The electronic structure of glassy uranium and thorium alloys

Journal of Non-Crystalline Solids 61 & 62 (1984) 1067-1072 North-Holland, Amsterdam

1067

THE ELECTRONIC STRUCTURE OF GLASSY URANIUM AND THORIUM ALLOYS P. OELH~FEN, G. INDLEKOFER, J. KRIEG, R. LAPKA, U.M. GUBLER and H.-J. GUNTHERODT, C.F. HAGUE* and J.-M. MARIOT* I n s t i t u t fur Physik der Universit~t Basel, Klingelbergstrasse 82 CH-4056 Basel, Switzerland *Laboratoire de Chimie Physique, Universit~ Pierre et Marie Curie F-75231 Paris Cedex 05, France We have studied the electronic structure of glassy Fe34U66, Co40U60, Ni33U67, Ni4oU60 and Fe45Th55 by photoelectron spectroscopy and X-ray emission spectroscopy. The uranium alloys e x h i b i t a narrow 5f band which remains pinned to the Fermi l e v e l . Also the binding energy s h i f t s of the U4f core levels on a l l o y i n g are rather small. Therefore, we conclude that uranium has the same e l e c t r o n i c configuration (5f 3) in these U rich alloys as in U metal. S i m i l a r l y to pure thorium, no occupied 5f states have been observed in Fe45Th55.

Photoelectron spectroscopy and X-ray emission spectroscopy have turned out to be e f f i c i e n t tools for the experimental study of the valence bands and core electrons of glassy alloys 1'2. Therefore, the a p p l i c a t i o n of these experiments to the recently reported a c t i n i d e glasses 3'4 is of great i n t e r e s t . Moreover the a c t i n i d e alloys in the c r y s t a l l i n e state e x h i b i t i n t e r e s t i n g e l e c t r o n i c properties 5 - 9 a s e . g . t h e d i f f e r e n t e l e c t r o n i c configurations f o r U (5f 3 and 5f 2) observed in various a l l o y s . In this context the degree of the l o c a l i z a t i o n of the 5f electrons is of p a r t i c u l a r i n t e r e s t . The m e t a l l i c glasses were prepared in high vacuum by rapid quenching from the l i q u i d state using a piston and anvil apparatus. The photoelectron spectroscopy measurements were performed with a Leybold-Heraeus spectrometer (EAIO/IO0). For f u r t h e r d e t a i l s concerning sample preparation and measurements see Ref.l and 2. The UPS valence band (VB) spectra of glassy Ni4oU60 and the p o l y c r y s t a l l i n e pure constituents are shown in F i g . l . The peak positions in U and Ni are close to EF (with binding energies of 0.I and 0 . 2 e V , respectively) and the state densities near EF are dominated by 6d and 5f electrons (5f 3 configuration) in U and by 3d electrons in Ni. In contrast to the pure metals the VB spectrum of the a l l o y e x h i b i t s two d i s t i n c t peaks, the f i r s t

one close to EF at a binding

energy of 0.I eV and the second located at 1.7 eV. 0022-3093/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

1068

P. Oelhafen et al. / The electronic structure o f glassy uranium and thorium alloys

Since the photoelectron VB spectra

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l

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I

are related to the t o t a l density of states the contributions from the sites of the two constituents to the a l l o y VB spectrum in general cannot be deduced from a single spectrum. We have used three d i f f e r e n t methods in order to elucidate the contributions of U and Ni to the a l l o y VB spectrum shown in F i g . l :

( i ) the v a r i a t i o n

of the a l l o y concentration,

(ii)

X-ray

emission spectroscopy, which is a local probe 2 and

(iii)

6

~

EF

2

BINDING ENERGY [eV]

the v a r i a t i o n

of the e x c i t a t i o n energy in UPS. The influence of a change in the a l l o y concentration is shown in Fig.2 .

FIGURE 1 UPS valence band spectra f o r glassy Ni4oU60 and the p o l y c r y s t a l l i n e cons t i t u e n t s Ni and U.

