Binary Actinide Oxides

227 Pages·1999·3.92 MB·English

by  R. Troc

Ref. p. 11] 2.2.1 Introduction 1
2 Magnetic and related properties of binary lanthanide and
actinide oxides
2.1 Binary lanthanide oxides
(See Subvolume III/27C1)
2.2 Binary actinide oxides
2.2.1 Introduction
2.2.1.1 General
Since the last publication of LBIII/12c containing literature through 1980, (section 6.5: Actinide
compounds with elements of group VI (O, S, Se, Te) by H. L(cid:1)tgemeier, 1982) the knowledge on actinide
oxides has increased largely. The number of figures (22) and references (52) previously presented is
small as compared to those presented in this contribution, only for the oxides i.e. 287 and 450,
respectively. However, in order to keep a continuation in the presentation we have included also some of
the figures already shown in the previous compilation, marking such cases by (LB12) in the survey
tables.
The actinide elements form numerous oxides. There exist several comprehensive descriptions of their
bulk chemical and physical properties, especially in the Gmelin’s series. As the vast number of literature
evidences, there has been in the recent 15 years a great deal of interest in these oxides. Apart from a
tremendous importance of actinide oxides in nuclear technique and technology they have also played a
large role in general solid state research of 5f-type materials. For example, the more localized nature of
the 5f levels in the actinide oxides than in other actinide binary compounds, of course except for the
actinide halides, makes them closer in magnetic and related behaviour to the respective rare-earth oxides
(see the last compilation on the rare-earth oxides edited in LBIII/27C1 (1997) by T.Palewski and
W.Suski). The development of single-crystal preparation and a large number of research tools applied
(especially different kind of spectroscopies) have shed new light on the complex problems connected
with the solid state chemistry and physics of these materials. Also new theoretical approaches have been
successfully applied to various aspects of the electronic structure of actinide oxides.
The first solid materials containing actinide elements that were investigated were oxides, since these
materials have not only been the easiest to prepare, but they have found a wide application in atomic
reactors as a nuclear fuel. This application concerns merely the oxides of thorium, uranium and
plutonium. The quantities available of transplutonium oxides diminish rapidly when progressing across
the series. This is due to their increasing radioactive and short-lived nature, which makes that those
beyond einsteinium can not be at all considered for any solid state work.
Landolt-Börnstein
New Series III/27C2
2 2.2.1 Introduction [Ref. p. 11
2.2.1.2 Chemical stability of binary actinide oxides
In Table 1i are shown the known stable phases of actinide oxides with different stoichiometry. In the
earlier literature several monoxides AnO of the lighter actinides had been reported as a condensed phase,
but later on they were shown to be ternary phases involving nitrogen and/or carbon [68FE]. Despite the
claim of the preparation on the microgram scale of AmO [67A] and BkO [72FPB], their existence has not
yet been quite substantiated. However, the highest potential for the existence of bulk monoxides is still
CfO and EsO, but to date no such an evidence has been reported.
Only three actinides, namely An = Pa, U and Np, form oxides with O/An ratio greater than two. This
means that these three actinides can have an oxidation state higher than +4 in such oxides. UO , in which
3
uranium is in the +6 oxidation state, is the highest oxygen stoichiometry in the binary An-O systems.
All actinides up to Cf are known to form dioxides, AnO , with the fluorite fcc-type structure.
2
Moreover, only UO can readily be obtained as hyperstoichiometric. However, the stability of the
2
dioxides decreases with the atomic number Z. This tendency is well seen in the almost linear decrease in
the known melting points from ThO to PuO . The decreasing thermodynamical stability of heavier
2 2
tetravalent actinide oxides usually limits the ability to prepare such pure phases. In addition, the short-
lived isotopes produce so called daughter products leading to increasing contamination of a given oxide
with another actinide content in the course of time. The least stable dioxides CmO and CfO easily
2 2
evolve to the respective sesquioxides Cm O and Cf O . The formation of CmO , in which Cm has the
2 3 2 3 2
5f6 electron configuration and the absence of such a dioxide for Gd (the most stable electronic
configuration for Cm as for its lanthanide homologue Gd should be just a half-filled f-orbital) show the
difference in energetics of the 5f7 and 4f7 configurations. Furthermore, the preparation of dioxides of
tetravalent Cf (f8) and Tb (f7) turned out to be more difficult than that of tetravalent Cm (f6). Also BkO
2
is much more stable and is formed more easily than TbO , despite the similar half-filled f-states in both
2
cases. Any existence of a tetravalent oxidation state of Es has not been found, so far.
