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The electronic structure of UAl2 and UCo2: A photoemission study
Physica 102B (1980) 122-125 North-Holland Publishing Company
THE ELECTRONIC STRUCTURE OF UAI2 AND UCo2: A PHOTOEMISSION STUDY J.R. N A E G E L E , L. MANES, J.C. SPIRLET, L. P E L L E G R I N I Commission of the European Communities, Joint Research Centre, Karlsruhe Establishment, European Institute for Transuranium Elements, Post[ach 2266, D-7500 Karlsruhe, Fed. Rep. Germany
J.M. F O U R N I E R Centre d'Etudes Nucl6aires de Grenoble, D6partement de Recherche Fondamentale, Section de Physique du Solide, 85X, 38041 Grenoble Cedex, France
Polycrystalline UAI2 and UCo2 samples have been studied by photoemission spectroscopy for different excitation energies (21.2, 40.8, 48.4, and 1486.6eV). For UAI2 a structure is found at 7.5 eV binding energy for only 21.2 eV excitation and is attributed to AI 3p, 3s electrons. U 5f electrons give rise to a very narrow and intense peak with an abrupt cutoff at the Fermi level. For UCo2 Co 3d electrons are found at 0.8 eV binding energy. The narrow peak at EF growing with excitation energy is associated with U 5f electrons. The weaker emission from U 5f states in UCo2 indicates a stronger hybridization between 5f and 3d states for UCo2. have been described in detail elsewhere [1]. The photoemission spectra were obtained with a Leybold spectrometer (AI X-ray source and a windowless U V H e discharge source), The vacuum in the measuring chamber was 5.5 × 10-9 Pa, which dropped to 6.5 × 10-8 Pa due to H e leakage during UPS work. The samples were cleaned in situ by argon ion sputtering and/or scraping with an A1203 file.
1. Introduction Photoemission measurements of intermetallic actinide compounds have been performed previously for UAI2 and UCo2 [1]. These measurements have been mainly performed for surface investigation. UAI2 and UCo2, which belong to the Laves phases, crystallize in the cubic MgCu2-type structure (C 15). Both materials show no magnetic ordering [2, 3]. The actinide-actinide spacing (UAI2:3.38/~; U C o 2 : 3 . 0 3 / ~ ) is below the 3.4-3.6 ~ separation proposed by Hill [4] as a cutoff for localization of 5f electrons in uranium compounds. The very high electronic specific heat coefficient of UAI2 (3' = 142 m J/mole. K 2) [5] can be understood by assuming a narrow 5f band at the Fermi level. For UCo2 the smaller lattice constant would induce a stronger hybridization between 5f and anion states. Photoemission distribution curves reflect directly the density of occupied states. The use of different excitation energies permits, in general, the separation of partial density of states contributing to band states [6, 7] since the excitation cross section is energy dependent [8].
3. Results and discussion The influence of different in situ surface preparation on core level and valence band spectra has been described in detail previously [1]. Scraping in comparison to sputtering of UAI2 does not induce a change of the energetic position of structures for the valence band, but lowers the emission at EF. For UCo2 this effect is much less pronounced. The O 2p signal around 5 eV binding energy could not completely be avoided. Therefore fig. 1 compares UPS spectra which have been recorded under similar surface conditions and for about the same He-source intensity for both compounds. The spectra have been normalized for constant analyzer transmission. The difference between the two curves in fig. 1 is striking: for H e I excitation UAI2 shows at
2. Experimental The sample preparation and surface treatment 122
J.R. Naegele et al./Electronic structure of UAi2 and UCo2
I--~--- He l, 21.2 eV
*1
,-t-
123
-'-,
Hell, 40,8 eV
o
i"/l
u~ Z 14.1 Z Z
g IE uJ
\
°i
\ \
\
t
02p
\ \\
I .... 10
I,, 5
I ~=0(Hen
I,, 15
,I, 10
, 5
EF=0(HelI)
BINDING ENERGY (eV} Fig. 1. Photoemission spectra for UAI2 and UC02 for He I and He II excitation (the spectra are normalized for constant analyzer transmission; secondary electron emission is indicated by the broken line). Resolution A E = 1 0 0 m e V at EF (HeI) and A E = 200 m e V at EF file II).
