LOCALIZATION OF 5f STATES IN VARIOUS URANIUM AND THORIUM OXIDES AND GLASSES

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LOCALIZATION OF 5f STATES IN VARIOUS

URANIUM AND THORIUM OXIDES AND GLASSES

J. Petiau, G. Calas, D. Petit-Maire, A. Bianconi, M. Benfatto, A. Marcelli

To cite this version:

J. Petiau, G. Calas, D. Petit-Maire, A. Bianconi, M. Benfatto, et al.. LOCALIZATION OF 5f STATES

IN VARIOUS URANIUM AND THORIUM OXIDES AND GLASSES. Journal de Physique Colloques,

1986, 47 (C8), pp.C8-949-C8-953. �10.1051/jphyscol:19868182�. �jpa-00226088�

JOURNAL DE PHYSIQUE

C o l l o q u e C 8 , s u p p l 6 m e n t au n o 12, Tome 47, d 6 c e m b r e 1 9 8 6

LOCALIZATION OF 5f STATES IN VARIOUS URANIUM AND THORIUM OXIDES AND GLASSES

J. PETIAU, G. CALAS, D. PETIT-MAIRE, A. BIANCONI*, M. BENFATTO*

and A. MARCELLI*

Laboratoire d e Mineralogie-Cristallographie, Universites Paris VI et VII,

U . A .

-C.N.R.S. 0 9 , T - 16, 4, place Jussieu,

F-75252 Paris Cedex 0 5 , France

' ~ i p a r t i m e n t o d i Fisica, Universita "La Sapienza", I-00185 Roma, Italy

RESUME - Le XANES a LtL mesurc a u x s e u i l s M e t L d e l ' u r a n i u m

e t du t h o r i u m d a n s d e s oxydes e t d e s v e r f e s pour 3 s o n d e r l e s L t a t s 6 l e c t r o n i q u e s inoccupLs 5f e t ( 6 d , 7 s ) e t e t u d i e r l a s t r u c t u r e l o c a l e d e s s i t e s de l ' u r a n i u m e t du t h o r i u m d a n s l e s v e r r e s . L ' h y b r i d a t i o n e n t r e l e s L t a t s U(6d,7s) e t U ( 5 f ) s e t r a d u i t p a r une a s s y m e t r i e t r G s marquLe du p i c p r i n c i p a l d ' a b s o r p t i o n v e r s l e s h a u t e s e n e r g i e s d a n s l e s oxydes. Les s p e c t r e s M4 e t M 5 _du t h o r i u m e t d e l ' u r a n i u m d a n s l e s v e r r e s m e t t e n t a u

c o n t r a i r e en e v i d e n c e une t r e s f o r t e l o c a l i s a t i o n d e s & t a t s 5f inoccupLs.

ABSTRACT - ^XANES s p e c t r o s c o p y a t Mg and L e d g e s of uranium and

t h o r i u m h a s been used t o probe the'ufioccupiea 5f e l e c t r o n i c s t a t e s and t h e l o c a l s t r u c t u r e of uranium and t h o r i u m s i t e s i n o x i d e s and g l a s s e s . The p r e s e n c e of 5f components i n t h e U(6d) c o n d u c t i o n band of o x i d e s , due t o h y b r i d i z a t i o n between U(bd,7s) and U ( 5 f ) i s shown by a l o n g asymmetric t a i l of t h e main a b s o r p t i o n peak a t i t s h i g h e n e r g y s i d e e x t e n d i n g up t o 12 eV. On t h e c o n t r a r y t h e p r e s e n c e of narrow 5f l o c a l i z e d unoccupied s t a t e s i s found i n t h o r i u m and uranium c o n t a i n i n g g l a s s e s .

INTRODUCTION

I n a c t i n i d e compounds 5f e l e c t r o n s a r e i n a n i n t e r m e d i a t e l o c a l i z a t i o n regime where t h e 5f c o r r e l a t i o n e n e r g y i s of t h e same o r d e r of magnitude a s t h e 5f bandwidth.

T h e r e f o r e uranium compounds have i n t e r m e d i a t e b e h a v i o u r between t h e s y s t e m s ( l i k e r a r e e a r t h compounds) e x h i b i t i n g l o c a l i z e d e l e c t r o n i c p r o p e r t i e s (e.g. magnetism) and t h e s y s t e m s e x h i b i t i n g d e l o c a l i z e d p r o p e r t i e s (e.g. s u p e r c o n d u c t i v i t y ) (1-2).

