Actinide Elements and their Compounds with other Elements. Part 1

197 Pages·1991·9.104 MB·English

by  R. Troc, W. Suski (auth.), H.P.J. Wijn (eds.)

Ref. p. 851 3.1.1 Actinide elements: introduction 1
3 Magnetic properties of actinide elements and compounds
3.1 Actinide elements
3.1.1 Introduction
Actinides, the elements with atomic numbers Z= 89...103 occupy a special and unique position in the
Periodic Table. To understand their extraordinary position among the chemical elements and recent
achievements in this field, we have to refer to several review articles, especially to those included in
comprehensive books, “The Actinides: Electronic Structure and Related Properties” edited by Freeman and
Darby in 1974: Vol. I [74 f I] and Vol. II [74f 21, and “Handbook on the Physics and Chemistry of the Actinides”
edited by Freeman and Lander in 1984: Vol. I [84f l] and 1985: Vol. II [SS f 11. See also the volumes: 85 f2,
86f1, 87f1, and 91fl.
All the elements in the actinide seriesa re radioactive and they have usually a large number of isotopes. Their
nuclei, especially those of the earlier members, undergo c1a nd p, sometimes also y, decays with a large spread in
the half-life times from E 1 year (24pBk) to lOlo years ( 232Th) (seeTable 1). Heavier nuclei have also an enhanced
tendency to spontaneous fission with increasing atomic number. For example, the stability towards spontaneous
fission is reduced from 238U to 256Fm by more than 101’ times.
From the point of view of handling materials, the work with the transuranium elements is very difficult and
hazardous. It is necessaryt o use hermetically closed glove-boxes with complex control systems.F urthermore, the
self-radiation of transuranium elements, determined in mW/g units (seeT able l), causess trong damages in the
crystal lattices, affecting also other physical properties. Moreover, there is a substantial problem with the purity
of samplesd ue to the occurrence of daughter content as a result of radioactive decay during the storage time. For
example, there appear r3% Pu/year in 244Cm,6 % Cf/month in 24pBk, and 3% Bk/day in 253Es.F inally, it is
worth of mentioning that the self-heating effect accompanying the radioactive decay process causes serious
problems in determining physical properties in the low-temperature region.
The elementso f the first half of the actinide series,e xcept for AC,a re available either in multiton (Th, U, Pu) or
gram (Pa, Np, Am, Cm) scale. In the second half of the series, Bk and Cf are attainable only in submilligram
quantities. The elements Th, Pa, and U are produced by chemical processing of ores. On the other hand, the
extremely rare occurrence of 237Np and 23pPui n uranium bearing minerals makes it necessaryt o produce these
elements artificially. They are, together with Am, extracted from the products of nuclear fuel reprocessing,w hile
the remaining available man-made elements AC, Bk, Cf and Es are obtained as the final products at high-flux
reactors.
3.1.1.1 Atomic properties
The electronic structure of the actinide atoms (Table 1) was established many years ago and the recent
theoretical achievements in this subject are summarized by Desclaux and Freeman [84D 11. In general, the
atomic electronic structure consists of a radon core (86 electrons) and of energetically close Sf’,6 dj and 7s’ shells
(Table 1).
From the point of view of the magnetism of the actinides the central feature is the progressive filling of the
incomplete 5f shell and its implication on the extent of the 5f wavefunctions. As an example, the orbital radial
densities of the valence electrons for the uranium atom are shown in Fig. 1. One notes that the 6p and 5f curves
cross. In consequencet his indicates a larger tail for the 5f orbitals. This effect can be related to mean values of
powers of the radial extension, (r”). Fig. 2 shows the radial extent (r4) for 5f and 4f wavefunctions, as well as for
Fe as one representative of the 3d transition metals, weighted by the interatomic spacing in the elemental metals.
The high values of this ratio for light actinides lead to a strong overlap of 5f electrons with orbitals of adjoining
actinide atoms. This effect changes suddenly at Am. Nevertheless, the degree of the localization off electrons
characteristic of the lanthanides is not reached even for Bk [83 P 21. Thus the great extent in spaceo f 5f electrons
plays a decisive role in determining the magnetic properties of the actinides.
Land&Bkmstein Trot, Suski
New Series IIUl9fl
2 3.1.1 Actinide eiements: introduction [Ref. p. 85
Table 1. Atomic properties of actinides.