The oxides having O/An intermediate stoichiometries with ratios between 2 and 1.5 are common to
both the heavier actinides and lanthanides forming e.g. the An O phases. The lowest O/An ratio of 1.5
7 12
yields the sesquioxides An O . Apart from Ac O , the first member of the actinide series to form a
2 3 2 3
sesquioxide is Pu. Beginning with this element the stability of the actinide sesquioxides (except for Bk)
tends to increase with Z and they are formed through Es. This tendency is well reflected in their melting
points T . The maximum T is found for Cm O . The less stable sesquioxides of Pu, Am and Bk readily
m m 2 3
take oxygen to become dioxides. The actinide sesquioxides in many cases show close similarities with the
analogous lanthanides, but there are also some significant differences. The similarities and differences of
the two f-series oxides were discussed in more detail by Haire and Eyring [94HE].
The most important problem with the higher actinide oxides is their self-irradiation. With increasing
time of storage this can lead to loss of oxygen in their oxides (change in stoichiometry), but also, e.g., to
the transformation of the C-form of the sesquioxides of Am [77HF] or Cm [70NFD] to the A-form at
ambient temperatures. The high specific activity of some isotopes of higher actinides, like Cm = 242 (a ,
t = 163 d) and Cm = 244 (a , t = 18 yr), causes that their oxides may be rapidly distroyed. This
1/2 1/2
usually swells the lattice and also causes reduction of Cm(IV) to Cm(III).
Landolt-Börnstein
New Series III/27C2
Ref. p. 11] 2.2.1 Introduction 3
Table 1i. The stable oxide phases of actinides indicated by the oxygen (O)/actinide (An) molecular ratio
in the compound.
O/An 3.0...2.5 2.5 2.25...2.0 2.0 2.0...1.5 1.5
Ac - - - - - Ac O
2 3
Th - - - ThO - -
2
Pa - Pa O - PaO - -
2 5 2
U UO ; U O U O UO ; U O UO UO -
3 3 8 2 5 2+x 4 9 2 2–x
Np - Np O - NpO NpO -
2 5 2 2–x
Pu - - - PuO PuO Pu O
2 2–x 2 3
Am - - - AmO - Am O
2 2 3
Am - - - CmO Cm O Cm O
2 7 12 2 3
Bk - - - BkO BkO Bk O
2 2–x 2 3
Cf - - - CfO Cf O Cf O
2 7 12 2 3
Es - - - - - Es O
2 3
2.2.1.3 Preparation technique and phase relations of binary actinide oxides
In general, if there are no special requirements for preparation, the actinide oxides can be obtained in the
process of calcination in air of more complex compounds, like a variety of salts: oxalates, hydroxides,
nitrates etc. For details see e.g. [82H] and [91M].
Several different techniques have been used to grow single crystals of the binary oxides: of Th, U, Np
and Pu, (see e.g. the following references: [62VSSA], [66FC], [81HB], [81NKN], [82V], [84SV] and
[85MS]).
A characteristic feature of the chemistry of actinide oxides is their great tendency to form various
stoichiometries, especially in the case of uranium oxides, as well as a large variety of nonstoichiometric
phases with a wide range of compositions or numerous solid solutions with the same crystal structure.
