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Photoacoustic spectroscopy of actinide oxides
Journal of the Less-Common
Metals, 122 (1986)
25
25 - 30
PHOTOACOUSTIC SPECTROSCOPY OF ACTINIDE OXIDES* G. HEINRICH, H. GkTEN Kernforschungszentrum Karlsruhe (F. R. G.)
and H. J. ACHE
Karlsruhe,
Znstitut fiir
Radiochemie,
Postfach
3640,
7500
Photoacoustic spectroscopy has been used to measure directly the absorption spectra of powdered actinide oxides (thorium, uranium, neptunium, plutonium, americium and curium). The photoacoustic spectra in the UV, visible and near-IR region were measured with a single beam instrument supplemented by a small glove box which contained the sample cell and the detector. In contrast with the photoacoustic spectra of the lanthanides, which show sharp, narrow absorption bands, the absorption spectra of the actinide oxides reveal quite different spectroscopic properties. All the spectra are characterized by intense and broad absorption bands, especially in the visible and UV ranges. The narrow, weak absorption bands, which are attributed to electron transitions within the 5f shells, cannot be observed in uranium, americium and curium dioxides. In neptunium and plutonium dioxide as well as in americium sesquioxide they are observable, but they are overlapped by broader absorption bands. Most of the 5f-electron levels are significantly hybridized and it seems that covalent metal-oxygen ligand mixing is an important factor. The intense, broad absorption bands in the UV and visible regions may be explained by electron transfer processes between 5f-electron states and oxygen molecular orbit&.
1. Introduction The lanthanides and actinides form two groups of similar elements which are characterized by successive filling of the 4f- and 5f-electron shells respectively. Accordingly, close similarities in the absorption spectra of the oxides of these two series of elements should be observed. Recently, photoacoustic spectroscopy has been used to measure directly the absorption spectra of powdered and sintered lanthanide oxides [ 1 - 43. The absorption spectra of the trivalent lanthanide oxides are characterized by sharp, narrow *Paper presented at Actinides 85, Aix en Provence, September 2 - 6, 1985. @ Elsevier Sequoia/Printed
in The Netherlands
26
bands in the UV, visible and IR ranges. Until now, little has been known about the spectroscopic properties of powdered actinide oxides. Only a few measurements of the optical reflectivity with dioxides of thorium, uranium and plutonium have been published [ 5 - 81. We report here the photoacoustic spectra of some actinide oxides (thorium to curium).
2. Experimental details The absorption spectra in the wavelength range 200 - 1500 nm have been measured with a home-built photoacoustic spectrometer, which is desribed elsewhere [ 41. Owing to the use of two different gratings blazed at 500 and 1000 nm with 1200 and 600 lines mm-‘, the bandpass in the 200 - 800 nm range is 6 run, and 12 nm in the IR 800 - 1500 nm range. The actinide dioxides (thorium to americium) were sintered in air at temperatures of 700 - 1000 “C directly before the photoacoustic absorption spectra were measured. To overcome signal saturation in the cases of the intensely coloured actinide oxides they were mixed with SiOZ (18 - 32 pm) in a vibrating mill. A sample of approximately eight years old curium-244 dioxide was measured after separating the daughter plutonium-240 from 244Cm02.
