- Email: [email protected]
Surface analysis of actinide materials
Journal of Nuclear Materials North-Holland, Amsterdam
SURFACE
59
166 (1989) 59-67
ANALYSIS
OF ACTINIDE
MATERIALS
J.R. NAEGELE Commission
of the European Communities,
D-7500 Karlstuhe,
Joint Research Centre, Institute for Transuranium Elements, Postfach 2340,
Fed. Rep. Germany
The application of highly surface sensitive electron spectroscopic methods to the determination of electronic structures and of the chemical composition of highly u-active solids is described. Selected results from photoelectron spectroscopy on UMn, and on actinide metals from Th to Am demonstrate the identification of the quantum character of valence electrons and the transformation of bandlike 5f electrons for a-f+ into localized Sf electrons for Am. For more applied purposes the high surface sensitivity of photoelectron spectroscopy was used to characterize surfaces during oxidation and gas adsorption. Characteristic examples are: (1) the oxidation of Np metal, for which it is shown that a boundary layer between the bulk Np metal and the final covering dioxide surface layer is composed of Np,O,, a phase that does not exist as bulk material; (2) the interaction of CO with the surface of clean U metal and UNi,.
The perspectives of combining basic research on electronic properties, chemical bonding etc. and characteristic surface studies in the field of actinides are described.
1. Introduction Highly surface sensitive electron spectroscopic methods as photoelectron spectroscopy (XPS: X-ray Photoelectron Spectroscopy; UPS: Ultraviolet Photoelectron Spectroscopy; ESCA-identical with XPS-: Electron Spectroscopy for Chemical Analysis), Auger Electron Spectroscopy (AES), Electron Energy Loss Spectroscopy (EELS) etc. have been applied in the last two decades intensively to the determination of the electronic structure and chemical composition of solid surfaces. This is largely due to the very straightforward way of analysis and the availability of several commercial spectrometers working under ultrahigh vacuum conditions (p < 1 X 10ms Pa). At the European Institute for Transuranium Elements (EITU) an electron spectrometer has been modified to meet safety and vacuum requirements for measurements of highly radioactive actinide materials [l]. Electron spectroscopies have been presented in many review articles and books [2-41. Therefore only a short description of the different spectroscopic techniques and their application to the work with actinide materials is given here. The study of the electron structure of actinides and their compounds was the original aim. Therefore specific results from photoelectron spectroscopy are selected to demonstrate the uniqueness of the actinide series in the
0022-3115/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
periodic table. Finally, results related to surface properties, e.g. surface reactivity complete the presentation of achievements.
2. Spectroscopic methods and experimental realization Electron spectroscopies are generally based on the detection of electrons (e-) in terms of their intensity, kinetic energy and angle distribution excited by different particles, e.g. electrons (e-), photons (hv) etc. from a sample surface under vacuum: Excitation:
e-,
hv, I+ -+ Detection:
e-,
I+.
Positive ions (I+) have been included here because many commercial spectrometers nowadays permit to run the detection system for positive particles and therefore prepare electron spectrometers for Ion Scattering Spectroscopy (KS). The following spectroscopic methods are employed at the EITU: A. Photoelectron Spectroscopy (XPS, UPS): The kinetic energy of the photoemitted (E,,,) is given by:
hv + eelectrons
Ekin = hv - E, - E,,,
wherin
B.V.
hv is the energy
of the exciting
photon,
E,
the
60
J. R. Naegele / SW-me analysts
binding energy of electrons in the solid under investigation and E,, the work function. Since the binding energy E, of core electrons is characteristic for each element, this technique is used for chemical analysis (ESCA). Electrons taking part in the chemical bonding, i.e. valence and conduction electrons, can be detected as well in XPS and UPS. Thus this technique permits the determination of the electronic structure (type of chemical bonding, density of occupied electronic states). The quantum character of the electrons is identified from the characteristic variation of the photoelectron excitation cross section with the energy of the exciting photons. B. Auger Electron Spectroscopy (AES) : e- -+ e- or hv +eElectrons or photons (not necessarily monoenergetic) produce a hole in an inner electron shell by ionization. This hole is filled by an electron from an energetically higher shell. The energy released in this process is used to eject a second electron, the Auger electron. The kinetic energy of this Auger electron is characteristic for the element. Since the electronic structure of the material under investigation only weakly influences the Auger emission, this technique is mainly used for chemical analysis. C. Electron Energy Loss Spectroscopy: em + em Monoenergetic electrons irradiating the sample loose energy by different processes: 1. electronic transitions from occupied to unoccupied states (similar to optical spectroscopy), 2. collective electron excitations, i.e. plasmons, 3. phonon excitations. Processes 1 and 2 can be observed at high electron excitation energies and relative poor monochromatization whereas process 3 because of the small phonon energy can only be observed at low excitation energy (some eV) and best monochromatization (- 5 mev). Due to the poor monochromatization (- 0.5 eV) and small excitation energy range (50-500 eV) of the electron source, originally used at the EITU for the compensation of charging effects particularly occurring in photelectron spectroscopy, mainly process 2 is studied. This gives complementary information on the electronic structure. D. Ion Scattering Spectroscopy (ISS) : I+ + I+ The primary, usually positive rare gas ions are scattered by the sample surface and carry after the collision with the surface atoms the information about the mass of these atoms. Thus, information on the
ofactmide materials
l-
t ,cus
,
5353 cus
ii+
TP
I
11SP
TP
J
L_-----_---------
Fig. 1 Scheme of the electron ACh EA EMP XRS ES uvs
Analyzing chamber Electron energy analyzer Electron multiplier X-ray source Electron source uv source
spectrometer.