For example, the hyperstoichiometric uranium oxides, UO , form a fluorite type structure with oxygen
2+x
atoms in excess up to x » 0.25 [82ATT]. The existence of a region of non-stoichiometry on the hypo-
(UO , NpO ) and hyper-stoichiometric sides (UO ) of the composition O/An = 2.00 is explained by
2–x 2–x 2+x
the transition of An4+ to An3+ or U4+ to higher oxidation states of uranium (U5+, U6+), respectively. Also
these dioxides form with various other oxides solid solutions of the M U O type (M = M4+, M3+ or
y 1–y 2+x
M2+; x ‡ 0). Such solid solutions are complete in the case, where M4+ = Th, Np, Pu, Zr and Hf, while
those with M3+ = Sc, Y or R (Ce, Pr, Nd, Gd and Er) and M2+ = Sr or Ca are limited (see a review by
[91FM]). In the cases of M3+ and M2+ this is possible due to the relevant oxidation of U4+ to higher
oxidation states. Phase relations and thermodynamics of all such systems have been extensively examined
in the past, finally leading to the construction of a different kind of phase diagram (see for example:
Gmelin’s Handbook of Inorganic Chemistry, Suppl. Ser., Uranium, C3 (1975), Berlin: Springer, pp.1-
360).
Landolt-Börnstein
New Series III/27C2
Ref. p. 11] 2.2.1 Introduction 3
Table 1i. The stable oxide phases of actinides indicated by the oxygen (O)/actinide (An) molecular ratio
in the compound.
O/An 3.0...2.5 2.5 2.25...2.0 2.0 2.0...1.5 1.5
Ac - - - - - Ac O
2 3
Th - - - ThO - -
2
Pa - Pa O - PaO - -
2 5 2
U UO ; U O U O UO ; U O UO UO -
3 3 8 2 5 2+x 4 9 2 2–x
Np - Np O - NpO NpO -
2 5 2 2–x
Pu - - - PuO PuO Pu O
2 2–x 2 3
Am - - - AmO - Am O
2 2 3
Am - - - CmO Cm O Cm O
2 7 12 2 3
Bk - - - BkO BkO Bk O
2 2–x 2 3
Cf - - - CfO Cf O Cf O
2 7 12 2 3
Es - - - - - Es O
2 3
2.2.1.3 Preparation technique and phase relations of binary actinide oxides
In general, if there are no special requirements for preparation, the actinide oxides can be obtained in the
process of calcination in air of more complex compounds, like a variety of salts: oxalates, hydroxides,
nitrates etc. For details see e.g. [82H] and [91M].
Several different techniques have been used to grow single crystals of the binary oxides: of Th, U, Np
and Pu, (see e.g. the following references: [62VSSA], [66FC], [81HB], [81NKN], [82V], [84SV] and
[85MS]).
A characteristic feature of the chemistry of actinide oxides is their great tendency to form various
stoichiometries, especially in the case of uranium oxides, as well as a large variety of nonstoichiometric
phases with a wide range of compositions or numerous solid solutions with the same crystal structure.
For example, the hyperstoichiometric uranium oxides, UO , form a fluorite type structure with oxygen
2+x
atoms in excess up to x » 0.25 [82ATT]. The existence of a region of non-stoichiometry on the hypo-
(UO , NpO ) and hyper-stoichiometric sides (UO ) of the composition O/An = 2.00 is explained by
2–x 2–x 2+x
the transition of An4+ to An3+ or U4+ to higher oxidation states of uranium (U5+, U6+), respectively. Also
these dioxides form with various other oxides solid solutions of the M U O type (M = M4+, M3+ or
y 1–y 2+x
M2+; x ‡ 0). Such solid solutions are complete in the case, where M4+ = Th, Np, Pu, Zr and Hf, while
those with M3+ = Sc, Y or R (Ce, Pr, Nd, Gd and Er) and M2+ = Sr or Ca are limited (see a review by
[91FM]). In the cases of M3+ and M2+ this is possible due to the relevant oxidation of U4+ to higher
oxidation states. Phase relations and thermodynamics of all such systems have been extensively examined
in the past, finally leading to the construction of a different kind of phase diagram (see for example:
Gmelin’s Handbook of Inorganic Chemistry, Suppl. Ser., Uranium, C3 (1975), Berlin: Springer, pp.1-
360).