3. Results and discussion The absorption spectra of the free ions of trivalent actinides and lanthanides in crystals show a remarkable similarity. They consist of sharp, clearly separated groups of lines. An energy level diagram of the free trivalent ions of actinide compounds has been published recently [ 9, lo]. The situation is much more complicated with the tetravalent actinides, because the ligand field interactions of tetravalent actinides are stronger than those of the trivalent ones. This makes the interpretation of these absorption spectra much more difficult. Furthermore, a predictive theoretical model for the tetravalent ions is more difficult to obtain than for the trivalent f-element spectra, because the degree of charge transfer and mixing with other states between metal and ligand is unknown. Nevertheless, a free ion energy level diagram of the tetravalent actinides was available for the interpretation of our photoacoustic absorption spectra [ 111. The spectroscopic properties of the actinide dioxide in the UV confirm the presence of band-like electrons. However, in contrast with this fact, the semiconducting character and the magnetic properties of the actinide oxides call for localized 5f states. All these circumstances complicate the interpretation of the tetravalent actinides and can easily be visualized by the absorption spectra of the actinide dioxides. The absorption spectrum of sintered, white ThO2 is quite simple to interpret, because there are no 5f electrons. Thus, there is no absorption in the IR and visible spectra (Fig. 1). The
27
200
300
100
SO0
600
700 800 Woudength
900 1000 I nm 1
1100
IZM
1300
1100
I IO
Fig. 1. Photoacousticspectraof ThOz,UOzand Us08 powders. increasing absorption starting at about 250 nm appears to be due to transitions from the top of the valence band, which is built up with 0-2~ levels, to the bottom of the conduction band, which is built up with empty 6d and probably 7s levels. Our photoacoustic absorption spectra of ThOz and UOz agree well with those measured by diffuse reflectance spectroscopy [ 5,6]. In UOz (Fig. 1) the absorption bands are broad and intensive. Narrow well-resolved absorption peaks as in the spectra of the lanthanide sesquioxides [l, 2, 41 cannot be observed. To overcome photoacoustic signal saturation the sample of U02 has to be “diluted” with silical gel or ThOz. The rapidly increasing absorption at 16700 cm-’ (600 nm) with small maxima at 473, 450, 406, 388 and 333 nm is consistent with optical spectra of UOz obtained by other spectroscopic techniques [6, 12, 131. Since the 6d electron band in actinides is shifted to lower energies, we attribute, with some uncertainty, the absorption bands between 600 and 350 nm to transitions from occupied 5f levels to the 6d band which consists of numerous individual crystal field split levels. An energy level scheme for UOz places the lowest-lying 5f2 --f 5f’ 6d transition near 590 nm [ 71. This assignment agrees with a first-principles molecular cluster calculation by Gubanov et al. on U02 [ 141. The assignment by Schoenes [ 71, however, is controversial [ 6, 151. Naegele et al. [6] and Catlow [ 151 attribute the intense absorption bands in the 32 000 - 64 000 cm-’ range to the 5f2 + 5f’6d transition. Another explanation for the broad absorption bands in the UV is the assumption of an electron transfer process which is produced by an electron being transferred from a molecular oxygen orbital to a partly filled or empty 5f electron state of the oxidizing central uranium atom. It has been shown that electron transfer bands compete with the 5f” + 5f”- ’ 6d bands, especially in the higher actinides [ 161. U308 shows an absorption spectrum similar to that of U02 (see Fig. 1). The absorption band between 600 and 300 nm is slightly suppressed relative to the absorption band in the UV. This fact is consistent with the observation that as the oxidation state in the binary oxides is increased, electrons are transferred from the localized 5f states into the bonding orbit& which are predominantly 0-2~ in character. The sini-
28
larity in the absorption spectra of the two uranium oxides shows that the stoiehiometry has almost no influence on the internal electronic transitions of uranium. The pho~acoustic absorption spectrum of neptunium dioxide (Fig. 2) is characterized by an intense and broad absorption band with a maximum near 350 nm and some less intensive but marked peaks in the visible and IR regions. The intense absorption band in the UV may be due to an electron transfer transition or an allowed 5f3 -+ 5f26d transition. The numerous sharp peaks of lower intensity in the visible region may be at~bu~ to transitions within the manifold of the 5f electron levels. Plutonium dioxide shows an absorption spectrum similar to that of Np02 (Fig. 2). The 5f4 =+5f 3 transitions are not as marked as in Np02. The only sharp and well resolved peak at about 1430 nm is due to adsorbed water molecules at the surface of the Pu0,j(Si02), mixture. The absorption spectrum of AmOz is very similar to that of Pu02 (Fig. 3). The tr~sitions within the 5f electron levels are overlapped by the intense broad absorption covering the whole spectral range. However, in the absorption spectrum of
20 10
*L_.,__ 200
300
I
I.00
-
330
600
Fig. 2. Photoacoustic
IQ0 801 WovPIengtk
900 1nna j “In I
1lUQ
12QO
_A_-.-i
1300
Ml0
1SOQ
spectra of NpOz and PM& powders,
IG / *
h_.-.A-.