The UPS VB spectrum of Ni4oU60 is plotted a f t e r subtraction of a l i n e a r background between EF and 5.0 eV. The difference spectrum was obtained by computing the difference o£ normalized VB spectra of Ni4oU60 and Ni33U67 . The normalization has been performed in such a way that the r a t i o of the areas of the VB spectra are equal to the r a t i o of t o t a l number of valence electrons in the two a l l o y s . Although we make the crude approximation of equal e x c i t a t i o n cross sections f o r a l l valence electrons by doing this normalization the difference spectrum of Fig.2 gives at least a good q u a l i t a t i v e picture f o r the increase of the Ni 3d states ( p o s i t i v e c o n t r i b u t i o n ) and the decrease of U5f/6d

states (negative

c o n t r i b u t i o n ) by comparing Ni33U67 with Ni4oU60 . The difference spectrum shows, that the Ni 3d band maximum is located at 1.7eV and the FWHM is about 1.5eV. The XES Ni L emission bands f o r Ni metal and glassy Ni33U67 are shown in Fig.3 . The spectra have not been measured with the best possible instrumental resolution in order to reduce the measuring time and to prevent the formation of a surface oxide layer I0. The s h i f t of the Ni 3d band to higher binding energies on a l l o y i n g is evident from Fig.3 . In the Ni33U67 a l l o y the 3d band peak is located at a binding energy of 2.0eV in reasonable agreement with the 1.7eV obtained from the difference spectrum in Fig.2 . Despite the f a c t of a poor resolution the Ni L band is d i s t i n c t l y narrower by about 0.6ev a l l o y than in Ni.

in the

P. Oelhafen et al. / The electronic structure o f glassy uranium and thorium alloys

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UPS hv=21.2e ~ j /

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1069

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XES

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Ni L~ ee Q I o

am

eQ

Ni >-

,.,j. ~l,e

eeoQ

.,,~-°"

°

~,* I

•

•

ee

F--

z

o°

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~.

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2 EF BINDIN6 ENERSYleVI

FIGURE 2 UPS valence band spectrum of Ni4oU60 a f t e r background subtraction. The difference spectrum was obtained by subtracting a normalized spectrum (see t e x t ) of Ni33U67 from that of Ni4oU60.

D%oo °*B~ e

Ni33U6"/...'"

spectrum(xS) '~ 10

B

6 ~ 2 BINDING ENERSY leVI

EF

FIGURE 3 Ni L~emission from Ni metal and glassy Ni33U67 . The Fermi level position was obtained by taking into account the Ni 2P3/2 binding energy and the core level s h i f t on a l l o y i n g .

The UPS VB of glassy Ni4oU60 measured with three d i f f e r e n t e x c i t a t i o n energies are shown in Fig.4 .The most marked difference in the VB spectra of Fig.4 is the r e l a t i v e i n t e n s i t y of the two VB peaks, i . e . the peak near EF becomes more dominant at higher e x c i t a t i o n energies. This behavior is typical for the U5f

states 5 ' I I and therefore these measurements c l e a r l y demonstrate the 5f

character of the narrow band at EF in Ni4oU60 and Ni33U67 . A comparison of three d i f f e r e n t glassy U alloys Fe34U66 , Co40U60 and Ni4oU60 measured with UPS ( h v = 2 1 . 2 e V ) is shown in Fig.5 . S i m i l a r l y to the VB spectrum of Ni4oU60 a two peak structure was found in Co40U60 and Fe34U66 . Again, a v a r i a t i o n of the e x c i t a t i o n energy demonstrates the 5f character of the narrow band at EF in Fe34U66 and Co40U60 . The peak related to the 3d electrons shows a decrease in the binding energy from Ni4oU60 to Co40U60 and Fe34U66 ( 1 . 7 e V , 1.2eV and < O,6eV, r e s p e c t i v e l y ) . A s i m i l a r trend has been found in the glassy Ni37Zr63, Co40Zr60 and Fe3oZr70 (1.85eV, I . I eV and 0 . 8 e V , r e s p e c t i v e l y ) . The UPS (hv=21.2eV)VB spectra of Fe34U66 and Fe45Th55 are compared in Fig.6. By analogy with the Zr alloys we a t t r i b u t e the peak at 0.6eV to Fe3d states. This i n t e r p r e t a t i o n is supported by the observed changes in the XPS core l i n e shapes on a l l o y i n g which w i l l be discussed below. The weak peak close to EF is j u s t present in the He I (h~=21.2eV)spectrum and is not v i s i b l e at h~=40.8eV. Therefore, we conclude that in contrast to Fe34U66 no occupied 5f