Landolt-Börnstein
New Series III/27C2
4 2.2.1 Introduction [Ref. p. 11
Sesquioxides and dioxides
A simple air-ignition of hydroxides, nitrates, carbonates or oxalates of (Ac...Es) leads on the one hand to
Ac O and Es O , similar as is the case for most rare-earth sesquioxides, and on the other hand to the
2 3 2 3
formation of higher oxides for the remaining elements. A further hydrogen reduction of the latter oxides
at higher temperatures finally yields the dioxides of Pa and U and the sesquioxides of (Pu...Cf). The
earlier members of the actinide series from Th to Np do not form sesquioxides. There exist no higher
oxides than ThO in the Th-O system.
2
Ac O crystallizes in the hexagonal-A form, while all the transplutonium sesquioxides (Am...Es) O
2 3 2 3
can be synthesized in all three crystallographic forms, i.e. hexagonal-A, monoclinic-B and cubic-C ones
(see Table 1). Stoichiometric Pu O is known in the A and C forms only. In addition, two
2 3
nonstoichiometric Pu-sesquioxides are also known. They are cubic and have an O/Pu ratio of 1.515 and
1.61. There is not a strong proof for the existence of a pure B phase in Am O . This modification is
2 3
stabilized in this compound probably by rare-earth impurities like Sm O [see 86K].
2 3
The air-stable dioxides of transuranium elements with the fluorite structure can be obtained by an
ignition of the relevant tetravalent (Np and Pu) or trivalent (Am and Bk) salts mentioned above. CmO
2
and BaCmO have been prepared by evaporation of a nitrate solution. Nevertheless, all Cm(IV)-
3
containing materials have also a small contamination of Cm(III)-content. By calcination of Cf materials
in air or oxygen one obtains Cf O . To yield CfO a high pressure of O or an atomic oxygen reaction
7 12 2 2
has to be applied.
Intermediate oxides An O
7 12
Intermediate oxides displaying O/An stoichiometries between sesquioxides and dioxides, are common to
lanthanides and some actinides. Only Cm and Cf form the stable An O (O/An = 1.714) phases, which
7 12
are isostructural with the lanthanide counterparts. Usually these rhombohedral phases are formed from
the cubic form of the relevant sesquioxide or from salts by heating them in air or oxygen followed by
rapid quenching. The absence of such a phase for Pu, Am and Bk is probably due to a fairly high stability
of their dioxides. Even if they are formed under the same conditions they are very easily oxidized to the
dioxide. This absence is somewhat surprising because both the Pu and Bk sesquioxides can take up
oxygen to reach O/An ratios being considerable higher than 1.5 (1.61 and 1.71, respectively) or e.g. the
Bk-dioxide can loose oxygen at high temperatures yielding the O/Bk ratio close to 1.8 but retaining the
fluorite structure (for references see [91M]).
Pentoxides
The air-stable Pa O is usually the product of heating in air or oxygen the Pa-hydrated oxide or its
2 5
peroxides. By various thermal treatments as many as five crystal modifications have been obtained and
reported in literature for Pa O (see [73K]), but only two of them, namely the tetragonal T3-phase and
2 5
hexagonal phase Pa O , being isostructural to U O , have been confirmed. The remaining phases, i.e., the
2 5 2 5
low-temperature cubic or orthorhombic and high-temperature rhombohedral phases have also been
reported but have not been studied in detail (for references see [91M]).
All three phases of U O , i.e. a , b and g have been obtained by applying high-pressure synthesis at
2 5
high temperatures. Only one phase of Np O exists, which is monoclinic. It was obtained by thermal
2 5
decomposition of neptunyl (V) nitrate in the presence of O and of neptunyl hydroxide or ammonium
2
dineptunate in NO atmosphere (for references see [91M]).
2
Higher oxides of uranium
Apart from the An O phases, there is a large number of complex oxides with an O/U ratio between 2 and
2 5
3 merely known for uranium (see reviews: [86W] and Gmelin’s Handbook of Inorganic Chemistry,
Landolt-Börnstein
New Series III/27C2
Ref. p. 11] 2.2.1 Introduction 5
Suppl. Ser. Uranium C3 (1975), C1 (1977), C2 (1978), C4 (1984) and C5 (1986). In this number (see e.g.
the U-O phase diagrams in [70HSG] and [73K]) some of them probably do not exist as a separate phase.