Of p a r t i c u l a r i n t e r e s t i s t h e q u e s t i o n of t h e l o c a l i z a t i o n v e r s u s i t i n e r a n c y of 5f e l e c t r o n s d e t e r m i n e d by d i f f e r e n t uranium oxygen bonding c o n f i g u r a t i o n s , by d i f f e r e n t c r y s t a l l i n e s t r u c t u r e s and by changing t h e U-U d i s t a n c e . I n o x i d e compounds, t h e open q u e s t i o n s c o n c e r n t h e h y b r i d i z a t i o n of 5f o r b i t a l s w i t h t h e O(2p) o r t h e U(6d, 7 s ) o r b i t a l s f o r v a r i o u s uranium-oxygen bonding con£ i g u r a t i o n s (3-4). It i s a l s o of p r e s e n t i n t e r e s t t o d i s c u s s t h e r o l e of f i n a l s t a t e e f f e c t s i n c o r e l e v e l s p e c t r o s c o p i e s a s compared t o t h e i r r o l e i n p h o t o e m i s s i o n ; t h e s e e f f e c t s c a n be v e r y i m p o r t a n t f o r l o c a l i z e d s t a t e s a s i t h a s been found i n r a r e e a r t h compounds.

I m p o r t a n t s t r u c t u r a l and e l e c t r o n i c changes a r e o b s e r v e d a l o n g t h e s e r i e s of UO o x i d e s (O<x<l). t h e most s t u d i e d (5-6). As w e l l a s

~ h 6 7 i t c r y s t a l l i z e s i:2 ^{th:' f:::%te} s t r u c t u r e w i t h e i g h t - f o l d uranium c o o r d i n a t i o n . I n t h e r a n g e 0<x<0.5 i n t e r s t i t i a l oxygens e n t e r t h e d i s t o r t e d

" f l u o r i t e type" s t r u c t u r e forming uranium s i t e s w i t h t e n - f o l d c o o r d i n a t i o n ( 7 ) . I n t h e r a n g e 0.5<x<1 t h e uranium l o c a l s t r u c t u r e c a n be d e s c r i b e d a s formed by d i s t o r t e d p e n t a g o n a l b i p y r a m i d s w i t h two s h o r t U-0 bonds and f i v e oxygens on t h e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868182

C8-950 JOURNAL DE PHYSIQUE

b a s a l plane. I n t h e s e oxides d i f f e r e n t s i t e s with d i f f e r e n t uranium e f f e c t i v e charge have been found ( 8 ) . I t i s commonly assumed t h a t f o r uranium c o o r d i n a t i o n number l a r g e r t h a n e i g h t 5f o r b i t a l s e n t e r more s t r o n g l y i n t h e chemical bond a s i t i s shown by t h e d e c r e a s i n g of t h e i n t e n s i t y of t h e mostly pure-5f occupied valence band a t -1.5eV. We r e p o r t r e s u l t s f o r t h e t h r e e o x i d e s , U02, U02,25 and

"2.66.

The g l a s s e s a r e b o r o s i l i c a t e g l a s s e s (60wt%Si02, 13wt%Na 0 and 7wt%ThO o r UO

).

yranium i s expected t o be

form o$ u r a n y l comp?exes 60

⁺

which a r e l i n e a r o r n e a r l y l i n e a r w i t h a very s h o r t U-0 d i s t a n c e (1.73-1.78~). An EXAFS s t u d y of t h e thorium and uranium s i t e s i n g l a s s e s i s d e s c r i b e d i n t h i s volume ( 9 ) . C r y s t a l l i n e u r a n y l n i t r a t e hexahydrate has been chosen a s a model f o r t h i s uranium bonding.

EXPERIMENTAL

The s p e c t r a were recorded by d i r e c t t r a n s m i s s i o n a t LURE-DCI (Orsay). For t h e M-edges i n t h e 3-4 keV range t h e monochromator was a double c r y s t a l ~ i ( l l 1 ) . The measurements were made e i t h e r w i t h t h e machine running a t low energy (1.5GeV) o r u s i n g a double f l a t r e f l e c t i n g m i r r o r t o avoid harmonic contamination when t h e machine was running a t 1.72GeV. The L-edges i n t h e range of 17keV were measured u s i n g a Si(400) "channel-cut" c r y s t a l with t h e machine running a t 1.85GeV. The i n s t r u m e n t bandwiths a r e p r i n c i p a l l y determined by t h e s i z e of t h e synchrotron source g i v i n g a f u l l width a t middle h e i g t h 0.7eV f o r M edges and 5eV f o r L edges.

The i n t r i n s i c c o r e - l e v e l widths a r e r e p o r t e d on t h e t a b l e . RESULTS

M4 and M t h r e s h o l d s a r e dominated by a "white l i n e " due t o t r a n s i t i o n s

from t h e gd c o r e l e v e l t o unoccupied £ - s t a t e s . The s t r o n g a b s o r p t i o n i s due t o t h e atomic resonance i n t h e 3d--5f atomic c r o s s s e c t i o n .