1070

P Oelhafen et al. / The electronic structure o f glassy uranium and thorium alloys

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/, 2 EF BINDING ENERGY[eV]

FIGURE 4 UPS valence band spectra of Ni4oU60 measured with d i f f e r e n t e x c i t a t i o n energies.

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~ 2 EF BINDING ENERGY[eV]

FIGURE A comparison of three U a l l o y s : UPS valence Fe34U66 , Co40U60 and

5 d i f f e r e n t glassy band spectra of Ni4oU60 .

states are present at EF in the glassy Th a l l o y . The core level binding energy s h i f t s AEB and the changes of the asymmetry Am of the core l i n e shapes on a l l o y i n g are presented in Table I. A p o s i t i v e value of AEB means an increase in the binding energy on a l l o y i n g . The + and - signs f o r A~ i n d i c a t e an increase and decrease of the core l i n e shape asymmetry, r e s p e c t i v e l y . Except f o r the Ni core l e v e l s a l l binding energy s h i f t s are r a t h e r small, of the order of 0.2eV or less. This indicates t h a t only a small charge t r a n s f e r occurs on a l l o y i n g . In a d d i t i o n the small binding energy s h i f t s of the U4f7/2

l e v e l s c l e a r l y show t h a t U has the same e l e c t r o n i c c o n f i g u r a t i o n

(5f 3) in the glassy a l l o y s as in U metal. This can be i l l u s t r a t e d

by taking UO2

as a reference f o r U in a t e t r a v a l e n t U4+ (5f 2) c o n f i g u r a t i o n where a U4f7/2 binding energy s h i f t of the order of 3eV 5'6 has been observed. The core l i n e asymmetry changes, Am, are most pronounced at the 3d metal s i t e and are somewhat less d i s t i n c t f o r the 4f l e v e l s at the a c t i n i d e s i t e . As discussed in previous p u b l i c a t i o n s I the asymmetry of a core l i n e is r e l a t e d to the local d e n s i t y of states at EF . Therefore, the d i s t i n c t decrease of the asymmetry of the core l i n e s of the 3d metals are r e l a t e d to a strong decrease of the state d e n s i t i e s at EF a t the t r a n s i t i o n metal s i t e due to a s h i f t of the 3d band to higher binding energies on a l l o y i n g . The same behavior has been

1071

P. Oelhafen et al. / The electronic structure o f glassy uranium and thorium alloys --I

observed in glassy Zr alloys with 3d

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UPS hv:2t2 eV

t r a n s i t i o n metals I .

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The s a t e l l i t e s observed in the 4f XPS core level spectra of actinide alloys and compounds are an i n t e r e s t i n g feature

h

closely related to the 5f electron con-

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f i g u r a t i o n 5 ' 7 ' 8 . No s a t e l l i t e s have been observed in the U4fcore level spectra in the glassy U alloys which is f u r t h e r evidence f o r believing that the e l e c t r o -

[

nic configuration of U in the alloys

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BINDLNG

and in U metal is the same. The Th 4f

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ENERGY [eV]

spectrum of Th metal e x h i b i t s a d i s t i n c t s a t e l l i t e at a 2.6eV higher binding energy than the main l i n e 5'12. This has been a t t r i b u t e d to the occupation of an

FIGURE 6 UPS valence band spectra of Fe45Th55 and Fe34U66 .

empty 5f state 8. In the Fe45Th55 a l l o y the i n t e n s i t y of t h i s s a t e l l i t e is strongly reduced by a f a c t o r of about 0.5 and is shifted by about 0.6eV to higher binding energy. The rather small s h i f t of the s a t e l l i t e indicates that only minor changes in the empty 5f states take place on a l l o y i n g . The d i s t i n c t change in the i n t e n s i t y of the s a t e l l i t e could be due to strong h y b r i d i z a t i o n effects between Th6d and Fe3d states. Core level binding energy s h i f t s (in eV) and changes of the core l i n e shapes on a l l o y i n g (see t e x t )