As mentioned above, UO accommodates oxygen up to UO (x = 0.25). To the latter composition
2 2+x
also the U O phase is assigned, which is a superstructure of the UO fluorite structure. A heating of all
4 9 2+x
kind of U-salts in air or oxygen at high temperatures always finally leads to the formation of U O
3 8
(O/U = 2.67), existing in two crystal modifications (Table 1). The a -form is the air-stable uranium oxide,
while the b -form is at high temperature a metastable material, which easily becomes the a -form upon
reheating. There are also reported two other U O forms, namely a trigonal a '- and an orthorhombic
3 8
superstructure-type a ''-U O (for references see [91M]). Another orthorhombic phase a -U O is probably
3 8 3 7
U O (O/U = 2.375) having the lattice parameters b and c doubled comparing to those of the a -form
8 19
displayed in Table 1.
The highest oxide of uranium is UO , usually obtained by using several different methods of
3
preparations, like e.g. by the treatment of lower oxides with high O -pressure, but at relatively low
2
temperatures (e.g. about 300 (cid:176) C). Preparation of amorphous A-UO is reached by heating several
3
hydrates or uranyl oxalate at about 400 (cid:176) C. A-UO easy converts to the a -phase at 470...500(cid:176) C, next to
3
the b -phase at 500...550 (cid:176) C and finally to the g -form at 650 (cid:176) C. By heating b -UO . H O at temperatures
3 2
below 375 (cid:176) C one obtains the d and e -forms of UO . The high pressure h -UO phase is formed at 30 kbar
3 3
and 1100 (cid:176) C (for references see [91M]). For more details about chemical and physical behaviour of all
actinide oxides one is referred to the book: „The Chemistry of the Actinide Elements”, Katz, J. J.,
Seaborg, G. T., Morss, L.R. (eds.), New York: Chapman and Hall, 1986.
2.2.1.4 Electronic structure of binary actinide oxides
An application of high resolution X-ray photoemission spectroscopy XPS and of inverse photoemission
spectroscopy BIS (bremsstrahlung isochromat spectroscopy) has provided the ground work for studying
electron states and electronic properties of the actinide metals and many actinide oxides, where the 5f
states are mainly localized. On the other hand, the XPS technique appears to be very useful for the study
of 5f electrons in establishing their role in chemical bonding. For example, in a study of the valence
bands of UO , U O and UO it was shown [76VRTP] that the localized U 5f peak can change its
2 3 8 3
intensity until it entirely disappears by the final oxidizing of uranium to the hexavalent state in UO . This
3
means that all the outer-most electrons of uranium (5f36d17s2) participate in the covalent metal-oxygen
bond [74VL1, 74VL2]. In other words there is a transfer of all three 5f electrons from a localized
nonbonding state into the covalent bond. An analysis of XPS spectra indicates that such 5f electrons,
which become bonding electrons, give up their 5f electron identity. The XPS of nonbonding 5f-electron
spectra in ionic systems, as are the actinide oxides, may be understood in terms of their final state
multiplet structure. For more details see the review [77VLDH].
Fig. 1i shows XPS spectra for the series of Th...Bk actinide oxides. Three peaks are apparent in each
of the oxides, i.e., An 6p , O 2s and An 6p , respectively. Comparison of the ThO (5f0) and UO (5f2)
1/2 3/2 2 2
spectra provides clear identification of the localized 5f states in the latter as a pronounced peak near E .
F
With increasing atomic number, the 5f spectra become more structured, and their linewidths much
enlarged due to the presence of the multiplets mentioned above.
A systematic theoretical analysis of actinide 4f core XPS spectra in the series of actinide dioxides
ThO ...BkO has been done with the use of the impurity Anderson Model including the exchange
2 2
interaction J between 5f electrons [93KOY]. Among others the charge transfer energy and the covalency
hybridization strength have been considered in this work as depending on the type of the actinide
element. The resulting values of correlation and charge transfer energies have been tabulated.