S GB
sample Glove box
PCh SR TP TiSP
Preparation chamber Sample rod Turbo pump Titanium sublimation pump
chemical composition is obtained. At present, this technique is in preparation at the EITU. A common feature of the spectroscopies given above is the extreme surface sensitivity of 0.5 to 2.5 nm, i.e. 1 to 5 monolayers. Thus ultrahigh vacuum (p < lop8 Pa, requires baking of the entire apparatus to 200 o C) and “in-situ” sample surface preparation are obligatory. These facts impose various problems on the application of these techniques to highly radioactive materials since the sample cannot be encapsulated. To meet safety requirements while keeping ultrahigh vacuum conditions the following concept based on a commercial spectrometer and shown as scheme in fig. 1 has been realized (a more detailed description is given in ref. [l]): _ The preparation chamber (PCh) was redesigned to achieve a pressure better than lo-* Pa and to allow for baking to 200 o C in a closed glove box system. _ The analysis chamber (ACh) with excitation sources (XRS, ES, UVS) and the detection system (EA, EMP) as well as the UHV pumping systems are mounted
J. R. Naegele
/ Surface analysis of aciinide
into open gfove box frames. Only in case of repair requiring an opening of the vacuum system the glove boxes have to be closed. - The “in-situ” sample surface preparation is solely performed in the PCh that takes up all the radioactive waste from the cleaning process. Afterwards, the sample is transferred via a lock system to the ACh just for the measurements. This way of operation keeps the radioactive contamination level low in the ACh. Fig. 2 shows a photograph of the modified spectrometer that offers the following spectroscopic measurements on highly radioactive materials: _ Auger electron spectroscopy; excitation either by electrons (0.5-5 keV) or X-rays (Mg and Al anodes with 1253.6 and 1486.6 eV, respectively);
materials
_ Electron
61
energy loss spectroscopy (So-500 eV, resolution A E = 0.5eV); _ Angle integrated photoelectron spectroscopy with Xray (XPS) and UV-light (UPS) excitation (XPS: 1253.6 and 1486.6 eV; UPS: energies of 16.7 (Ne I), 21.2 (He I), 40.8 (He II), and 48.4 (He II *) provided by a rare gas discharge source). The ISS technique is currently under test on a second cold machine (i.e. for non-radioactive materials): it is considered to give ~mplementary information on the surface composition because of its sensitivity to just the topmost surface atoms. The performance of the spectrometer is demonstrated in fig. 3. The high counting rate at high resolution of 45 meV in UPS with its high surface sensitivity is of particular importance since it permits short record-
Fig. 2. Photograph of the spectrometer (same view as for fig. 1)
J. R. Naegele / Surface analysis of actinide materials tion of A E = 20 meV thus demonstrating
that
the mea-
meV is mainly caused by the temperature broadening; a modification of the cooling system to use liquid He instead of liquid N, is presently under test. sured
half
width
of gold
at
T=
80
K
of 45
3. Selected results and discussion Originally, electron spectroscopy was dedicated at the EITU to the investigation of electronic properties of actinide containing materials, in particular to elucidate the contribution of the actinide 5f electrons to the chemical bonding. The basic question was: Are the 5f electrons in actinides localized, i.e. not taking part in the chemical bonding, as the 4f electrons in the lanthanides, or are they delocalized, i.e. taking part in the chemical bonding like the d electrons in the transition metals? Photoemission was particularly considered to be best suited to answer this question since this technique gives quite a straight forward view to the electronic states. Thus the first part of this section will deal with the clarification of the electronic structure of actinides. Because of the high surface sensitivity of the spectroscopic techniques the investigations were later extending more and more into applied areas dealing with specific surface effects, e.g. corrosion, surface reactivity etc. This aspect of the work is presented in section 3.2. 3. I. Electronic properties
89
I
I
06
81
I 66
BINDING
85
ENERGY,
04
03
eV
Fig. 3. Performance of the electron spectrometer. (a) XPS (Mg K,: 1253.6 ev): Gold 4f core levels; photoelectron pass energy for the energy analyzer and recording time are: curve 1: 20 eV, 10 min; curve 2: 5 eV, 20 min. (b) UPS (HeI: 21.2 eV): Gold at T= 80 K; photoelectron pass Fermi edge, E, recorded energy for the energy analyzer, recording time and maximum emission intensity in the valence band are: curve 1: 5 eV, 10 min., 4 X 10’ counts/s; curve 2: 2 eV, 15 min., 5 X lo4 counts/s.
ing times
for chemically
highly
build-up
of
contamination
UPS
chemical
measurements
reactive
on Ar gas reveal
materials on
the
a machine
without surface.
resolu-
From a viewpoint of electron spectroscopy, recently the electronic properties of actinides have been reviewed [5,6]. As explained above the contribution of actinide 5f electrons to the chemical bonding is of main interest. Energy dependent photoelectron spectroscopy is a very appropriate tool to identify this contribution because of the characteristic photoexcitation cross section of f states: simply speaking, the 5f excitation probability is growing strongly with increasing photon energy in the UV range compared to the cross section of s, p and d states. This has been nicely demonstrated in the case of UO, that has a nearly pure localized U 5f state at about 1.5 eV below the Fermi energy E, and a mainly oxygen derived 2p valence band extending from 4 to 9 eV below E, [6]. Recently, quantitative photoexcitation cross section calculations have been presented for a wide range of excitation energies [7]. These cross sections, modified to account for resonant effects at the 3p 3d threshold energy (- 50 ev) that have not been included in the
63
J. R. Naegele / Surface analysis of actinide materials
calculation, have been used for a more quantitative analysis of photoelectron conduction band spectra of UMn, [8] shown in fig. 4. Due to the increase of emission intensity at E, (structure C) with photon excitation energy, the U5f states are identified at E, and are delocalized, i.e. they are taking part in the chemical bonding. The other structures A and B have been identified in a similar way to be dominated by Mn 3d and U 6d states, respectively. This interpretation confirms a previous analysis on the basis of synchrotron light-induced photoelectron spectra (91.
V=
LO.8 eV (LIPS).;; ,’
Am
d,.,A”‘s
‘I ocalized 5f states “multiplet” lelocalized
5f states
A.\-
Th
Jr’
,o occupied 5f states ,lIII 1 LI 9
5 BINDING
ENERGY
L 11 E, (ev)
Fig. 5. UPS conduction band spectra for actinide Th to Am for 40.8 eV excitation.
I
L
.
.
BINDING
.
.
I
,
EF=O
5 ENERGY
(eV)
Fig. 4. UPS conduction band spectra for UMn, recorded at T = 80 K for photon energies of 16.7, 21.2, 40.8, and 48.4 eV (curves a, b, c, d, respectively).