200
300
100
5QQ
600
700 nun Wwelength
L
908 1QnO I nm 1
,,PQ
~~.~~~~
i2QQ
,300
,‘OQ
1500
Fig. 3. Photoacoustic spectra of AmOz, Am203 and Cm& levels of Am3+ are indicated at the top).
powders (the free-ion energy
29
the trivalent americium oxide there are some narrow peaks. The three peaks about 1100 run, which superimpose the broad absorption common in all actinide oxides, can be assigned to a ‘F, + ‘Fe transition. The free ion energy levels of the trivalent americium [lo] are indicated at the top of Fig. 3. Curium dioxide shows a nearly continuous absorption (Fig. 3) in the IR and in the visible regions. 4. Conclusions Compared with the absorption spectra of the lanthanide oxides [ 1 - 41, the absorption spectra of the oxides of the actinides are far from similar. The narrow weak absorption bands, which are attributed to electron transitions within the 5f” shell, cannot be observed in uranium, americium and curium dioxides. In neptunium and plutonium dioxides as well as in americium(II1) oxide they are observable, but are overlapped by very broad absorption bands extending from the UV to the visible and sometimes to the IR. The conclusion is that the 5f electrons in the actinide dioxide are not well localized as the 4f electrons in the lanthanide sesquioxides. This produces the condition of strong competition between the 5f electrons and the 6d and 7s electrons and their participation in bonding. There is also evidence for electron transfer transitions in the actinide oxides which cause intense and broad absorption bands in the UV and visible where they produce the reddishbrown and black colours of the actinide dioxides. Acknowledgments We thank Dr. H. Bokelund for a sample of curium-244 dioxide and Mr. U. Bemdt for the separation of plutonium-240 from this curium dioxide sample. References 1 J. R. Schoonover, Y.-L. Lee, S. N. Su, S. H. Lin and L. Eyring, Appl. Spectrosc., 38 (1984) 154. 2 R. Tilgner, Appl. Opt., 20 (1981) 3780. 3 A. C. Tam and C. K. N. Patel,AppL Phys. Lett., 35 (1979) 843. 4 G. Heinrich, H. Giisten and H. J. Ache, Apple Spectrosc., 40 (1986) 363. 5 P. Imris and D. Imris, J. Znorg. Nucl. Chem., 27 (1965) 2135. 6 J. Naegele, L. Manes and H. Winkelmann, in J. Mulak, W. Silski and R. Trod6 (eds.), Proc. 2nd Znt. Co& Electron Structure of the Actinides, 1976, Wrodhw, Poland, Polish Academy of Science, Warwaw, 1977, p. 163. 7 J. Schoenes, J. Apple Phys., 49 (1978) 1463. 8 D. T. Hodul, Spectrosc. Lett., 16 (1983) 181. 9 J. P. Hessler and W. T. CarnaIl, in Lanthanide and Actinide Chemistry and Spectroscopy, ACS symp. Ser., 131 (1980) Chapter 17. 10 W. T. Carnal1 and H. M. Crosswhite, in J. J. Katz, G. T. Seaborg and L. R. Morss (eds.), The Chemistry of the Actinide Elements, Chapman and Hall, London, 1985.
30
11 W. T. Carnal& personal communication. 12 J. Ackermann, R. J. Thorn and G. H. Winslow, J. Opt. Sot. Am., 49 (1959) 1107. 13 L. Manerr and J. Naegele, in H. Blank and R. Lindner (eds.), Plutonium 1975 and Other Actinidee, North-Holland, Amsterdam, 1976, p. 363. 14 V. A. Guhanov, A. Rosen and D. E. Ellis, Solid-State Commun., 22 (1977) 219. 15 C. R. A. Catlow, J. Chem. Sot., Faraday Trans. 2, 74 (1978) 1901. 16 L. J. Nugent, R. D. Baybarz, J. L. Burnett and J. L. Ryan,J. Phys. Chem., 77 (1973) 1528.