Fig.l- Thorium and uranium Mq-edge XANES spectra of Tho2 and U02 (the absorption maximum is chosen a s a common zero energy).

In t h e upper part modulations of the ab- sorption spectra i n the range 10-50eV above the absorption maximum for U02,U02,25 and U02.66.

A s an example edge s p e c t r a of Tho2, UO , uo2 25 and uo2. 66 ;Ze t o 50 ev above t a e

a b s d r p t i o n maxlmum r e p o r t e d i n Fig.1. I n t h e range 10-50eV above t h e a b s o r p t i o n maximum t h e f e a t u r e s a r e mainly determined by m u l t i p l e s c a t t e r i n g e f f e c t s and t h e r e f o r e by t h e atomic arrangement ( 1 0 ) . The s i m i l a r i t y between U02 and Tho2 s p e c t r a i s expected s i n c e t h e c r y s t a l l i n e s t r u c t u r e s a r e t h e same. The s h i f t s i n energy of t h e maxima a and b going from U02 t o Tho2 a r e c o n s i s t e n t with t h e M-O=d i s t a n c e v a r i a t i o n according t o t h e (E-E )dq

=

c o n s t . formula. The peak b ' i n !he spectrum of

UO can be assigned t o ten-fold uranium

s i g ~ g ~ formed by i n t e r s t i t i a l oxygens. The peak b of ::2. 25iS which c o i n c i d e s with t h e peak b of

due t o u n d i s t o r t e d e i g h t - f o l d uranium

sigis. ^The energy s e p a r a t i o n between b and b ' i n d i c a t e s a c o n t r a c t i o n of about 5% of t h e n e a r e s t U-0 d i s t a n c e i n t h e ten-fold s i t e s .

This i s i n agreement with c r y s t a l l o g r a p h i c r e s u l t s . The widths of t h e white l i n e s a r e always broader t h a n t h e experimental r e s o l u t i o n convoluted by t h e core l e v e l width. The widths and shapes a r e very d i f f e r e n t i n d i f f e r e n t compounds and t h a t c o n t r a s t s with r a r e e a r t h compounds where t h e widths of t r a n s i t i o n s from c o r e s t a t e s t o unoccupied atomic-like 4 f - s t a t e s a r e n e a r l y c o n s t a n t .

Except f o r thorium i n g l a s s , t h e peaks a r e s t r o n g l y asymmetric a s shown i n t h e t a b l e

where t h e h a l f widths i n t h e low and i n t h e h i g h energy s i d e s a r e r e p o r t e d s e p a r a t e l y .

The asymmetries a r e however s m a l l e r i n t h e s e oxide compounds t h a n t h o s e observed i n uranium i n t e r m e t a l l i c compounds by Lawrence e t a 1 . ( 2 ) . I n f i g . 2 a we r e p o r t t h e M w h i t e l i n e s of thorium i n t h e b o r o s i l i c a t e g l a s s and i n Ih02. The f i n a l s t a t e s 4 a r e 3d 5f and no m u l t i p l e t s could be p r e s e n t .

-

1

( T.1 = half width at middle height in the low energy side, T2 in the high energy side). All energies in eV.

'*

M5

L,

No s h i f t of t h e energy p o s i t i o n of t h e thorium white l i n e i s observed going from t h e c r y s t a l l i n e oxide t o t h e g l a s s . It i s symmetric i n t h e g l a s s and i t s h a l f width i s only 1.9eV. An i n t r i n s i c width of l e v f o r unoccupied 5f s t a t e s can be deduced.

I n c r y s t a l l i n e Tho2 t h e h a l f width rl i n c r e a s e s t o 2.8eV showing an i n c r e a s e of l e v of t h e 5f l o c a l i z e d unoccupied band width and t h e white l i n e becomes s t r o n g l y asymmetric. This i n d i c a t e s t h a t i n t h e g l a s s , t h e unoccupied S f - s t a t e s a r e confined i n a narrow band, being l o c a l i z e d a t l e a s t a t molecular l e v e l i n t h e c l u s t e r formed by t h e thorium atom and i t s f i r s t oxygen neighbors. On t h e c o n t r a r y 5f unoccupied s t a t e s form a broad band i n t h e c r y s t a l l i n e oxide ( l e v i n t h e g l a s s and 5-6eV i n t h e c r y s t a l ) . The main d i f f e r e n c e i s t h e long t a i l on t h e high energy s i t e f o r t h e c r y s t a l l i n e oxide which i n d i c a t e s t h a t t h e 5f o r b i t a l s c o n t r i b u t e t o t h e mainly ( 6 d , 7 s ) conduction band extending a t - h i g h e r energy up t o 12eV.