I
EF=O
metals
from
The electronic structure of actinide metals from Th to Am has been investigated in a similar way by using energy-dependent photoemission spectroscopy. Fig. 5 shows the conduction band spectra of Am [lo], a-Pu [lo], Np [ll], U [12] and Th [13] for 40.8 eV excitation. Th is known to have no occupied 5f states and thus shows a structured but low emission intensity extending to E,; the observed structures are maxima in the density of Th 6d and 7s electron states. The spectra for U, Np and a-Pu are similar to each other and show a characteristic strong increase of emission intensity up to E, that has its origin in 5f states. Since these 5f states are found to peak at E,, they must take part in the electrical conduction, i.e. they take part in the chemical bonding, they are delocalized. The appearance of the conduction band spectrum changes completely when reaching Am. The strong 51 emission at E, as found for U through a-Pu is missing. Instead, a triplet structure is found around 2.5 eV below E, and clearly identified to have 5f character whereas the emission at E, is dominated by Am 6d electrons [lo]. This behaviour is characteristic for localized f electrons. Fig. 6 makes this even clearer by presenting the spectra of a-Pu and Am together with the one for Sm recorded at the EITU on
J. R. Naegele / Surface anaiys~sofrrctinide mairrruls
64
I
’
’
1~~40.8
’
’
I
’ .+-, I ’
eV (UPS1
/
i .
’
I
:
: k!
I I
A modelling of the experimental conduction band shape measured for Am [fig. 61 based on a theoretical multiplet calculation [15] evoked a contribution from divalent surface atoms with a localized 5f’ state [16]. Very recently photoemission studies on binary Am, Pu , __: alloys [17] have demonstrated the gradual increase of Pu 5f electron localization with decreasing Pu content in the binary alloy. 3.2. Surface properties The high surface sensitivity of electron spectroscopy has been used to investigate the oxidation of U [18.19], Np[ll] and (Y-Pu [6]. U and a-Pu form on the surface only layers of those oxide phases that are also existent as bulk phases: U builds up on its surface solely the dioxide UO, whereas a-Pu forms either the sesquioxide Pu,O, or the dioxide PuO, depending on the experimental conditions, e.g. sample temperature, oxygen par-
BINDING
ENERGY
Eg (eV)
Fig. 6. UPS conduction band spectra for Sm, a-Pu and Am for 40.8 eV excitation.
I
hv.
1253.6
Np 4f
Sm films evaporated under ultrahigh vacuum conditions. Sm is quite a useful metal to compare with Am and Pu because it is a related 4f electron system having localized 4f electrons. The 4f electron occupation number of the trivalent Sm is 5 for bulk f states (4f ‘) as also predicted for a-Pu (5f5); in addition, Sm has in its topmost surface layer divalent atoms that have 6 electrons in the 4f shell (4f 6), the same number of electrons as expected in the 5f shell of bulk Am. As seen in the spectrum of Sm (fig. 6) the localization of 4f electrons results in a characteristic set of structures, the so-called “multiplet”. The difference between cu-Pu and Am is striking: a-Pu has just an increasing emission intensity peaking at E, and shows no structures comparing to the 4f 5 --* 4f4 multiplet of Sm, whereas Am shows a similar triplet structure (“multiplet”) as the 4f6 + 4f 5 multiplet of Sm. This is an unambiguous experimental proof of the transition from delocalized 5f electron behaviour in (Y-Pu to localized behaviour in Am as previously predicted from theoretical considerations [14]. This transition is also clearly visible in the change of the shape of the 4f core levels of a-Pu and Am measured by XPS [lo]. Thus Am is the first lanthanide like metal in the actinide series.
eV (XPS)
(NpO,
)
NOT EXISTENT AS BULK !
Np 4f ( Np -metal
)
a I 1I40
I 420 BINDING
ENERGY
t 400
L
EB (eV1
Fig. 7. XPS 4f core level spectra (hv = 1253.6 eV) for increasing surface oxidation of Np metal: (a) clean metal, (b), (c) intermediate oxidation, (d) final oxidation.
J. R. Naegele / Surface analysis of actinide materials
hv
hv
hv
e-
Fig. 8. Scheme of the surface oxidation steps of Np metal corresponding to curves a, b, c, and d of fig. 7.
tial pressure etc. In contrast to U and Pu, Np behaves in quite a different way. From phase studies of the Np-0 system [20] the formation of a NpO, surface layer is expected upon oxidation of Np metal because it is the only stable oxide phase, i.e. Np should behave similar as U metal [18,19]. In contrast to the expected behaviour, Np forms in its initial oxidation stage a thin layer of Np,O, that does not exist as bulk phase. Only upon further oxidation NpO, is formed on the surface. The formation of a Np,O, boundary layer between Np metal and NpO, is unambigously observed in the XPS 4f core level spectra shown in fig. 7. In addition to the lines for clean Np metal (curve a) two main distinct emission signals appear at higher binding energy. The main lines in curve d are shifted by about 3.9 eV and
65
represent clearly the formation of NpO, because of the characteristic actinide dioxide signal with satellites at 6.8 eV higher binding energy, as also found for bulk UO,, NpO, and PuO, [6]. Since the value of the energy shift (“chemical shift”) of core lines measured by XPS is directly related to the oxidation state, the additional lines occurring in curves b and c of fig. 7 in between the dioxide (curve d) and metal (curve a) line must be due to a lower oxide than NpO, and are therefore attributed to Np,O,. This picture as schematically given in fig. 8 is also confirmed by UPS valence band spectra [ll] which show with the increase of the oxygen 2p signal around 6 eV binding energy two distinct 5f electron derived structures at 1.5 and 3 eV binding energies being attributed to the formation of Np,O, and NpO,, respectively. Electron spectroscopic studies have been performed to investigate the adsorption of H,, CO, CO, and C,H, gases on U [18] and UFe, [21]. As an example for this type of study, photoelectron spectra (UPS, 40.8 eV excitation energy) for the interaction of CO with the surface of clean U metal and UNi, at T = 75 K [22] are presented in fig. 9. The results show quite different adsorption mechanisms for U and UNi,. In the case of U metal (fig. 9a) increasing CO dosage (1 L = 10m6 mbar s., e.g. 10 s at a pressure of lo-’ mbar) results in a growth of the 0 2p emission line at about 6 eV binding energy. This is a clear indication of CO dissociation and the formation of UO,; a formation of additional UC,O,_, might be possible, too. At a dosage of about 8 L saturation occurs. Beyond 8 L three additional emission lines appear with intensities and an energy separation as found for gaseous CO. These lines are therefore attributed to 40, la and 5a orbitals of physisorbed CO. Chemisorption was never observed. In the case of UNi, (fig. 9b) CO adsorbs at low dosage dissociatively as indicated by the growth of the 0 2p emission. But in contrast to U metal, additional lines appear which are characteristic for chemisorbed CO. At higher dosage lines for physisorbed CO appear, similar to the case of U metal. From these results it is concluded that CO is dissociated and partially chemisorbed resulting in an oxidation of the U atoms, i.e. the formation of UO, and the breaking of the U-Ni bonds with the formation of metallic Ni particles in an UO, matrix.