Metals, 122 (1986)
25
25 - 30
PHOTOACOUSTIC SPECTROSCOPY OF ACTINIDE OXIDES* G. HEINRICH, H. GkTEN Kernforschungszentrum Karlsruhe (F. R. G.)
and H. J. ACHE
Karlsruhe,
Znstitut fiir
Radiochemie,
Postfach
3640,
7500
Photoacoustic spectroscopy has been used to measure directly the absorption spectra of powdered actinide oxides (thorium, uranium, neptunium, plutonium, americium and curium). The photoacoustic spectra in the UV, visible and near-IR region were measured with a single beam instrument supplemented by a small glove box which contained the sample cell and the detector. In contrast with the photoacoustic spectra of the lanthanides, which show sharp, narrow absorption bands, the absorption spectra of the actinide oxides reveal quite different spectroscopic properties. All the spectra are characterized by intense and broad absorption bands, especially in the visible and UV ranges. The narrow, weak absorption bands, which are attributed to electron transitions within the 5f shells, cannot be observed in uranium, americium and curium dioxides. In neptunium and plutonium dioxide as well as in americium sesquioxide they are observable, but they are overlapped by broader absorption bands. Most of the 5f-electron levels are significantly hybridized and it seems that covalent metal-oxygen ligand mixing is an important factor. The intense, broad absorption bands in the UV and visible regions may be explained by electron transfer processes between 5f-electron states and oxygen molecular orbit&.
1. Introduction The lanthanides and actinides form two groups of similar elements which are characterized by successive filling of the 4f- and 5f-electron shells respectively. Accordingly, close similarities in the absorption spectra of the oxides of these two series of elements should be observed. Recently, photoacoustic spectroscopy has been used to measure directly the absorption spectra of powdered and sintered lanthanide oxides [ 1 - 43. The absorption spectra of the trivalent lanthanide oxides are characterized by sharp, narrow *Paper presented at Actinides 85, Aix en Provence, September 2 - 6, 1985. @ Elsevier Sequoia/Printed
in The Netherlands
26
bands in the UV, visible and IR ranges. Until now, little has been known about the spectroscopic properties of powdered actinide oxides. Only a few measurements of the optical reflectivity with dioxides of thorium, uranium and plutonium have been published [ 5 - 81. We report here the photoacoustic spectra of some actinide oxides (thorium to curium).
2. Experimental details The absorption spectra in the wavelength range 200 - 1500 nm have been measured with a home-built photoacoustic spectrometer, which is desribed elsewhere [ 41. Owing to the use of two different gratings blazed at 500 and 1000 nm with 1200 and 600 lines mm-‘, the bandpass in the 200 - 800 nm range is 6 run, and 12 nm in the IR 800 - 1500 nm range. The actinide dioxides (thorium to americium) were sintered in air at temperatures of 700 - 1000 “C directly before the photoacoustic absorption spectra were measured. To overcome signal saturation in the cases of the intensely coloured actinide oxides they were mixed with SiOZ (18 - 32 pm) in a vibrating mill. A sample of approximately eight years old curium-244 dioxide was measured after separating the daughter plutonium-240 from 244Cm02.