about 7.5 e V a structure which is missing for He II and also missing in UCo2. The emission at EF is quite different and shows for UAI2 a very intense increase up to EF followed by an abrupt cutoff at EF. For UCo: a pronounced peak is found at 0.8 eV for He I excitation, whereas for He II excitation the peak has decreased relatively to the emission at EF. The ratio of emission intensity at EF for He II and He I excitation is much larger for UAI2 than for UCo2. Fig. 2 summarizes the photoemission results for UAI2 and UCo2. Comparing with recent high resolution photoemission data on U [9, 10], Co [11-13], and AI [14, 15] the following observations are made:
(1) the photoemission spectra of UA12 look very similar to those of U except the structure at 7.5 eV, and (2) the He I excited spectrum for UCo2 is very similar to the He I excited spectrum for Co. Taking into account the energy dependence of the photoemission cross section [6,8, 16] the photoemission spectra of UA12 can be explained by a superimposition of the valence bands of AI (3s, 3p electrons) and U (5f, 6d, 7s electrons). The valence band for AI [14, 15] shows a weak, nearly free electron density of states (DOS). Thus, the UAI2 spectrum (He I excitation) is dominated by the emission of U 6d, 5f electrons. The weak structure at 7.5 eV is attributed to a
124
J.R. Naegele et al./Electronic structure of UAI2 and UCo2
21.2eV
\
t~
>-
UAI 2
~,
i-or)
UAI2
U5f
CoZ,s?
U6d,~~ ~ ,\
j~
\
\
\
z~
\
UC°2 \/
Co3d .... ~
z LIJ Z
UC°2
-vl
', /~
I I
o tn 2E Z
10
O3
I.U
5 EF BINDING ENERGY(eV)
Fig. 3. Tentative density of states scheme for UAI2 and UCo2. 48,4eV
\
lz.86.6eV
10
5
EF=0 10
5
F_,-=0
BINDINGENERGY (eV) Fig. 2. Photoemission spectra for UAI2 and UCo2 (the spectra are normalized for constant analyzer transmission;
secondary electron emission is indicated by the broken line).
maximum in the DOS of the sp-band of AI. Its disappearance for higher excitation energy is explained by cross-section effects. The emission at EF, which increases with excitation energy (the O 2p emission can be used as a reference), is explained by a narrow 5f band consistent with the very high electronic specific heat coefficient. For He I excitation the UCo2 spectrum is dominated by the emission of a nearly filled Co 3d band with a maximum at 0.8 eV. Owing to the smaller lattice constant of UCo2, the O 5f electrons can be assumed to hybridize strongly
with Co 3d states. Thus, the 5f intensity at EF is expected to be weaker than in UAI2, in agreement with the observation for He I excitation (fig. 1). With increasing excitation energy the relative 5f emission increases; the peak, which develops at EF (fig. 2) is therefore attributed to 5f electrons. Unfortunately no specific heat measurements exist for UCo2. However, resistivity measurements indicate that it is an exchange reinforced paramagnet, as is also YCo2. T h u s , it must have a rather large density of states at the Fermi level. The above proposed picture would predict a smaller electronic specific heat coefficient for UCo2 compared with that for UAI2. From the above consideration a tentative DOS picture is derived and shown in fig. 3.
Acknowledgement We are grateful to Mr. N. Nolte for his expert technical support. References [I] J.R. Naegele, L. Manes, J.C. Spirlet and J.M. Fournier, to appear in Appl. Surf. Sci. 4 (1980).
J.R. Naegele et al./Electronic structure of UAI2 and UCo2 [2] J.M. Fournier, Solid State Commun. 29 (1979) 111. [3] D.J. Lam and A.T. Aldred, AIP Conf. Proc. 24 (1974) 349. [4] H.H. Hill, in: Plutonium 1970 and Other Actinides, W.N. Miner, ed. (AIME, New York, 1970) p. 2. [5] R.J. Trainor, M.B. Brodsky and H.V. Culbert, Phys. Rev. Lett. 34 (1975) 1019. [6] D.E. Eastman and M. Kuznietz, J. Appl. Phys. 42 (1971) 1396. [7] J. Tajeda, N.J. Shevchik, W. Braun, A. Goldmann and M. Cardona, Phys. Rev. B12 (1975) 1557. [8] D.J. Kennedy and S.T. Manson, Phys. Rev. A5 (1972) 227.
125
[9] H. Grohs, H. H6chst, P. Steiner, S. Hiifner and K.H.J. Buschow, Solid State Commun. 33 (1980) 573. [10] Y. Baer and J.K. Lang, to be published in Phys. Rev. [11] P. Heimann, E. Marschall, H. Neddermeyer, M. Pessa and H.F. Roloff, Phys. Rev. B16 (1977) 2575. [12] A. Amamou and G. Krill, Solid State Commun. 31 (1979) 971. [13] D.E. Eastman, F.J. Himpsel and J.A. Knapp, J. Appl. Phys. 50 (1979) 7423. [14] Y. Baer and G. Busch, Phys. Rev. Lett. 30 (1973) 280. [15] W. Eberhardt and F.J. Himpsel, Phys. Rev. Lett. 42 (1979) 1375. [16] J.H. Scofield, J. Electron. Spectrosc. 8 (1976) 129.
