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Actinide Elements 221

these delocalized or itinerant 5 f electrons are involved most cases notoriously difficult. Therefore, data on physi-
in the metallic bonding as a part of the conduction band cal properties of the actinide metals are very limited. Data
formed together with the 6d and 7s electrons. The band on selected physical and thermodynamic properties are
character of the delocalized 5 f electrons is inhibitory to presented in Table V.
the development of magnetism. Within the framework of Proceeding along the 5 f series, the high melting points
a simple model of the metallic bond, the metal is an array of Th and Pa reflect their transition metal character, Np
of ions held together by quasi-free conduction electrons, and Pu have very low melting points due to f -orbital re-
and a metallic valence can be defined as the contribution flection, the melting points rise over Am to Cm, and they
of outer electrons each atom gives to the “sea” of bonding then again decrease. The maximum at Cm reflects both its
conduction electrons. Conversely, the metallic valence is half-filled 5 f shell and the presence of a d-type valence
the charge left per atom when the bonding electrons have electron. The decreasing melting points of the transcurium
been stripped off. In this approach, the first five actinides elements reflect the onset of s-type bonding and the loss
after actinium, thorium up to plutonium, are considered of d bonding in the divalent metals. The melting point
as having metallic valences greater than three. of Lr is expected to be as high as that of Cm, assuming d
As the atomic number increases, the radial extension bonding, but should be lower if it behaves like a p element
and the bandwidth of the 5 f electrons decreases. From due to relativistic effects.
americium on the 5 f electrons are localized, nonbond- Looking at transport and magnetic properties along
ing, and carry a magnetic moment. The actinide metals the actinide series, superconductivity under atmospheric
americium to californium and lawrencium are trivalent pressure (Th, Pa), superconductivity under high pressure
metals. Einsteinium to nobelium are divalent metals due (U), exchange reinforced Pauli paramagnetism without
to very high promotion energies needed to promote one superconductivity (Np, Pu), superconductivity under at-
f electron to the metallic bonding state as known from mospheric pressure (Am), and finally magnetic ordering
ytterbium in the lanthanide series. Thus, the actinide se- and absence of superconductivity (Cm, Bk, Cf) are succes-
ries displays more complex electronic structures than does sively encountered. Measurements of electrical, magnetic,
the lanthanide series; not only in the first half of thek or electronic properties of the heaviest actinides beyond
series. californium have been missing up to now.

2. Crystal Structures 4. Thermodynamic Properties
Actinide crystal structures are more complicated and di- One of the fundamental properties of a metal is its enthalpy
versified than the corresponding lanthanide metal struc- of sublimation. The enthalpy of sublimation of a metal,
tures. Information about the crystal structures of the ac- which is a measure of its cohesive energy, is related to the
tinide metals is given in Table IV. electronic structure in both the solid and its vapor. The
Actinium and thorium have no f electrons and behave enthalpies of sublimation of the actinide metals thorium
like transition metals with a body-centered cubic structure through californium have been determined directly by va-
of thorium. Neptunium and plutonium have complex, low- por pressure measurements using the pure metals, those of
symmetry, room-temperature crystal structures and ex- einsteinium and fermium by measuring partial pressures
hibit multiple phase changes with increasing temperature over alloys. Estimates of the enthalpies of sublimation for
duetotheirdelocalized5 f electrons.Forplutoniummetal, the actinide metals californium through nobelium have
up to six crystalline modifications between room tempera- also been made based on thermochromatographic mea-
ture and 915 K exist. The f electrons become localized for surements of the adsorption of actinide atoms on metals.
the heavier actinides. Americium, curium, berkelium, and The experimental enthalpies of sublimation clearly reflect
californiumallhaveroom-temperature,doublehexagonal, the trends and changes in the electronic properties of the
close-packed phases and high-temperature, face-centered actinide metals when progressing across the series. Thus,
cubic phases. Einsteinium, the heaviest actinide metal there is further evidence for metallic divalency well before
available in quantities sufficient for crystal structure stud- the end of the actinide series.
ies on at least thin films, has a face-centered cubic structure
as typical for a divalent metal.
5. Alloying Behavior
Experimental studies of actinide alloys have been carried
3. Physical Properties
out with Np, Am, Cm, Bk, Es, and Fm, and far more
The radioactivity of the actinides along with their lim- extensive studies have been carried out with the actinide
ited availability makes their experimental investigation in metals of technological importance, Th, U, and Pu. The