Atomic Most Half-life Specific Decay Self- Coherent Avail-
electronic available time activity mode heating neutron ability
structure isotopes a Ci/g mW/g scattering
A length
fm
AC 6d17s2 227 21.7 72 UTl -k Y 50 milligram
Th 6d27s2 232 1.4.10’0 1.1 .lO-’ a 0 10.08 ton
Pa 5f26d’7s2 231 3.3. lo4 4.7.10-2 0: y 1.5 gram
U 5f36d17s2 238 4.5.109 - 01 0 8.53 ton
8.417(5)‘ )
NP 5f46d’7s2 237 2.2 IO6 7.1.10-4 CL 0.02 10.15 kilogram
PU 5f66d07s2 239 2.4. lo4 6.1 .10-2 u 1.9 8.7 kilogram
242 3.8’10’ 3.8.10-3 c1 0.11 8.1
Am 5f76d07s2 241 4.3.102 3.2 % Y 115 gram
243 7.4.103 0.2 a 6.2 7.6 2, gram
Cm 5f76d17s2 244 18 83 a, Y 2670 7*) gram
248 4.4. IO5 4.2.10-3 u 0.1 milligram
Bk 5fs6d17s2 247 1.0.104 CL 1000 milligram
249 0.86 1.7.10-3 01, p 360 milligram
Cf 5fg6d’7s2 249 5.6. lo2 4.1 a 100 milligram
252 2.6 u, SF 3, milligram
‘) [82B3].
2, C74M2-J.
3, SF is the spontaneous fission.
1.0
0.9
I 0.8
2
2 0.i
\
z!
L
5 0.6
L
”
0.5
OA
0.3
PO U Np Pu Am Cm I
z-
Fig. 1. Radial densityQ ,o f the Sf, 6p, 6d and 7s orbitals as Fig. 2. Ratio of average 5f electron radial extension,
a function of radius, in arbitrary units, for a uranium [(r4),,]1/4, to the interatomic separation,d ,,-,,“, as a
atom with a 5f36d27s1e lectron configuration [85 A 23. function of atomic number Z of the actinides,c ompared
to those of the lanthanide metals (41o rbital) and Fe (3d
orbital) [83 PZ]. For the radial expectationsv alues (r”)
of 5f states see[ 84 D 11.
Trot, Suski Land&BBmstein
New Series III:19fl

4 3.1.1 Actinide elements: introduction [Ref. p. 85
Table 2 summarizes the free-ion magnetic properties of the actinides. As seen,s pin-orbit coupling is very large
and of the order of (2...3). IO3 cm-‘. Hence, it is necessary to introduce mixing between various LS states
(intermediate coupling scheme). As one can deduce from Table 2, the departure from the pure Russell-Saunders
ground state, indicated in the correction to the Russel-Saunders g,-factor and thus to the magnetic moment,
becomes important for the 5fS configuration. Also the crystalline electric field is much stronger for the actinides
than for the lanthanides. The detailed treatment of the crystal-field splitting in the presence of a comparable
magnitude of spin-orbit coupling was made by Chan and Lam [74C I].
The relative expansion of the wavefunctions in the solid is reflected in the so-called neutron magnetic form
factor (see Fig. 21). Theoretically, this involves a modification of the radial integrals &), which for U4+ are
plotted in Fig. 3 as a function of scattering vector (see Desclaux and Freeman in [84 D 11). It is also worth
noticing a large contribution of nonspherical terms with k>O to the magnetic form factor. The study of the
coeflicient ck of a given integral (j,) in the expansion of the form factor allows one to study the ground state of
the magnetic ion.
‘TV, I U”‘1HI OF)I I
Fig. 3. Different radial integrals, (h), of the 5f orbital
contributing to the neutron magnetic form factor, f(Q)
(Q=(4n/%) sin0: scattering vector), as a function of
sin O/i. for U4+(5f2), calculated by using the mixed-
configuration Dirac-Fock (MDF) method. The dashed
line represents the angle variation of the experimental
form factor of the U4+ ion in US according to the
equation shown, with coefficients cI: defined by the
electronic configuration of the magnetic ion; for valuess ee
the original paper [76 F 11. For the integrals of the other
actinides on various oxidation states, see tables in
[84 D 11.