4. Perspectives The actinide series, as shown above, exhibits many interesting aspects, not only from an electronic but also from a surface structure point of view. The uniqueness
J.R. Naegele / Surface analym
66
I’.‘.!“’ ,y;
I’.‘.I.
CO
phys.
u
~
u 5 f r-
@ >,.
1.
ofactrnidemaferials
‘.
.
1”’
*
IO
15
BINDING
ENERGY
I”*
*”
5
Efz=O
, .
‘.
1.
‘.
15 BINDING
(eV)
.
1..
. . 1..
10 ENERGY
s
. 1.
EF= 0
(eV)
Fig. 9. UPS conduction band spectra (h Y= 40.8 eV) for CO adsorbed on U (a) and UNi, (b) at T = 75 K.
of the actinides is nicely demonstrated by a modified periodic table [23] for elements with unsaturated electron shells as shown in fig. 10. The actinides form the only series of elements that shows the transition from electron delocalization (“Bonding”) to localization (“Magnetic Moments”) around halffilling of the unsaturated shell. This crossover has been experimentally clearly demonstrated to occur between cr-Pu and Am
Mugnetic
partially
filled shell
Moments
full
Cd 5d
Bal Lu
Hf /
Ta
W Re OS Ir
Pt \Au Hg
Bonding Fig. 10. Modified periodic table of elements with uncompletely filled electronic shells (ref. [23]). The dashed region represents the crossover between delocalized (“Bonding”, electrons take part in the chemical bonding) and localized electron character (“Magnetic
Moments”,
electrons
form magnetic
moments).
(figs. 5 and 6). Therefore these elements are very suitable to study basic electronic properties, e.g. electron localization effects and their influence on other physical properties. To study localization effects in more detail electron spectroscopic measurements on systems like S-Pu, Pu,Am,_, and selected compounds of Pu and Am are of particular interest and will be therefore continued. On the other hand, the technique of electron spectroscopy will be used more extensively in applied research areas. The modified periodic table (fig. 10) contains many elements that are of importance in catalysis, e.g. CO, Ni, Pd, Pt etc. The actinides with their unique electronic structure that even permits a controlled variation of electronic properties might therefore be considered to be used as model substances to study basic correlations between electronic structure and surface activity including catalytic activity. Catalytic studies on actinide containing materials have already been started at the EITU in a catalytic flow reactor, on U-Ni and U-Fe systems. Adsorption studies described above show on the other hand that the surface of these materials is easily decomposed into metallic Ni or Fe particles in an UO, matrix. This means that the catalytic activity is mainly due to metallic Ni or Fe. Therefore one has to extend these studies to actinide containing systems which produce metallic actinide particles in the surface. Th,Am, and La,PuY are potential candidates. The high surface sensitivity could be used in the future to analyze the surfaces of actinide containing materials. Corrosion effects of nuclear fuels. interac-
J. R. Naegele / Surface analysis of actinide materials
between the fuel and its cladding, leaching of glasses for nuclear waste disposal, dissolution properties of oxide fuel elements [l] etc. are considered to be subjects of future electron spectroscopic studies. tions
Acknowledgements The excellent assistance of Mr. H. Winkelmann, who took part in the initial construction of the electron spectrometer for a-active materials, of Mr. N. Nolte, Miss F. Schiavo and Dr. T. Gouder is gratefully acknowledged.
References [l] J.R. Naegele, J. Phys. (Paris) 45, C2 (1984) 841. [2] Electron Spectroscopy for Surface Analysis, Ed. H. Ibach, Topics in Current Physics, Vol. 4 (Springer Verlag, Berlin, 1977). [3] Electron Spectroscopy: Theory, Techniques and Applications, Eds. C.R. Brundle and A.D. Baker, Vols. l-4 (Academic Press, London, 1977). [4] T.A. Carlson, Photoelectron and Auger Spectroscopy (Plenum Press, New York, 1975). [5] Y. Baer, Handbook on the Physics and Chemistry of the Actinides, Eds. A.J. Freeman and G.H. Lander, Vol. 1 (North-Holland, Amsterdam, 1984) p. 271. [6] J.R. Naegele, J. Ghijsen and L. Manes, Actinides - Chemistry and Physical Properties, in: Structure and Bonding Ed. L. Manes, Vols. 59/60 (Springer, Berlin, 1985) p. 197.
61
[7] J.J. Yeh and I. Lindau, Atomic Data and Nuclear Data Tables 32 (1985) 1. [8] J.R. Naegele, V. Sechovsky and L. Havela, to be published. [9] J.R. Naegele, J. Ghijsen, R.L. Johnson and V. Sechovsky, Phys. Scripta 35 (1987) 877. [lo] J.R. Naegele, L. Manes, J.C. Spirlet and W. Mtiller, Phys. Rev. Lett. 52 (1984) 1834. [ll] J.R. Naegele, L.E. Cox and J.W. Ward, Inorg. Chim. Acta 139 (1987) 327. [12] T. Gouder and J.R. Naegele, unpublished results. [13] J.R. Naegele, T. Gouder and D.D. Sarma, to be published. [14] H.L. Skriver, O.K. Andersen and B. Johansson, Phys. Rev. Lett. 44 (1980) 1230. [15] F. Gerken and J. Schmidt-May, J. Phys. F13 (1983) 1571. [16] N. Martensson, B. Johansson and J.R. Naegele, Phys. Rev. B35 (1987) 1437. [17] J.R. Naegele and J.C. Spirlet, to be published. 1181 T. Gouder, J.R. Naegele, C.A. Colmenares and J.J. Verbist, Inorg. Chim. Acta 140 (1987) 35. [19] T. Gouder, C. Colmenares, J.R. Naegele and J. Verbist, submitted to Surf. Sci. [20] K. Richter and C. Sari, J. Nucl. Mater. 148 (1987) 266. [21] J. Schultz, C.A. Cqlmenares, J.R. Naegele and J.C. Spirlet, Inorg. Chim. Acta 140 (1987) 37. 1221 T. Gouder, J.R. Naegele, C. Colmenares, J.C. Spirlet and J. Verbist, to be published. [23] J.L. Smith and E.A. Kmetko, J. Less-Comm. Met. 90 (1983) 83.