3. Results and discussion The absorption spectra of the free ions of trivalent actinides and lanthanides in crystals show a remarkable similarity. They consist of sharp, clearly separated groups of lines. An energy level diagram of the free trivalent ions of actinide compounds has been published recently [ 9, lo]. The situation is much more complicated with the tetravalent actinides, because the ligand field interactions of tetravalent actinides are stronger than those of the trivalent ones. This makes the interpretation of these absorption spectra much more difficult. Furthermore, a predictive theoretical model for the tetravalent ions is more difficult to obtain than for the trivalent f-element spectra, because the degree of charge transfer and mixing with other states between metal and ligand is unknown. Nevertheless, a free ion energy level diagram of the tetravalent actinides was available for the interpretation of our photoacoustic absorption spectra [ 111. The spectroscopic properties of the actinide dioxide in the UV confirm the presence of band-like electrons. However, in contrast with this fact, the semiconducting character and the magnetic properties of the actinide oxides call for localized 5f states. All these circumstances complicate the interpretation of the tetravalent actinides and can easily be visualized by the absorption spectra of the actinide dioxides. The absorption spectrum of sintered, white ThO2 is quite simple to interpret, because there are no 5f electrons. Thus, there is no absorption in the IR and visible spectra (Fig. 1). The
27
200
300
100
SO0
600
700 800 Woudength
900 1000 I nm 1
1100
IZM
1300
1100
I IO
Fig. 1. Photoacousticspectraof ThOz,UOzand Us08 powders. increasing absorption starting at about 250 nm appears to be due to transitions from the top of the valence band, which is built up with 0-2~ levels, to the bottom of the conduction band, which is built up with empty 6d and probably 7s levels. Our photoacoustic absorption spectra of ThOz and UOz agree well with those measured by diffuse reflectance spectroscopy [ 5,6]. In UOz (Fig. 1) the absorption bands are broad and intensive. Narrow well-resolved absorption peaks as in the spectra of the lanthanide sesquioxides [l, 2, 41 cannot be observed. To overcome photoacoustic signal saturation the sample of U02 has to be “diluted” with silical gel or ThOz. The rapidly increasing absorption at 16700 cm-’ (600 nm) with small maxima at 473, 450, 406, 388 and 333 nm is consistent with optical spectra of UOz obtained by other spectroscopic techniques [6, 12, 131. Since the 6d electron band in actinides is shifted to lower energies, we attribute, with some uncertainty, the absorption bands between 600 and 350 nm to transitions from occupied 5f levels to the 6d band which consists of numerous individual crystal field split levels. An energy level scheme for UOz places the lowest-lying 5f2 --f 5f’ 6d transition near 590 nm [ 71. This assignment agrees with a first-principles molecular cluster calculation by Gubanov et al. on U02 [ 141. The assignment by Schoenes [ 71, however, is controversial [ 6, 151. Naegele et al. [6] and Catlow [ 151 attribute the intense absorption bands in the 32 000 - 64 000 cm-’ range to the 5f2 + 5f’6d transition. Another explanation for the broad absorption bands in the UV is the assumption of an electron transfer process which is produced by an electron being transferred from a molecular oxygen orbital to a partly filled or empty 5f electron state of the oxidizing central uranium atom. It has been shown that electron transfer bands compete with the 5f” + 5f”- ’ 6d bands, especially in the higher actinides [ 161. U308 shows an absorption spectrum similar to that of U02 (see Fig. 1). The absorption band between 600 and 300 nm is slightly suppressed relative to the absorption band in the UV. This fact is consistent with the observation that as the oxidation state in the binary oxides is increased, electrons are transferred from the localized 5f states into the bonding orbit& which are predominantly 0-2~ in character. The sini-
28
larity in the absorption spectra of the two uranium oxides shows that the stoiehiometry has almost no influence on the internal electronic transitions of uranium. The pho~acoustic absorption spectrum of neptunium dioxide (Fig. 2) is characterized by an intense and broad absorption band with a maximum near 350 nm and some less intensive but marked peaks in the visible and IR regions. The intense absorption band in the UV may be due to an electron transfer transition or an allowed 5f3 -+ 5f26d transition. The numerous sharp peaks of lower intensity in the visible region may be at~bu~ to transitions within the manifold of the 5f electron levels. Plutonium dioxide shows an absorption spectrum similar to that of Np02 (Fig. 2). The 5f4 =+5f 3 transitions are not as marked as in Np02. The only sharp and well resolved peak at about 1430 nm is due to adsorbed water molecules at the surface of the Pu0,j(Si02), mixture. The absorption spectrum of AmOz is very similar to that of Pu02 (Fig. 3). The tr~sitions within the 5f electron levels are overlapped by the intense broad absorption covering the whole spectral range. However, in the absorption spectrum of
20 10
*L_.,__ 200
300
I
I.00
-
330
600
Fig. 2. Photoacoustic
IQ0 801 WovPIengtk
900 1nna j “In I
1lUQ
12QO
_A_-.-i
1300
Ml0
1SOQ
spectra of NpOz and PM& powders,
IG / *
h_.-.A-.