Trot, Suski Land&-A6mstein
Neu Series 111:1911
Ref. p. 851 3.1.1 Actinide elements: introduction 5
3.1.1.3 Properties of actinide metals
In the metallic state the orbitals mentioned above broaden into bands and their mutual overlap gives rise to
the hybridization of different kinds of orbitals (i.e., a quantum-mechanical mixing of wavefunctions for various
electronic states).I n pure metals, this effect has also bearing on the symmetry of the crystal structures formed and
the interatomic distance in a given metal.
From the point of view of recent achievementsi n studies of the band structures (seeF reeman and Koelling in
[74 F I]), the actinide metals can be classified into three subgroups:
(1) Th: a typical tetravalent transition metal, through some f character of this metal has been postulated.
(2) Pa, U, Np, Pu: these elements form a new kind of transition seriesw here the 5f electrons are delocalized
into bands and have significant contribution to the metallic bonding in similar way as do the d electrons in the
transition element series.
(3) Am, Cm, Bk, Cf: this trivalent subgroup of metals is characterized by forming energetically fairly sharp 5f
levels, and thus this subgroup can be regarded as a new lanthanide-like series,w ith nonbonding and localized 5f
states.
The most interesting of actinide elements are those placed in subgroup 2. There is much experimental
evidence for the Sf-electron contribution to chemical bonding, such as
(a) occurrence of exotic low-symmetry crystal structures with many allotropies (see Table 3 and Fig. 4),
(b) transition-metal-like dependence in atomic volumes (Fig. 5),
(c) rather complex cohesive properties (Fig. 36),a nd
(d) characteristic optical, electrical and magnetic behaviour (Figs. 64...127).
As shown in Table 3, only the high-temperature crystal modifications have lattices of the samet ype as those of
other metals. High-density structures occurring at lower temperatures are characterized by abnormally short
distances between atoms in certain directions in the crystal. The bonds realized in this type of structures are not
purely metallic and involve a 6d-5f covalent bonding. Unfortunately, the quantitative theoretical understand-
ing of these exotic structures is still insufficient and such ph’asesre sult probably from the low symmetry of f-type
bonding. The linear combination of asymmetric f-orbitals with more symmetric d-orbitals may give rise to highly
directional hybride orbitals, like those of a “sp’‘-type.
The most studied crystal structure among the actinide metals has been the orthorhombic a-phase of uranium,
being stable below 662 “C (Fig. 9). This phase is quite unique and has been found only for Ce and Am (Fig. 31) as
well as for Cm (Fig. 32), Bk (Fig. 33) and Cf (Fig. 33) under high pressure (see[ 87 B 11).V arious anomalies in the
physical properties found for this phase at 43 K has been the subject of numerous investigations. Quite recently
it has been stated [82 M I] that a-U is the first element to develop an incommensurate charge density wave
below this temperature, (see Fig. 19).
As Fig. 26 illustrates, the most complex pressure-temperature diagram is for Pu. Its b-phase has extremely
peculiar properties, being the densestk nown structure with abnormal negative coefficient of thermal expansion
(see Fig. 22).
The atomic volumes, together with the melting temperatures for the actinide metals available up to date, are
shown in Fig. 5, in comparison with a “normal” behaviour of these quantities for the lanthanide metals. It is seen
first a large transition-metal-like decreaseu p to Pu and then a suddenj ump in theseq uantities for Am. It may be
supposed that the values of the atomic volumes of elements beyond Bk follow the lanthanide-like, trivalent
contraction behaviour. The sudden change in this behaviour at Am and the occurrence of the lanthanide-like
crystal structures provide the first experimental evidence for the localization of 5f electrons, starting from Am.
Furthermore, the observed low melting points (e.g., 637 “C for Np and 641 “C for Pu) were associated to
f-electron chemical bonding. This seemst o be in agreement with the complicated angular dependence of the
f-electron wavefunctions. Thus the liquid state permits better bonding than the highly symmetric bee phases
occurring just before melting.
Another direct experimental proof of the localization effect at Am is evidenced by the photoemission studies,
recently reviewed in [84 B 51.T he results of theses tudies are schematically shown in Fig. 58. As seeni n this figure,
the 5f band for Am is shifted down from the Fermi level, E,.
In the past decade and a half, an enormous progress has been made in the theoretical approach to the light
actinide metals, especially in the calculations of their energy bands (seeF igs. 38.. .53). These calculations, made
first of all by Freeman and Koelling (for completenesss ee[ 74 F I]), who used the RAPW (relativistic augmented
plane wave) method, had to overcome several complications like the relativistic and convergence problems.