Landolt-Börnstein
New Series III/27C2
Ref. p. 11] 2.2.1 Introduction 5
Suppl. Ser. Uranium C3 (1975), C1 (1977), C2 (1978), C4 (1984) and C5 (1986). In this number (see e.g.
the U-O phase diagrams in [70HSG] and [73K]) some of them probably do not exist as a separate phase.
As mentioned above, UO accommodates oxygen up to UO (x = 0.25). To the latter composition
2 2+x
also the U O phase is assigned, which is a superstructure of the UO fluorite structure. A heating of all
4 9 2+x
kind of U-salts in air or oxygen at high temperatures always finally leads to the formation of U O
3 8
(O/U = 2.67), existing in two crystal modifications (Table 1). The a -form is the air-stable uranium oxide,
while the b -form is at high temperature a metastable material, which easily becomes the a -form upon
reheating. There are also reported two other U O forms, namely a trigonal a '- and an orthorhombic
3 8
superstructure-type a ''-U O (for references see [91M]). Another orthorhombic phase a -U O is probably
3 8 3 7
U O (O/U = 2.375) having the lattice parameters b and c doubled comparing to those of the a -form
8 19
displayed in Table 1.
The highest oxide of uranium is UO , usually obtained by using several different methods of
3
preparations, like e.g. by the treatment of lower oxides with high O -pressure, but at relatively low
2
temperatures (e.g. about 300 (cid:176) C). Preparation of amorphous A-UO is reached by heating several
3
hydrates or uranyl oxalate at about 400 (cid:176) C. A-UO easy converts to the a -phase at 470...500(cid:176) C, next to
3
the b -phase at 500...550 (cid:176) C and finally to the g -form at 650 (cid:176) C. By heating b -UO . H O at temperatures
3 2
below 375 (cid:176) C one obtains the d and e -forms of UO . The high pressure h -UO phase is formed at 30 kbar
3 3
and 1100 (cid:176) C (for references see [91M]). For more details about chemical and physical behaviour of all
actinide oxides one is referred to the book: „The Chemistry of the Actinide Elements”, Katz, J. J.,
Seaborg, G. T., Morss, L.R. (eds.), New York: Chapman and Hall, 1986.
2.2.1.4 Electronic structure of binary actinide oxides
An application of high resolution X-ray photoemission spectroscopy XPS and of inverse photoemission
spectroscopy BIS (bremsstrahlung isochromat spectroscopy) has provided the ground work for studying
electron states and electronic properties of the actinide metals and many actinide oxides, where the 5f
states are mainly localized. On the other hand, the XPS technique appears to be very useful for the study
of 5f electrons in establishing their role in chemical bonding. For example, in a study of the valence
bands of UO , U O and UO it was shown [76VRTP] that the localized U 5f peak can change its
2 3 8 3
intensity until it entirely disappears by the final oxidizing of uranium to the hexavalent state in UO . This
3
means that all the outer-most electrons of uranium (5f36d17s2) participate in the covalent metal-oxygen
bond [74VL1, 74VL2]. In other words there is a transfer of all three 5f electrons from a localized
nonbonding state into the covalent bond. An analysis of XPS spectra indicates that such 5f electrons,
which become bonding electrons, give up their 5f electron identity. The XPS of nonbonding 5f-electron
spectra in ionic systems, as are the actinide oxides, may be understood in terms of their final state
multiplet structure. For more details see the review [77VLDH].
Fig. 1i shows XPS spectra for the series of Th...Bk actinide oxides. Three peaks are apparent in each
of the oxides, i.e., An 6p , O 2s and An 6p , respectively. Comparison of the ThO (5f0) and UO (5f2)
1/2 3/2 2 2
spectra provides clear identification of the localized 5f states in the latter as a pronounced peak near E .
F
With increasing atomic number, the 5f spectra become more structured, and their linewidths much
enlarged due to the presence of the multiplets mentioned above.