THE ELECTRONIC STRUCTURE OF UAI2 AND UCo2: A PHOTOEMISSION STUDY J.R. N A E G E L E , L. MANES, J.C. SPIRLET, L. P E L L E G R I N I Commission of the European Communities, Joint Research Centre, Karlsruhe Establishment, European Institute for Transuranium Elements, Post[ach 2266, D-7500 Karlsruhe, Fed. Rep. Germany
J.M. F O U R N I E R Centre d'Etudes Nucl6aires de Grenoble, D6partement de Recherche Fondamentale, Section de Physique du Solide, 85X, 38041 Grenoble Cedex, France
Polycrystalline UAI2 and UCo2 samples have been studied by photoemission spectroscopy for different excitation energies (21.2, 40.8, 48.4, and 1486.6eV). For UAI2 a structure is found at 7.5 eV binding energy for only 21.2 eV excitation and is attributed to AI 3p, 3s electrons. U 5f electrons give rise to a very narrow and intense peak with an abrupt cutoff at the Fermi level. For UCo2 Co 3d electrons are found at 0.8 eV binding energy. The narrow peak at EF growing with excitation energy is associated with U 5f electrons. The weaker emission from U 5f states in UCo2 indicates a stronger hybridization between 5f and 3d states for UCo2. have been described in detail elsewhere [1]. The photoemission spectra were obtained with a Leybold spectrometer (AI X-ray source and a windowless U V H e discharge source), The vacuum in the measuring chamber was 5.5 × 10-9 Pa, which dropped to 6.5 × 10-8 Pa due to H e leakage during UPS work. The samples were cleaned in situ by argon ion sputtering and/or scraping with an A1203 file.
1. Introduction Photoemission measurements of intermetallic actinide compounds have been performed previously for UAI2 and UCo2 [1]. These measurements have been mainly performed for surface investigation. UAI2 and UCo2, which belong to the Laves phases, crystallize in the cubic MgCu2-type structure (C 15). Both materials show no magnetic ordering [2, 3]. The actinide-actinide spacing (UAI2:3.38/~; U C o 2 : 3 . 0 3 / ~ ) is below the 3.4-3.6 ~ separation proposed by Hill [4] as a cutoff for localization of 5f electrons in uranium compounds. The very high electronic specific heat coefficient of UAI2 (3' = 142 m J/mole. K 2) [5] can be understood by assuming a narrow 5f band at the Fermi level. For UCo2 the smaller lattice constant would induce a stronger hybridization between 5f and anion states. Photoemission distribution curves reflect directly the density of occupied states. The use of different excitation energies permits, in general, the separation of partial density of states contributing to band states [6, 7] since the excitation cross section is energy dependent [8].
3. Results and discussion The influence of different in situ surface preparation on core level and valence band spectra has been described in detail previously [1]. Scraping in comparison to sputtering of UAI2 does not induce a change of the energetic position of structures for the valence band, but lowers the emission at EF. For UCo2 this effect is much less pronounced. The O 2p signal around 5 eV binding energy could not completely be avoided. Therefore fig. 1 compares UPS spectra which have been recorded under similar surface conditions and for about the same He-source intensity for both compounds. The spectra have been normalized for constant analyzer transmission. The difference between the two curves in fig. 1 is striking: for H e I excitation UAI2 shows at
2. Experimental The sample preparation and surface treatment 122
J.R. Naegele et al./Electronic structure of UAi2 and UCo2
I--~--- He l, 21.2 eV
*1
,-t-
123
-'-,
Hell, 40,8 eV
o
i"/l
u~ Z 14.1 Z Z
g IE uJ
\
°i
\ \
\
t
02p
\ \\
I .... 10
I,, 5
I ~=0(Hen
I,, 15
,I, 10
, 5
EF=0(HelI)
BINDING ENERGY (eV} Fig. 1. Photoemission spectra for UAI2 and UC02 for He I and He II excitation (the spectra are normalized for constant analyzer transmission; secondary electron emission is indicated by the broken line). Resolution A E = 1 0 0 m e V at EF (HeI) and A E = 200 m e V at EF file II).
