele, J. Ghijsen and L. Manes, Localization and hybridization of 5f states in the metallic and ionic bond as investigated by photoelectron spectroscopy. In Structure and Bonding 59/60, Springer, Berlin, 1985, Chapter E, p. 197. 16 C. D. Shushakov, A. G. Selezner, N. S. Kosulin and T. V. Shushakova, Fiz. Met. Metalloved, 55 (1983) 405 (in Russian).