SURFACE
59
166 (1989) 59-67
ANALYSIS
OF ACTINIDE
MATERIALS
J.R. NAEGELE Commission
of the European Communities,
D-7500 Karlstuhe,
Joint Research Centre, Institute for Transuranium Elements, Postfach 2340,
Fed. Rep. Germany
The application of highly surface sensitive electron spectroscopic methods to the determination of electronic structures and of the chemical composition of highly u-active solids is described. Selected results from photoelectron spectroscopy on UMn, and on actinide metals from Th to Am demonstrate the identification of the quantum character of valence electrons and the transformation of bandlike 5f electrons for a-f+ into localized Sf electrons for Am. For more applied purposes the high surface sensitivity of photoelectron spectroscopy was used to characterize surfaces during oxidation and gas adsorption. Characteristic examples are: (1) the oxidation of Np metal, for which it is shown that a boundary layer between the bulk Np metal and the final covering dioxide surface layer is composed of Np,O,, a phase that does not exist as bulk material; (2) the interaction of CO with the surface of clean U metal and UNi,.
The perspectives of combining basic research on electronic properties, chemical bonding etc. and characteristic surface studies in the field of actinides are described.
1. Introduction Highly surface sensitive electron spectroscopic methods as photoelectron spectroscopy (XPS: X-ray Photoelectron Spectroscopy; UPS: Ultraviolet Photoelectron Spectroscopy; ESCA-identical with XPS-: Electron Spectroscopy for Chemical Analysis), Auger Electron Spectroscopy (AES), Electron Energy Loss Spectroscopy (EELS) etc. have been applied in the last two decades intensively to the determination of the electronic structure and chemical composition of solid surfaces. This is largely due to the very straightforward way of analysis and the availability of several commercial spectrometers working under ultrahigh vacuum conditions (p < 1 X 10ms Pa). At the European Institute for Transuranium Elements (EITU) an electron spectrometer has been modified to meet safety and vacuum requirements for measurements of highly radioactive actinide materials [l]. Electron spectroscopies have been presented in many review articles and books [2-41. Therefore only a short description of the different spectroscopic techniques and their application to the work with actinide materials is given here. The study of the electron structure of actinides and their compounds was the original aim. Therefore specific results from photoelectron spectroscopy are selected to demonstrate the uniqueness of the actinide series in the
0022-3115/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
periodic table. Finally, results related to surface properties, e.g. surface reactivity complete the presentation of achievements.
2. Spectroscopic methods and experimental realization Electron spectroscopies are generally based on the detection of electrons (e-) in terms of their intensity, kinetic energy and angle distribution excited by different particles, e.g. electrons (e-), photons (hv) etc. from a sample surface under vacuum: Excitation:
e-,
hv, I+ -+ Detection:
e-,
I+.
Positive ions (I+) have been included here because many commercial spectrometers nowadays permit to run the detection system for positive particles and therefore prepare electron spectrometers for Ion Scattering Spectroscopy (KS). The following spectroscopic methods are employed at the EITU: A. Photoelectron Spectroscopy (XPS, UPS): The kinetic energy of the photoemitted (E,,,) is given by:
hv + eelectrons
Ekin = hv - E, - E,,,
wherin
B.V.
hv is the energy
of the exciting
photon,
E,
the
60
J. R. Naegele / SW-me analysts
binding energy of electrons in the solid under investigation and E,, the work function. Since the binding energy E, of core electrons is characteristic for each element, this technique is used for chemical analysis (ESCA). Electrons taking part in the chemical bonding, i.e. valence and conduction electrons, can be detected as well in XPS and UPS. Thus this technique permits the determination of the electronic structure (type of chemical bonding, density of occupied electronic states). The quantum character of the electrons is identified from the characteristic variation of the photoelectron excitation cross section with the energy of the exciting photons. B. Auger Electron Spectroscopy (AES) : e- -+ e- or hv +eElectrons or photons (not necessarily monoenergetic) produce a hole in an inner electron shell by ionization. This hole is filled by an electron from an energetically higher shell. The energy released in this process is used to eject a second electron, the Auger electron. The kinetic energy of this Auger electron is characteristic for the element. Since the electronic structure of the material under investigation only weakly influences the Auger emission, this technique is mainly used for chemical analysis. C. Electron Energy Loss Spectroscopy: em + em Monoenergetic electrons irradiating the sample loose energy by different processes: 1. electronic transitions from occupied to unoccupied states (similar to optical spectroscopy), 2. collective electron excitations, i.e. plasmons, 3. phonon excitations. Processes 1 and 2 can be observed at high electron excitation energies and relative poor monochromatization whereas process 3 because of the small phonon energy can only be observed at low excitation energy (some eV) and best monochromatization (- 5 mev). Due to the poor monochromatization (- 0.5 eV) and small excitation energy range (50-500 eV) of the electron source, originally used at the EITU for the compensation of charging effects particularly occurring in photelectron spectroscopy, mainly process 2 is studied. This gives complementary information on the electronic structure. D. Ion Scattering Spectroscopy (ISS) : I+ + I+ The primary, usually positive rare gas ions are scattered by the sample surface and carry after the collision with the surface atoms the information about the mass of these atoms. Thus, information on the
ofactmide materials
l-
t ,cus
,
5353 cus
ii+
TP
I
11SP
TP
J
L_-----_---------
Fig. 1 Scheme of the electron ACh EA EMP XRS ES uvs
Analyzing chamber Electron energy analyzer Electron multiplier X-ray source Electron source uv source
spectrometer.