TABLE I

Alloy A-B

a)

A:

2P3/2 AEB

B: A~

4f7/2 AEB

A

Fe34U66

0.17-+0.03

0.12 -+0.03

+

Co40U60

0.20 -+0.05 a)

0.17 ±0.05

+

Ni33U67

0.80±0.I0

0

+

Ni4oU60

0.75 ±0.05

Fe45Th55

0.20 ± 0.05 b)"

_+0.05

0.12 ±0.05

+

-0.28 + 0.05

c)

Co2pv21evel , b) Fe3p level , c) change in the T h 4 f s a t e l l i t e

1072

P. Oelhafen et al. / The electronic structure o f glassy uranium and thorium alloys

In conclusion, the e l e c t r o n i c structure of glassy U and Th alloys with 3d t r a n s i t i o n metal have been studied by photoelectron and X-ray emission spectros. copy. The 3d elements contribute to the valence band structure in a s i m i l a r manner as in glassy t r a n s i t i o n metal alloys with 3d bands located at about the same binding energies. The a c t i n i d e metal plays a s i m i l a r role as the early t r a n s i t i o n metal in the TE-TL alloys (T E and TL are t r a n s i t i o n metals with a less than h a l f and more than h a l f f i l l e d d band, r e s p e c t i v e l y ) . Therefore, the e l e c t r o n i c structure at EF is dominated by the a c t i n i d e metal with a narrow 5f band in U alloys comparable with that in U metal and 6d states in Fe45Th55 . Uranium has the same e l e c t r o n i c configuration (5f 3) in a l l the glassy U alloys studied here. Future investigations w i l l show whether this observation is a general property of glassy U alloys or i f d i f f e r e n t e l e c t r o n i c configurations do e x i s t in more d i l u t e alloys or with other elements as the second c o n s t i t u e n t We would l i k e to thank Dr. F. Greuter f o r valuable suggestions and stimul a t i n g discussions. We are also indebted to P. Reimann f o r s k i l l f u l

preparation

work. Financial support of the Swiss National Science Foundation is g r a t e f u l l y acknowledged. REFERENCES I ) P. Oelhafen, in: Glassy Metals I I , ed. H. Beck and H.-J. GUntherodt, Topics in Applied Physics, Voi.53 (Springer, Heidelberg 1983) 2) C.F. Hague, P. Oelhafen and H.-J. GUntherodt, in: Amorphous M e t a l l i c Alloys, ed. F.E. Luborsky (Butterworths, London 1983) pp.126 3) B.C. Giessen and R.O. E l l i o t t , in: Rapidly Quenched Metals Vol. I , B. Cantor (The Metals Society, 1978) p.406

ed. by

4) K.H.J. Buschow, Solid State Commun° 43 (1982) 171 5) P.R. Norton, R.L. Tapping, D.K. Greber and W.J.Lo Buyers, Phys.Rev. B 21 (1980) 2572 6) H. Grohs, H. H~chst, P. Steiner and S. HUfner, Solid State Commun. 33 (1980) 573 7) Y. Baer, H.R. Ott and K. Andres, Solid State Commun. 36 (1980) 387 8) Y. Baer, Physica I02B (1980) 104 9) W.D. Schneider and C. Laubschat, Phys.Rev. B 23 (1981) 997 IO) P.J. Durham, D. Ghaleb, B.L. Gy~rffy, C.F. Hague, J.-M. Mariot, G.M. Stocks and W.M. Temmerman, J. Phys. F ,g (1979) 1719 11)F. Greuter, E. Hauser, P. Oelhafen, H.-J. GUntherodt, B. Reihl and O. Vogt, Physica 102B (1980) 117 12)J.C. Fuggle, A.F. Burr, W. Lang, L.M. Watson and D.J. Fabian, J. Phys. F , 4 (1974) 335