A systematic theoretical analysis of actinide 4f core XPS spectra in the series of actinide dioxides
ThO ...BkO has been done with the use of the impurity Anderson Model including the exchange
2 2
interaction J between 5f electrons [93KOY]. Among others the charge transfer energy and the covalency
hybridization strength have been considered in this work as depending on the type of the actinide
element. The resulting values of correlation and charge transfer energies have been tabulated.
Landolt-Börnstein
New Series III/27C2
6 2.2.1 Introduction [Ref. p. 11
In the past, the most frequently studied material by different photoemission techniques was, of course,
UO . In a simple ionic model the 5f2 valence states corresponding to the U4+ ion are located in a clear gap
2
between E (see Fig. 49) and a filled oxygen 2p band. However, any realistic model for UO should
F 2
include both the hybridization between the uranium 5f and oxygen 2p electrons and the 5f Coulomb
interaction, U [92A]. Thus, the most appropriate model which can successfully describe the real UO
ff 2
valence and 4f-core level spectra appears to be the impurity Anderson Hamiltonian [88GSHS]. In this model
the ionic groundstate is corrected by including an admixture of electrons transferred from oxygen to uranium
atoms. This admixture can be simply presented by the configurations: f2+np6–n, where n = 0, 1 or 2. The
weights of the 5f2, 5f3 and 5f4 configurations in the ground state were found to be 0.85, 0.14 and 0.01,
respectively. According to good agreement with experiment on UO (Fig. 49), the above hybridization
2
effect resulted in that the main 5f PES peak appeared to be shifted by about 2.5 eV to a smaller binding
energy, its spectral weight is spread over the p-band region and the 5f BIS peak is shifted to a higher
energy by about 1 eV compared to the single-valence model [80BS], where the final-state multiplet
spectra are described by simple 5f2fi 5f1 and 5f2fi 5f3 transitions in LS coupling.
The electronic structure of actinide oxides is essentially that of an ionic insulator. Nevertheless,
experimental spectroscopic evidences as well as theoretical approaches of the most studied actinide
oxide, UO , point to some covalent effect in its chemical bonding. In such a situation, where an effective
2
overlap of the participating atomic orbitals of similar energy takes place, somewhat different energy
levels than in atoms (or ions) are formed. For example, molecular orbital cluster calculations by Gubanov
et al. (see references in a review [82E]) have allowed the authors not only to present the molecular level
structure of UO , but also to demonstrate some f-p and d-p covalency contributions to the total chemical
2
bonding in UO . The latter contribution in the form of a strong U 6d-O 6p hybridization has also been
2
derived in the band structure calculations [83BK].
The latter calculations, e.g. in the case of UO were based on the density functional theory (DFT)
2
within the local spin density approximation (LSDA) [87KB, 92B, 96PMLP]. This allows one to include
only to some extent electron-electron correlation. However, these conventional calculations incorrectly
predict a non-magnetic metallic ground state in UO . Since UO orders antiferromagnetically at low
2 2
temperatures [85FT] we deal here with strong electron-electron correlations. Very recently Dudarev et al.
[97DNS] have included to LSDA an additional term U that describes the Hubbard on-site repulsion
between the 5f electrons. This results in a large modification of the Heitler-London type of hybridisation
between the 5f orbitals and gives rise to a better agreement with experimental observations.
Techniques, which are useful for elucidating electronic structure of actinide materials, are at present
very numerous. Some of the most powerful experimental methods for electronic structure studies, which
should be mentioned here are those based on the X-ray absorption near-edge structure (XANES)
measurements. (For calculated 5d- X-ray absorption spectra of trivalent actinide ions and for Th4+ see
e.g. [93KOY]). A large number of experimental results obtained by means of all kind of techniques
available at present are thus presented in this compilation. These techniques have recently been reviewed
thoroughly by L. Manes (Gmelin’s Handbook of Inorganic Chemistry, Suppl. Ser., Uranium, C5, (1986)
(Springer, Berlin), Chapter 6, pp. 229-274). The above review, which extensively treats the optical and
spectroscopic properties of UO and UO (U O ), gives a rather complicated picture of the electronic
2 2+x 4 9
structure, which is not yet quite understood. The recent XANES data and LSDA + U approach have
shown that UO can be classified as a Mott-Hubbard insulator with an f-f gap [97JPGT].