S GB
sample Glove box
PCh SR TP TiSP
Preparation chamber Sample rod Turbo pump Titanium sublimation pump
chemical composition is obtained. At present, this technique is in preparation at the EITU. A common feature of the spectroscopies given above is the extreme surface sensitivity of 0.5 to 2.5 nm, i.e. 1 to 5 monolayers. Thus ultrahigh vacuum (p < lop8 Pa, requires baking of the entire apparatus to 200 o C) and “in-situ” sample surface preparation are obligatory. These facts impose various problems on the application of these techniques to highly radioactive materials since the sample cannot be encapsulated. To meet safety requirements while keeping ultrahigh vacuum conditions the following concept based on a commercial spectrometer and shown as scheme in fig. 1 has been realized (a more detailed description is given in ref. [l]): _ The preparation chamber (PCh) was redesigned to achieve a pressure better than lo-* Pa and to allow for baking to 200 o C in a closed glove box system. _ The analysis chamber (ACh) with excitation sources (XRS, ES, UVS) and the detection system (EA, EMP) as well as the UHV pumping systems are mounted
J. R. Naegele
/ Surface analysis of aciinide
into open gfove box frames. Only in case of repair requiring an opening of the vacuum system the glove boxes have to be closed. - The “in-situ” sample surface preparation is solely performed in the PCh that takes up all the radioactive waste from the cleaning process. Afterwards, the sample is transferred via a lock system to the ACh just for the measurements. This way of operation keeps the radioactive contamination level low in the ACh. Fig. 2 shows a photograph of the modified spectrometer that offers the following spectroscopic measurements on highly radioactive materials: _ Auger electron spectroscopy; excitation either by electrons (0.5-5 keV) or X-rays (Mg and Al anodes with 1253.6 and 1486.6 eV, respectively);
materials
_ Electron
61
energy loss spectroscopy (So-500 eV, resolution A E = 0.5eV); _ Angle integrated photoelectron spectroscopy with Xray (XPS) and UV-light (UPS) excitation (XPS: 1253.6 and 1486.6 eV; UPS: energies of 16.7 (Ne I), 21.2 (He I), 40.8 (He II), and 48.4 (He II *) provided by a rare gas discharge source). The ISS technique is currently under test on a second cold machine (i.e. for non-radioactive materials): it is considered to give ~mplementary information on the surface composition because of its sensitivity to just the topmost surface atoms. The performance of the spectrometer is demonstrated in fig. 3. The high counting rate at high resolution of 45 meV in UPS with its high surface sensitivity is of particular importance since it permits short record-
Fig. 2. Photograph of the spectrometer (same view as for fig. 1)
J. R. Naegele / Surface analysis of actinide materials tion of A E = 20 meV thus demonstrating
that
the mea-
meV is mainly caused by the temperature broadening; a modification of the cooling system to use liquid He instead of liquid N, is presently under test. sured
half
width
of gold
at
T=
80
K
of 45
3. Selected results and discussion Originally, electron spectroscopy was dedicated at the EITU to the investigation of electronic properties of actinide containing materials, in particular to elucidate the contribution of the actinide 5f electrons to the chemical bonding. The basic question was: Are the 5f electrons in actinides localized, i.e. not taking part in the chemical bonding, as the 4f electrons in the lanthanides, or are they delocalized, i.e. taking part in the chemical bonding like the d electrons in the transition metals? Photoemission was particularly considered to be best suited to answer this question since this technique gives quite a straight forward view to the electronic states. Thus the first part of this section will deal with the clarification of the electronic structure of actinides. Because of the high surface sensitivity of the spectroscopic techniques the investigations were later extending more and more into applied areas dealing with specific surface effects, e.g. corrosion, surface reactivity etc. This aspect of the work is presented in section 3.2. 3. I. Electronic properties
89
I
I
06
81
I 66
BINDING
85
ENERGY,
04
03
eV
Fig. 3. Performance of the electron spectrometer. (a) XPS (Mg K,: 1253.6 ev): Gold 4f core levels; photoelectron pass energy for the energy analyzer and recording time are: curve 1: 20 eV, 10 min; curve 2: 5 eV, 20 min. (b) UPS (HeI: 21.2 eV): Gold at T= 80 K; photoelectron pass Fermi edge, E, recorded energy for the energy analyzer, recording time and maximum emission intensity in the valence band are: curve 1: 5 eV, 10 min., 4 X 10’ counts/s; curve 2: 2 eV, 15 min., 5 X lo4 counts/s.
ing times
for chemically
highly
build-up
of
contamination
UPS
chemical
measurements
reactive
on Ar gas reveal
materials on
the
a machine
without surface.
resolu-
From a viewpoint of electron spectroscopy, recently the electronic properties of actinides have been reviewed [5,6]. As explained above the contribution of actinide 5f electrons to the chemical bonding is of main interest. Energy dependent photoelectron spectroscopy is a very appropriate tool to identify this contribution because of the characteristic photoexcitation cross section of f states: simply speaking, the 5f excitation probability is growing strongly with increasing photon energy in the UV range compared to the cross section of s, p and d states. This has been nicely demonstrated in the case of UO, that has a nearly pure localized U 5f state at about 1.5 eV below the Fermi energy E, and a mainly oxygen derived 2p valence band extending from 4 to 9 eV below E, [6]. Recently, quantitative photoexcitation cross section calculations have been presented for a wide range of excitation energies [7]. These cross sections, modified to account for resonant effects at the 3p 3d threshold energy (- 50 ev) that have not been included in the
63
J. R. Naegele / Surface analysis of actinide materials
calculation, have been used for a more quantitative analysis of photoelectron conduction band spectra of UMn, [8] shown in fig. 4. Due to the increase of emission intensity at E, (structure C) with photon excitation energy, the U5f states are identified at E, and are delocalized, i.e. they are taking part in the chemical bonding. The other structures A and B have been identified in a similar way to be dominated by Mn 3d and U 6d states, respectively. This interpretation confirms a previous analysis on the basis of synchrotron light-induced photoelectron spectra (91.
V=
LO.8 eV (LIPS).;; ,’
Am
d,.,A”‘s
‘I ocalized 5f states “multiplet” lelocalized
5f states
A.\-
Th
Jr’
,o occupied 5f states ,lIII 1 LI 9
5 BINDING
ENERGY
L 11 E, (ev)
Fig. 5. UPS conduction band spectra for actinide Th to Am for 40.8 eV excitation.
I
L
.
.
BINDING
.
.
I
,
EF=O
5 ENERGY
(eV)
Fig. 4. UPS conduction band spectra for UMn, recorded at T = 80 K for photon energies of 16.7, 21.2, 40.8, and 48.4 eV (curves a, b, c, d, respectively).