However, the most difficult problems still exist and are associated with the validity of the band description (a
single-particle approach) in view of the progressive decreasing of the 5f bandwidth, W,, with increasing atomic
number of the light actinides and hencet he increasing role of the Coulomb correlation energy, U,. As known, the
band calculations are based on the one-electron approach and Uc can not be included into the calculations.
Land&-Bhstein Tro6, Suski
New Series IIIjl9fl
6 3.1.1 Actinide elements: introduction [Ref. p. 85
However, the basic result of these calculations is that the physical properties of the light actinide metals are
governed by the complicated “5f6d7s” conduction band, containing all the outermost electrons. In such a
situation any assignment of a valence to a metal becomes pointless.
Most recently Skriver et al. and Brooks (see[ 84 B 4]), using the RLMTO (relativistic linear muffin-tin orbital)
method, have given a better understanding of the light actinide metals by carrying out calculations of their
atomic volume variation and cohesive properties whose results are in good agreement with experiment (see
Fig. 6). Generally, the cohesive energies of the light actinides can be related to those of the 3d transition metals,
whereas for the heavy actinides they can be related to those ofthe lanthanides. For full referencess eeB rooks et al.
in [84B4].
The magnetic properties of lighter actinide metals were recognized many years ago. They are summarized in
Table 4. Unlike the corresponding rare earth metals, the light actinides from Th to Pu are paramagnetic, being
only weakly temperature-dependent (Th, a-U) or almost temperature-independent (Pa, Np, Pu, Am) (seeF ig. 64).
Although there is an increase in the value of x with atomic number, all these values are almost constant, except for
the low-temperature region of Pu and Am. There is also no distinct change in X(T), for example, in the case of Pu
allotropies a, l3 and G(forthe &phase see Fig. 80), the two latter forms being stabilized at low temperatures by a
slight alloying with other metals.
Initially, the observed almost temperature-independent paramagnetism (TIP) for the light actinides
contradicted the actinide concept and rather supported the idea of the development of the 6d electronic shell by
its successivef illing with electrons.
Presently, we know that such magnetic behaviour results from the formation of a hybridized “5f6d7s” band,
which leads to important differences with respect to the normal transition metals, especially apparent in the
transport properties (the 5f electrons are in the conduction band and have essentially no magnetic moment).
In the case of a narrow hybridized 5f-6d band, the magnetic susceptibility can be expressed as follows:
where the terms are the Landau, spin-orbital, orbital, and enhanced Pauli term, respectively, with S being the
so-called Stoner exchange-enhanced factor. A weak temperature dependence of x, observed for some light
actinide metals, likely arises from the thermal broading effect of the electron density of states (EDOS) at the
Fermi level (Er), directly associated with the Pauli term. The absence of magnetic ordering in the sense of
itinerant electron-magnetism (IEM) for the light actinides results from band structures for which the over-all
effective bandwidth, W,,,, becomes too great to produce any magnetization by spin polarization, i.e., the spin-
up and spin-down alignment in the band.
As shown in Fig. 139, one observes a regular increase in x measured at 300 K, and in y determined at T= 0 K,
with increasing atomic number. Since both quantities are linked to N(E,), we can expect a nearly magnetic state
for Np and Pu. However, in the IEM model the transition from the nonmagnetic to the magnetic state is not
abrupt, but is preceded by an intermediate state known as the spin-fluctuation state. This effect is certainly possible
also in the case of some actinide metals, and their compounds, due to a progressive narrowing of the Sf-bands,
when U,,,/W,,,-+l (where Ucrl is the effective intra-atomic Coulomb correlation energy). In fact, however, for Np
and Pu we deal with thermodynamic fluctuations of the magnetic moments that are too short to allow in a direct
way any anomalies in the temperature dependence ofthe magnetic susceptibility, heat capacity, NMR relaxation
time. etc., to be detected.
In a system being nearly magnetic, the measured magnetic susceptibility becomes greatly enhanced by the
Stoner exchange factor S, compared to the Pauli susceptibility, xp,
x=S.xr=[l -I.N(E,)]-’ ‘Xp.