200
300
100
5QQ
600
700 nun Wwelength
L
908 1QnO I nm 1
,,PQ
~~.~~~~
i2QQ
,300
,‘OQ
1500
Fig. 3. Photoacoustic spectra of AmOz, Am203 and Cm& levels of Am3+ are indicated at the top).
powders (the free-ion energy
29
the trivalent americium oxide there are some narrow peaks. The three peaks about 1100 run, which superimpose the broad absorption common in all actinide oxides, can be assigned to a ‘F, + ‘Fe transition. The free ion energy levels of the trivalent americium [lo] are indicated at the top of Fig. 3. Curium dioxide shows a nearly continuous absorption (Fig. 3) in the IR and in the visible regions. 4. Conclusions Compared with the absorption spectra of the lanthanide oxides [ 1 - 41, the absorption spectra of the oxides of the actinides are far from similar. The narrow weak absorption bands, which are attributed to electron transitions within the 5f” shell, cannot be observed in uranium, americium and curium dioxides. In neptunium and plutonium dioxides as well as in americium(II1) oxide they are observable, but are overlapped by very broad absorption bands extending from the UV to the visible and sometimes to the IR. The conclusion is that the 5f electrons in the actinide dioxide are not well localized as the 4f electrons in the lanthanide sesquioxides. This produces the condition of strong competition between the 5f electrons and the 6d and 7s electrons and their participation in bonding. There is also evidence for electron transfer transitions in the actinide oxides which cause intense and broad absorption bands in the UV and visible where they produce the reddishbrown and black colours of the actinide dioxides. Acknowledgments We thank Dr. H. Bokelund for a sample of curium-244 dioxide and Mr. U. Bemdt for the separation of plutonium-240 from this curium dioxide sample. References 1 J. R. Schoonover, Y.-L. Lee, S. N. Su, S. H. Lin and L. Eyring, Appl. Spectrosc., 38 (1984) 154. 2 R. Tilgner, Appl. Opt., 20 (1981) 3780. 3 A. C. Tam and C. K. N. Patel,AppL Phys. Lett., 35 (1979) 843. 4 G. Heinrich, H. Giisten and H. J. Ache, Apple Spectrosc., 40 (1986) 363. 5 P. Imris and D. Imris, J. Znorg. Nucl. Chem., 27 (1965) 2135. 6 J. Naegele, L. Manes and H. Winkelmann, in J. Mulak, W. Silski and R. Trod6 (eds.), Proc. 2nd Znt. Co& Electron Structure of the Actinides, 1976, Wrodhw, Poland, Polish Academy of Science, Warwaw, 1977, p. 163. 7 J. Schoenes, J. Apple Phys., 49 (1978) 1463. 8 D. T. Hodul, Spectrosc. Lett., 16 (1983) 181. 9 J. P. Hessler and W. T. CarnaIl, in Lanthanide and Actinide Chemistry and Spectroscopy, ACS symp. Ser., 131 (1980) Chapter 17. 10 W. T. Carnal1 and H. M. Crosswhite, in J. J. Katz, G. T. Seaborg and L. R. Morss (eds.), The Chemistry of the Actinide Elements, Chapman and Hall, London, 1985.
30
11 W. T. Carnal& personal communication. 12 J. Ackermann, R. J. Thorn and G. H. Winslow, J. Opt. Sot. Am., 49 (1959) 1107. 13 L. Manerr and J. Naegele, in H. Blank and R. Lindner (eds.), Plutonium 1975 and Other Actinidee, North-Holland, Amsterdam, 1976, p. 363. 14 V. A. Guhanov, A. Rosen and D. E. Ellis, Solid-State Commun., 22 (1977) 219. 15 C. R. A. Catlow, J. Chem. Sot., Faraday Trans. 2, 74 (1978) 1901. 16 L. J. Nugent, R. D. Baybarz, J. L. Burnett and J. L. Ryan,J. Phys. Chem., 77 (1973) 1528.