I
EF=O
metals
from
The electronic structure of actinide metals from Th to Am has been investigated in a similar way by using energy-dependent photoemission spectroscopy. Fig. 5 shows the conduction band spectra of Am [lo], a-Pu [lo], Np [ll], U [12] and Th [13] for 40.8 eV excitation. Th is known to have no occupied 5f states and thus shows a structured but low emission intensity extending to E,; the observed structures are maxima in the density of Th 6d and 7s electron states. The spectra for U, Np and a-Pu are similar to each other and show a characteristic strong increase of emission intensity up to E, that has its origin in 5f states. Since these 5f states are found to peak at E,, they must take part in the electrical conduction, i.e. they take part in the chemical bonding, they are delocalized. The appearance of the conduction band spectrum changes completely when reaching Am. The strong 51 emission at E, as found for U through a-Pu is missing. Instead, a triplet structure is found around 2.5 eV below E, and clearly identified to have 5f character whereas the emission at E, is dominated by Am 6d electrons [lo]. This behaviour is characteristic for localized f electrons. Fig. 6 makes this even clearer by presenting the spectra of a-Pu and Am together with the one for Sm recorded at the EITU on
J. R. Naegele / Surface anaiys~sofrrctinide mairrruls
64
I
’
’
1~~40.8
’
’
I
’ .+-, I ’
eV (UPS1
/
i .
’
I
:
: k!
I I
A modelling of the experimental conduction band shape measured for Am [fig. 61 based on a theoretical multiplet calculation [15] evoked a contribution from divalent surface atoms with a localized 5f’ state [16]. Very recently photoemission studies on binary Am, Pu , __: alloys [17] have demonstrated the gradual increase of Pu 5f electron localization with decreasing Pu content in the binary alloy. 3.2. Surface properties The high surface sensitivity of electron spectroscopy has been used to investigate the oxidation of U [18.19], Np[ll] and (Y-Pu [6]. U and a-Pu form on the surface only layers of those oxide phases that are also existent as bulk phases: U builds up on its surface solely the dioxide UO, whereas a-Pu forms either the sesquioxide Pu,O, or the dioxide PuO, depending on the experimental conditions, e.g. sample temperature, oxygen par-
BINDING
ENERGY
Eg (eV)
Fig. 6. UPS conduction band spectra for Sm, a-Pu and Am for 40.8 eV excitation.
I
hv.
1253.6
Np 4f
Sm films evaporated under ultrahigh vacuum conditions. Sm is quite a useful metal to compare with Am and Pu because it is a related 4f electron system having localized 4f electrons. The 4f electron occupation number of the trivalent Sm is 5 for bulk f states (4f ‘) as also predicted for a-Pu (5f5); in addition, Sm has in its topmost surface layer divalent atoms that have 6 electrons in the 4f shell (4f 6), the same number of electrons as expected in the 5f shell of bulk Am. As seen in the spectrum of Sm (fig. 6) the localization of 4f electrons results in a characteristic set of structures, the so-called “multiplet”. The difference between cu-Pu and Am is striking: a-Pu has just an increasing emission intensity peaking at E, and shows no structures comparing to the 4f 5 --* 4f4 multiplet of Sm, whereas Am shows a similar triplet structure (“multiplet”) as the 4f6 + 4f 5 multiplet of Sm. This is an unambiguous experimental proof of the transition from delocalized 5f electron behaviour in (Y-Pu to localized behaviour in Am as previously predicted from theoretical considerations [14]. This transition is also clearly visible in the change of the shape of the 4f core levels of a-Pu and Am measured by XPS [lo]. Thus Am is the first lanthanide like metal in the actinide series.
eV (XPS)
(NpO,
)
NOT EXISTENT AS BULK !
Np 4f ( Np -metal
)
a I 1I40
I 420 BINDING
ENERGY
t 400
L
EB (eV1
Fig. 7. XPS 4f core level spectra (hv = 1253.6 eV) for increasing surface oxidation of Np metal: (a) clean metal, (b), (c) intermediate oxidation, (d) final oxidation.
J. R. Naegele / Surface analysis of actinide materials
hv
hv
hv
e-
Fig. 8. Scheme of the surface oxidation steps of Np metal corresponding to curves a, b, c, and d of fig. 7.
tial pressure etc. In contrast to U and Pu, Np behaves in quite a different way. From phase studies of the Np-0 system [20] the formation of a NpO, surface layer is expected upon oxidation of Np metal because it is the only stable oxide phase, i.e. Np should behave similar as U metal [18,19]. In contrast to the expected behaviour, Np forms in its initial oxidation stage a thin layer of Np,O, that does not exist as bulk phase. Only upon further oxidation NpO, is formed on the surface. The formation of a Np,O, boundary layer between Np metal and NpO, is unambigously observed in the XPS 4f core level spectra shown in fig. 7. In addition to the lines for clean Np metal (curve a) two main distinct emission signals appear at higher binding energy. The main lines in curve d are shifted by about 3.9 eV and
65
represent clearly the formation of NpO, because of the characteristic actinide dioxide signal with satellites at 6.8 eV higher binding energy, as also found for bulk UO,, NpO, and PuO, [6]. Since the value of the energy shift (“chemical shift”) of core lines measured by XPS is directly related to the oxidation state, the additional lines occurring in curves b and c of fig. 7 in between the dioxide (curve d) and metal (curve a) line must be due to a lower oxide than NpO, and are therefore attributed to Np,O,. This picture as schematically given in fig. 8 is also confirmed by UPS valence band spectra [ll] which show with the increase of the oxygen 2p signal around 6 eV binding energy two distinct 5f electron derived structures at 1.5 and 3 eV binding energies being attributed to the formation of Np,O, and NpO,, respectively. Electron spectroscopic studies have been performed to investigate the adsorption of H,, CO, CO, and C,H, gases on U [18] and UFe, [21]. As an example for this type of study, photoelectron spectra (UPS, 40.8 eV excitation energy) for the interaction of CO with the surface of clean U metal and UNi, at T = 75 K [22] are presented in fig. 9. The results show quite different adsorption mechanisms for U and UNi,. In the case of U metal (fig. 9a) increasing CO dosage (1 L = 10m6 mbar s., e.g. 10 s at a pressure of lo-’ mbar) results in a growth of the 0 2p emission line at about 6 eV binding energy. This is a clear indication of CO dissociation and the formation of UO,; a formation of additional UC,O,_, might be possible, too. At a dosage of about 8 L saturation occurs. Beyond 8 L three additional emission lines appear with intensities and an energy separation as found for gaseous CO. These lines are therefore attributed to 40, la and 5a orbitals of physisorbed CO. Chemisorption was never observed. In the case of UNi, (fig. 9b) CO adsorbs at low dosage dissociatively as indicated by the growth of the 0 2p emission. But in contrast to U metal, additional lines appear which are characteristic for chemisorbed CO. At higher dosage lines for physisorbed CO appear, similar to the case of U metal. From these results it is concluded that CO is dissociated and partially chemisorbed resulting in an oxidation of the U atoms, i.e. the formation of UO, and the breaking of the U-Ni bonds with the formation of metallic Ni particles in an UO, matrix.