For example, the experimental S values for Np and Pu are in the range of 3...4 (see Fournier and Trod: in
[SS F 11). Usually, the spin fluctuations below the characteristic temperature T,, (= TJS, where TF is the Fermi
temperature of a narrow band) drive x as T2, with the complete saturation at T=O. In the temperature range
above T,, a normal Curie-Weiss-type behaviour is expected. The absenceo f an apparent temperature dependence
of the magnetic susceptibility for the nearly magnetic actinide metals considered here could be accounted for by
the so-called antiferromagnetic-like spin-fluctuation state. However, the most striking anomalies do occur for
Np and Pu in their electrical resistivity curves (S-shaped),a s demonstrated in Fig. 112. As shown in this figure, the
e vs. T curves for Np and Pu are much more anomalous than those for Th, Pa and U. Unlike the latter metals,
they start as a T2 dependence at low temperatures (Fig. 114) and either saturate (for Np) or go through a
maximum (for Pu) at higher temperatures.
It should be also mentioned that the experimental evidence of higher thermopower values Q for Np (at low
temperatures) and Pu (Fig. 123) in comparison to those ofthe other light actinides indicates again the presence of
the spin-fluctuation state in these metals.
Trot, Suski Land&-Bhctein
New Series 111119fl
Ref. p. 851 3.1.1 Actinide elements: introduction 7
Finally, it is worth noting in the description of this subgroup that Th, Pa and a-U (under pressure) are
superconducting materials below z 2 K. On the contrary, the spin-fluctuations suppress these properties in Np
and Pu. At present, it seemst hat in order to produce a possible superconducting phase in these two metals a very
high pressure, at least 200 kbar, would be required.
In conclusion, one can say that the nature of the 5f electrons in the metals Pa through Pu and their effects on
crystal structures, melting points, magnetism and superconductivity are in many ways quite unique in the
Periodic Table.
As we have seen before, Am becomes the first in the actinide series to exhibit the rare-earth-like behaviour.
Unlike europium, it is trivalent and possessesa nonmagnetic 5f6 ground state with J = 0, like that of the divalent
samarium. An excitation level J = I, is at least g 3000 K above the ground state and therefore x is independent of
temperature. The observed low-temperature x upturn (seeF ig. 83) is not yet clear: either it is due to an intrinsic
behaviour or due to some defects. Both a-Am (dhcp) and stabilized P-Am (fee) are superconductors with T, = 0.75
and 1.05 K, respectively (Table 4). However, the Q vs. T curve of Am is concave with respect to the temperature
axis, suggesting that a very narrow 5f level is energetically not far below E, and, as the temperature rises, the
thermal broadening of the 5f level causes an overlap with E,.
The actinide metals beyond Am which are yet available in measurable quantities, namely Cm, Bk and Cf, all
are magnetically ordered at low temperatures, owing to their trivalent lanthanide-like character. As shown in
Fig. 84, Cm in its dhcp phase is antiferromagnetic with TN= 55 K. This behaviour has been confirmed by both
neutron-diffraction and electrical resistivity measurements (Fig. 127). Surprisingly, the latter measurements
(corrected on phonon contribution) have indicated, except for a typical magnetic scattering run of the electrical
resistivity below TN,a In T-dependence above this temperature in agreement with the Kondo theory [85 F I].
Conversely, the fee phase of Cm is ferromagnetic (Fig. 87) with a rather small value of the ordered magnetic
moment (p,=O.4 u&m) but with high T,=205 K. The low value of p, may indicate the occurrence of a
noncollinear magnetic structure.
Recent magnetic data obtained for Bk containing predominantly the dhcp phase have shown that it is also an
antiferromagnet, with TN somewhere between 22 to 35K according to the results of different authors (see
Fig. 89a).
After earlier misleading reports concerning Cf, recent data for the dhcp phase [85 N I] point to its
ferromagnetic behaviour below x 51 K (Fig. 90).
Furthermore, the magnetic susceptibility measurements made for all these three metals (Cm, Bk, Cf) in the
paramagnetic state (seeT able 4) have demonstrated that the Curie-Weiss law is approximately fulfilled with the
effective magnetic moments Pen being very close to those expected for the free-ion values with the f’, f* and fg
electronic configuration (Table 2), respectively. For the references to the above data, see Fournier and TroC in
[85 F I] and more recent review by Huray and Nave in [87 H I].
Specific heat data for the actinide metals are summarized in Table 5 and shown in Figs. 128...138.
The results obtained for the low-temperature electronic heat capacity coefficient y are especially emphasized
in order to associate their values with the electronic structure of the actinide metals.
As Fig. 139 illustrates, this quantity increasesw ith increasing atomic number up to Pu and suddenly decreases
at Am in accordance with expectation.