2
Landolt-Börnstein
New Series III/27C2
Ref. p. 11] 2.2.1 Introduction 7
An O
X
An=Th
Am
U
I I
y y
nsit nsit
nte Np nte Cm
I I
Pu Bk
E E
F F
40 30 20 10 0 40 30 20 10 0
a Binding energy E [eV] b Binding energy E [eV]
b b
Fig. 1i. AnO . XPS spectra for thin films of actinide E are caused by 5f-electron excitations. Up to Pu the
x F
oxides (Th...Bk) within » 40 eV of the Fermi level spectra represent dioxides; beyond Pu the stoichiometry
taken at (cid:1)w = 1486.6 eV. The prominent features near is more close to the sesquioxides (An O ) [77VLDH].
2 3
2.2.1.5 Magnetic and related properties of binary actinide oxides
Fig. 2i shows the effective magnetic moment as a function of An ion configuration for actinide oxides.
They are compared with the so-called free-ion moment values, i.e. for cases where the applicability of LS
coupling (Russell-Saunders) and Hund’s rules are true. As seen from this figure, except for PaO , PuO
2 2
and CmO , the remaining actinide oxides have experimentally determined paramagnetic moments in
2
reasonable agreement with the free-ion values. Of course, the best agreement is found for the
transplutonium sesquioxides or for the An O phases, which are the rare-earth-like compounds.
2 7
A less good fit is found for the light-actinide oxides due to rather large crystal electrical field (CEF)
interactions and to the well known fact that the 5f electrons of the earlier actinides have energies in close
proximity to the 6d and 7s electrons and therefore the crystal field interactions cause a larger perturbation
also to the spin-orbit interactions. As a result, the mixing of higher J' states into the ground state J takes
place, which finally leads to the so-called JJ' intermediate coupling (I.C.) (see Table 2i, where the free
ion values of magnetic moments in both cases: LS and I.C. coupling schemes are compared).
Landolt-Börnstein
New Series III/27C2
Ref. p. 11] 2.2.1 Introduction 7
An O
X
An=Th
Am
U
I I
y y
nsit nsit
nte Np nte Cm
I I
Pu Bk
E E
F F
40 30 20 10 0 40 30 20 10 0
a Binding energy E [eV] b Binding energy E [eV]
b b
Fig. 1i. AnO . XPS spectra for thin films of actinide E are caused by 5f-electron excitations. Up to Pu the
x F
oxides (Th...Bk) within » 40 eV of the Fermi level spectra represent dioxides; beyond Pu the stoichiometry
taken at (cid:1)w = 1486.6 eV. The prominent features near is more close to the sesquioxides (An O ) [77VLDH].
2 3
2.2.1.5 Magnetic and related properties of binary actinide oxides
Fig. 2i shows the effective magnetic moment as a function of An ion configuration for actinide oxides.
They are compared with the so-called free-ion moment values, i.e. for cases where the applicability of LS
coupling (Russell-Saunders) and Hund’s rules are true. As seen from this figure, except for PaO , PuO
2 2
and CmO , the remaining actinide oxides have experimentally determined paramagnetic moments in
2
reasonable agreement with the free-ion values. Of course, the best agreement is found for the
transplutonium sesquioxides or for the An O phases, which are the rare-earth-like compounds.
2 7
A less good fit is found for the light-actinide oxides due to rather large crystal electrical field (CEF)
interactions and to the well known fact that the 5f electrons of the earlier actinides have energies in close
proximity to the 6d and 7s electrons and therefore the crystal field interactions cause a larger perturbation
also to the spin-orbit interactions. As a result, the mixing of higher J' states into the ground state J takes
place, which finally leads to the so-called JJ' intermediate coupling (I.C.) (see Table 2i, where the free
ion values of magnetic moments in both cases: LS and I.C. coupling schemes are compared).
Landolt-Börnstein
New Series III/27C2