4. Perspectives The actinide series, as shown above, exhibits many interesting aspects, not only from an electronic but also from a surface structure point of view. The uniqueness
J.R. Naegele / Surface analym
66
I’.‘.!“’ ,y;
I’.‘.I.
CO
phys.
u
~
u 5 f r-
@ >,.
1.
ofactrnidemaferials
‘.
.
1”’
*
IO
15
BINDING
ENERGY
I”*
*”
5
Efz=O
, .
‘.
1.
‘.
15 BINDING
(eV)
.
1..
. . 1..
10 ENERGY
s
. 1.
EF= 0
(eV)
Fig. 9. UPS conduction band spectra (h Y= 40.8 eV) for CO adsorbed on U (a) and UNi, (b) at T = 75 K.
of the actinides is nicely demonstrated by a modified periodic table [23] for elements with unsaturated electron shells as shown in fig. 10. The actinides form the only series of elements that shows the transition from electron delocalization (“Bonding”) to localization (“Magnetic Moments”) around halffilling of the unsaturated shell. This crossover has been experimentally clearly demonstrated to occur between cr-Pu and Am
Mugnetic
partially
filled shell
Moments
full
Cd 5d
Bal Lu
Hf /
Ta
W Re OS Ir
Pt \Au Hg
Bonding Fig. 10. Modified periodic table of elements with uncompletely filled electronic shells (ref. [23]). The dashed region represents the crossover between delocalized (“Bonding”, electrons take part in the chemical bonding) and localized electron character (“Magnetic
Moments”,
electrons
form magnetic
moments).
(figs. 5 and 6). Therefore these elements are very suitable to study basic electronic properties, e.g. electron localization effects and their influence on other physical properties. To study localization effects in more detail electron spectroscopic measurements on systems like S-Pu, Pu,Am,_, and selected compounds of Pu and Am are of particular interest and will be therefore continued. On the other hand, the technique of electron spectroscopy will be used more extensively in applied research areas. The modified periodic table (fig. 10) contains many elements that are of importance in catalysis, e.g. CO, Ni, Pd, Pt etc. The actinides with their unique electronic structure that even permits a controlled variation of electronic properties might therefore be considered to be used as model substances to study basic correlations between electronic structure and surface activity including catalytic activity. Catalytic studies on actinide containing materials have already been started at the EITU in a catalytic flow reactor, on U-Ni and U-Fe systems. Adsorption studies described above show on the other hand that the surface of these materials is easily decomposed into metallic Ni or Fe particles in an UO, matrix. This means that the catalytic activity is mainly due to metallic Ni or Fe. Therefore one has to extend these studies to actinide containing systems which produce metallic actinide particles in the surface. Th,Am, and La,PuY are potential candidates. The high surface sensitivity could be used in the future to analyze the surfaces of actinide containing materials. Corrosion effects of nuclear fuels. interac-
J. R. Naegele / Surface analysis of actinide materials
between the fuel and its cladding, leaching of glasses for nuclear waste disposal, dissolution properties of oxide fuel elements [l] etc. are considered to be subjects of future electron spectroscopic studies. tions
Acknowledgements The excellent assistance of Mr. H. Winkelmann, who took part in the initial construction of the electron spectrometer for a-active materials, of Mr. N. Nolte, Miss F. Schiavo and Dr. T. Gouder is gratefully acknowledged.
References [l] J.R. Naegele, J. Phys. (Paris) 45, C2 (1984) 841. [2] Electron Spectroscopy for Surface Analysis, Ed. H. Ibach, Topics in Current Physics, Vol. 4 (Springer Verlag, Berlin, 1977). [3] Electron Spectroscopy: Theory, Techniques and Applications, Eds. C.R. Brundle and A.D. Baker, Vols. l-4 (Academic Press, London, 1977). [4] T.A. Carlson, Photoelectron and Auger Spectroscopy (Plenum Press, New York, 1975). [5] Y. Baer, Handbook on the Physics and Chemistry of the Actinides, Eds. A.J. Freeman and G.H. Lander, Vol. 1 (North-Holland, Amsterdam, 1984) p. 271. [6] J.R. Naegele, J. Ghijsen and L. Manes, Actinides - Chemistry and Physical Properties, in: Structure and Bonding Ed. L. Manes, Vols. 59/60 (Springer, Berlin, 1985) p. 197.
61
[7] J.J. Yeh and I. Lindau, Atomic Data and Nuclear Data Tables 32 (1985) 1. [8] J.R. Naegele, V. Sechovsky and L. Havela, to be published. [9] J.R. Naegele, J. Ghijsen, R.L. Johnson and V. Sechovsky, Phys. Scripta 35 (1987) 877. [lo] J.R. Naegele, L. Manes, J.C. Spirlet and W. Mtiller, Phys. Rev. Lett. 52 (1984) 1834. [ll] J.R. Naegele, L.E. Cox and J.W. Ward, Inorg. Chim. Acta 139 (1987) 327. [12] T. Gouder and J.R. Naegele, unpublished results. [13] J.R. Naegele, T. Gouder and D.D. Sarma, to be published. [14] H.L. Skriver, O.K. Andersen and B. Johansson, Phys. Rev. Lett. 44 (1980) 1230. [15] F. Gerken and J. Schmidt-May, J. Phys. F13 (1983) 1571. [16] N. Martensson, B. Johansson and J.R. Naegele, Phys. Rev. B35 (1987) 1437. [17] J.R. Naegele and J.C. Spirlet, to be published. 1181 T. Gouder, J.R. Naegele, C.A. Colmenares and J.J. Verbist, Inorg. Chim. Acta 140 (1987) 35. [19] T. Gouder, C. Colmenares, J.R. Naegele and J. Verbist, submitted to Surf. Sci. [20] K. Richter and C. Sari, J. Nucl. Mater. 148 (1987) 266. [21] J. Schultz, C.A. Cqlmenares, J.R. Naegele and J.C. Spirlet, Inorg. Chim. Acta 140 (1987) 37. 1221 T. Gouder, J.R. Naegele, C. Colmenares, J.C. Spirlet and J. Verbist, to be published. [23] J.L. Smith and E.A. Kmetko, J. Less-Comm. Met. 90 (1983) 83.