3.1.1.4 Arrangement of tables and figures
This compilation contains, apart from the main subject, i.e. the magnetic behaviour of actinide metals, also
their relevant properties, like crystal and electronic structure, as well as transport properties and specific heat
data. Knowledge of all these properties is needed for a proper understanding of the unusual place of actinide
elements in the Periodic Table.
The numerical data of the electronic, nuclear and other properties of actinide atoms are shown in Table 1.
The ground-state configurations of the free ions of these elements with corresponding values of their magnetic
moments are given in Table 2. One can then compare these theoretical values with those found experimentally.
Furthermore in Figs. 1.. .3 are visualized some basic behaviour of the above properties.
The remaining data concerning exclusively actinide metals, like crystal and electronic structures, magnetic
and transport ones, and also some basic information on the specific heat measurement results, are all gathered in
Tables 3.~~5a nd Figs. 4...140.
In order to have the possibility to compare given properties for all the series of actinide metals available up to
now, the latter have been described separately. For example, the crystallographic property details, comprising
the structure types with all known allotropic modifications and transition temperatures, densities, lattice and
other physical parameters, and their variation with temperature and pressure, are given in Table 3 and
Figs. 4...36 along the increasing atomic number of the actinide elements.
Land&Biirnstein Tro6, Suski
New Series IIU19fl
Ref. p. 851 3.1.1 Actinide elements: introduction 7
Finally, it is worth noting in the description of this subgroup that Th, Pa and a-U (under pressure) are
superconducting materials below z 2 K. On the contrary, the spin-fluctuations suppress these properties in Np
and Pu. At present, it seemst hat in order to produce a possible superconducting phase in these two metals a very
high pressure, at least 200 kbar, would be required.
In conclusion, one can say that the nature of the 5f electrons in the metals Pa through Pu and their effects on
crystal structures, melting points, magnetism and superconductivity are in many ways quite unique in the
Periodic Table.
As we have seen before, Am becomes the first in the actinide series to exhibit the rare-earth-like behaviour.
Unlike europium, it is trivalent and possessesa nonmagnetic 5f6 ground state with J = 0, like that of the divalent
samarium. An excitation level J = I, is at least g 3000 K above the ground state and therefore x is independent of
temperature. The observed low-temperature x upturn (seeF ig. 83) is not yet clear: either it is due to an intrinsic
behaviour or due to some defects. Both a-Am (dhcp) and stabilized P-Am (fee) are superconductors with T, = 0.75
and 1.05 K, respectively (Table 4). However, the Q vs. T curve of Am is concave with respect to the temperature
axis, suggesting that a very narrow 5f level is energetically not far below E, and, as the temperature rises, the
thermal broadening of the 5f level causes an overlap with E,.
The actinide metals beyond Am which are yet available in measurable quantities, namely Cm, Bk and Cf, all
are magnetically ordered at low temperatures, owing to their trivalent lanthanide-like character. As shown in
Fig. 84, Cm in its dhcp phase is antiferromagnetic with TN= 55 K. This behaviour has been confirmed by both
neutron-diffraction and electrical resistivity measurements (Fig. 127). Surprisingly, the latter measurements
(corrected on phonon contribution) have indicated, except for a typical magnetic scattering run of the electrical
resistivity below TN,a In T-dependence above this temperature in agreement with the Kondo theory [85 F I].
Conversely, the fee phase of Cm is ferromagnetic (Fig. 87) with a rather small value of the ordered magnetic
moment (p,=O.4 u&m) but with high T,=205 K. The low value of p, may indicate the occurrence of a
noncollinear magnetic structure.
Recent magnetic data obtained for Bk containing predominantly the dhcp phase have shown that it is also an
antiferromagnet, with TN somewhere between 22 to 35K according to the results of different authors (see
Fig. 89a).
After earlier misleading reports concerning Cf, recent data for the dhcp phase [85 N I] point to its
ferromagnetic behaviour below x 51 K (Fig. 90).
Furthermore, the magnetic susceptibility measurements made for all these three metals (Cm, Bk, Cf) in the
paramagnetic state (seeT able 4) have demonstrated that the Curie-Weiss law is approximately fulfilled with the
effective magnetic moments Pen being very close to those expected for the free-ion values with the f’, f* and fg
electronic configuration (Table 2), respectively. For the references to the above data, see Fournier and TroC in
[85 F I] and more recent review by Huray and Nave in [87 H I].