about 7.5 e V a structure which is missing for He II and also missing in UCo2. The emission at EF is quite different and shows for UAI2 a very intense increase up to EF followed by an abrupt cutoff at EF. For UCo: a pronounced peak is found at 0.8 eV for He I excitation, whereas for He II excitation the peak has decreased relatively to the emission at EF. The ratio of emission intensity at EF for He II and He I excitation is much larger for UAI2 than for UCo2. Fig. 2 summarizes the photoemission results for UAI2 and UCo2. Comparing with recent high resolution photoemission data on U [9, 10], Co [11-13], and AI [14, 15] the following observations are made:
(1) the photoemission spectra of UA12 look very similar to those of U except the structure at 7.5 eV, and (2) the He I excited spectrum for UCo2 is very similar to the He I excited spectrum for Co. Taking into account the energy dependence of the photoemission cross section [6,8, 16] the photoemission spectra of UA12 can be explained by a superimposition of the valence bands of AI (3s, 3p electrons) and U (5f, 6d, 7s electrons). The valence band for AI [14, 15] shows a weak, nearly free electron density of states (DOS). Thus, the UAI2 spectrum (He I excitation) is dominated by the emission of U 6d, 5f electrons. The weak structure at 7.5 eV is attributed to a
124
J.R. Naegele et al./Electronic structure of UAI2 and UCo2
21.2eV
\
t~
>-
UAI 2
~,
i-or)
UAI2
U5f
CoZ,s?
U6d,~~ ~ ,\
j~
\
\
\
z~
\
UC°2 \/
Co3d .... ~
z LIJ Z
UC°2
-vl
', /~
I I
o tn 2E Z
10
O3
I.U
5 EF BINDING ENERGY(eV)
Fig. 3. Tentative density of states scheme for UAI2 and UCo2. 48,4eV
\
lz.86.6eV
10
5
EF=0 10
5
F_,-=0
BINDINGENERGY (eV) Fig. 2. Photoemission spectra for UAI2 and UCo2 (the spectra are normalized for constant analyzer transmission;
secondary electron emission is indicated by the broken line).
maximum in the DOS of the sp-band of AI. Its disappearance for higher excitation energy is explained by cross-section effects. The emission at EF, which increases with excitation energy (the O 2p emission can be used as a reference), is explained by a narrow 5f band consistent with the very high electronic specific heat coefficient. For He I excitation the UCo2 spectrum is dominated by the emission of a nearly filled Co 3d band with a maximum at 0.8 eV. Owing to the smaller lattice constant of UCo2, the O 5f electrons can be assumed to hybridize strongly
with Co 3d states. Thus, the 5f intensity at EF is expected to be weaker than in UAI2, in agreement with the observation for He I excitation (fig. 1). With increasing excitation energy the relative 5f emission increases; the peak, which develops at EF (fig. 2) is therefore attributed to 5f electrons. Unfortunately no specific heat measurements exist for UCo2. However, resistivity measurements indicate that it is an exchange reinforced paramagnet, as is also YCo2. T h u s , it must have a rather large density of states at the Fermi level. The above proposed picture would predict a smaller electronic specific heat coefficient for UCo2 compared with that for UAI2. From the above consideration a tentative DOS picture is derived and shown in fig. 3.
Acknowledgement We are grateful to Mr. N. Nolte for his expert technical support. References [I] J.R. Naegele, L. Manes, J.C. Spirlet and J.M. Fournier, to appear in Appl. Surf. Sci. 4 (1980).
J.R. Naegele et al./Electronic structure of UAI2 and UCo2 [2] J.M. Fournier, Solid State Commun. 29 (1979) 111. [3] D.J. Lam and A.T. Aldred, AIP Conf. Proc. 24 (1974) 349. [4] H.H. Hill, in: Plutonium 1970 and Other Actinides, W.N. Miner, ed. (AIME, New York, 1970) p. 2. [5] R.J. Trainor, M.B. Brodsky and H.V. Culbert, Phys. Rev. Lett. 34 (1975) 1019. [6] D.E. Eastman and M. Kuznietz, J. Appl. Phys. 42 (1971) 1396. [7] J. Tajeda, N.J. Shevchik, W. Braun, A. Goldmann and M. Cardona, Phys. Rev. B12 (1975) 1557. [8] D.J. Kennedy and S.T. Manson, Phys. Rev. A5 (1972) 227.
125
[9] H. Grohs, H. H6chst, P. Steiner, S. Hiifner and K.H.J. Buschow, Solid State Commun. 33 (1980) 573. [10] Y. Baer and J.K. Lang, to be published in Phys. Rev. [11] P. Heimann, E. Marschall, H. Neddermeyer, M. Pessa and H.F. Roloff, Phys. Rev. B16 (1977) 2575. [12] A. Amamou and G. Krill, Solid State Commun. 31 (1979) 971. [13] D.E. Eastman, F.J. Himpsel and J.A. Knapp, J. Appl. Phys. 50 (1979) 7423. [14] Y. Baer and G. Busch, Phys. Rev. Lett. 30 (1973) 280. [15] W. Eberhardt and F.J. Himpsel, Phys. Rev. Lett. 42 (1979) 1375. [16] J.H. Scofield, J. Electron. Spectrosc. 8 (1976) 129.