Specific heat data for the actinide metals are summarized in Table 5 and shown in Figs. 128...138.
The results obtained for the low-temperature electronic heat capacity coefficient y are especially emphasized
in order to associate their values with the electronic structure of the actinide metals.
As Fig. 139 illustrates, this quantity increasesw ith increasing atomic number up to Pu and suddenly decreases
at Am in accordance with expectation.
3.1.1.4 Arrangement of tables and figures
This compilation contains, apart from the main subject, i.e. the magnetic behaviour of actinide metals, also
their relevant properties, like crystal and electronic structure, as well as transport properties and specific heat
data. Knowledge of all these properties is needed for a proper understanding of the unusual place of actinide
elements in the Periodic Table.
The numerical data of the electronic, nuclear and other properties of actinide atoms are shown in Table 1.
The ground-state configurations of the free ions of these elements with corresponding values of their magnetic
moments are given in Table 2. One can then compare these theoretical values with those found experimentally.
Furthermore in Figs. 1.. .3 are visualized some basic behaviour of the above properties.
The remaining data concerning exclusively actinide metals, like crystal and electronic structures, magnetic
and transport ones, and also some basic information on the specific heat measurement results, are all gathered in
Tables 3.~~5a nd Figs. 4...140.
In order to have the possibility to compare given properties for all the series of actinide metals available up to
now, the latter have been described separately. For example, the crystallographic property details, comprising
the structure types with all known allotropic modifications and transition temperatures, densities, lattice and
other physical parameters, and their variation with temperature and pressure, are given in Table 3 and
Figs. 4...36 along the increasing atomic number of the actinide elements.
Land&Biirnstein Tro6, Suski
New Series IIU19fl
8 3.1.1 Actinide elements: introduction [Ref. p. 85
In turn, the electronic structure information, including such details as the calculated band structure, density
of states, experimentally determined deHaas-van Alphen frequencies and various spectroscopic or optical data,
appears in Table 4 (before the magnetic specifications) and in Figs. 37...63.
Consequently, the magnetic data presentation, included in Table 4, depends on the atomic number of a given
actinide element. For the lighter actinides Th-Am, Table 4a, it is limited mainly to the values of the magnetic
susceptibility at room temperature RT, whereas for the heavier ones Cm Cf the characteristic quantities used in
Table 4(b) are: the effective paramagnetic moment pFlf calculated either from the pure or a modified Curie-Weiss
law, saturation magnetic moment p, based on magnetization studies, or ordered magnetic moment p, found in
neutron-diffraction or Mossbauer effect studies, the paramagnetic Curie temperature 0 and magnetic phase
transition temperatures Tc or TN. All these magnetic quantities have been previously defined in LB, NS
vol.III/12c, subsect. 6.1.0. The magnetically ordered states are characterized by the spin order labelled in
Table 4(b) by F (ferromagnetic), AF (antiferromagnetic) and Fi (ferrimagnetic). The temperature dependences of
the magnetic susceptibility of lighter actinide metals are summarized in Fig. 64, and its reciprocal dependences
for the heavier ones in Fig. 93. Details of the magnetic characteristics in hierarchy ofincreasing atomic number of
actinide metals are subsequently presented in Figs. 65...92. The results shown in these figures are presently
regarded as the best ones, but one should realize that they are far from being final, particularly for the heaviest
actinides, for the reason mentioned at the beginning of this introductory part.
Considerable interest is also attached to the presentation of the transport properties which comprise the
electrical resistivity, magnetoresistivity, Hall effect, thermoelectric power, etc., whose numerical data are also
placed in Table 4. The temperature dependences of these quantities are illustrated in Figs. 94. f .127. For some
cases also the field or pressure characteristics are shown. Some particular transport behaviours of a series of
actinide metals are summarized in Figs. 94, 122, 123, 126 and 140.
Finally, in Table 5 are listed the main characteristics of specific heat measurements, like the values of heat
capacity and entropy at 298.15 K, together with the values of the coeficient of the electronic specific heat at 0 K
and Debye or anomaly temperatures. The most interesting examples of C, behaviour at low temperatures or near
the magnetic transition temperature are displayed in Figs. 128...138. Figures 139 and 140 summarize magnetic,
electrical and specific heat data, as well as transport heat properties, respectively.
Tro6, Suski Land&-B6rnskin
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