F m - h'hite lines at Mq-edges for a-Tho2 and the thorium-containing glass, b-UO2 and the uranium-containing glass

-

at Mg edge for c-the uranyl nitrate, U02 and the uranyl-glass (the absorption maxima of Tho2 and U02 are chosen as zero energy).

bore l e v e l

half-rridth

~h

1 ^u

(3d3I2) 1.6

/

^1.7

( (2p3I2)

13.7

Same comparisons a r e r e p o r t e d f o r t h e uranium compounds i n fig.2b,c. Things a r e more complicated f o r uranium than f o r thorium s i n c e t h e o x i d a t i o n number, and 5f-occupation, i s not unique. The white l i n e i s narrower and more symmetrical i n t h e g l a s s t h a n i n any c r y s t a l l i n e compound though a s m a l l asymmetry i s p r e s e n t which does not e x i s t i n t h e thorium-containing g l a s s .

Absorption peaks half-widths

glT:s T ~ O ~ I

uo2 U02.25 U02.66 uranyl glass T.1

r2

.I r2 11

1.9 2.8j3.0 3.0 2.9 2.6 2.5 1.9 4,2i 5 5 5 shoulder 3.1

I S at 4

/

^{3 3 3 2.6 2.6}

1

4.8 4.8 4.5 shoulder 2.7 S at 4

f 6.6 6.6 7.1 5.6 6.2

I

C8-952 JOURNAL DE PHYSIQUE

The energy p o s i t i o n of t h e

M~

white l i n e moves from t h e energy p o s i t i o n i n U02 towards h i g h e r energy i n h y p 6 r s t o i c h i o m e t r i c oxides i n c r e a s i n g by 0.8+0.2eV i n UO and by 1+0.2eV i n UO i n u r a n y l n i t r a t e and i n t h e g l a s s

( F L ~ . ~ $ J ! ~ This s h i f t i s s i m i l a r t o $hg61-1.5e~ measured f o r core l e v e l s h i h t i n XPS s p e c t r a going from formal U(V1) t o U(IV) i o n s i n oxides. f 1 i n c r e a s e s from 2.5 t o 3eV going from t h e g l a s s t o U02 and t h e l i n e becomes more asymmetric. The

a b s o r p t i o n peaks of c r y s t a l l i n e u r a n y l n i t r a t e and of uranium i n t h e g l a s s

E295 ^{t h e} same widths i n t h e low energy s i d e . They a r e l i k e l y t o be due t o 36

5f f i n a l s t a t e s . The narrow bandwidth of unoccupied 5f s t a t e s i n d i c a t e s t h a t they a r e w e l l l o c a l i z e d a s they a r e i n t h e thorium c o n t a i n i n g g l a s s . On t h e c o n t r a r y t h e 5f band i s broader and very asymmetric i n U02, U02 and

The M4 s p e c t r a of c r y s t a l l i n e u r a n y l n i t r a t e e x h i b i t a s h o ~ f a e r s

r:2.%rkt ^3,5-PeV above t h e a b s o r p t i o n maximum which i s not p r e s e n t i n g l a s s e s . Following t h e above d i s c u s s i o n we a s s i g n t h i s shoulder t o the f-components of t h e U(6d,7s) conduction band. The c h a r a c t e r i s t i c spectrum of u r a n y l i s determined by t h e f a c t t h a t t h e unoccupied 5 f - s t a t e s a r e more l o c a l i z e d t h a n i n uranium o x i d e s and appear s e p a r a t e d from t h e U6d conduction band. However an important h y b r i d i z a t i o n between U(5f) and U(6d,7s) o r b i t a l s i s p r e s e n t a s i n d i c a t e d by t h e o r e t i c a l c a l c u l a t i o n s which show a l s o a l a r g e mixing of U5f and 02p o r b i t a l s . The non-bonding occupied s t a t e s of almost pure 5f c h a r a c t e r , which a r e a t -1.5eV i n

uo2, a r e not p r e s e n t i n t h e valence band s p e c t r a of u r a n y l compounds. The o p t i c a l spectrum of u r a n y l n i t r a t e does n o t r e v e a l f--f t r a n s i t i o n s and shows a charge t r a n s f e r gap a t about 3eV t h a t can be assigned t o the t r a n s i t i o n from t h e mostly 02p band t o t h e unoccupied U5f s t a t e s . Therefore t h e h y p o t h e s i s t h a t t h e shoulder a t 4 e ~ a t t h e M4-edge i s due t o a charge t r a n s f e r e x c i t a t i o n of a valence e l e c t r o n should be d i s c u s s e d . The charge t r a n s f e r gap ( 0 2 ~ - - ~ 5 f ) = 3 e V i s c l o s e c o n f i g u r a t i o n s 5f t o t h e Uaf and c o r r e l t i o n energy 5f 7 L can be expected a s i n Ce02 and N i O t h a t (2eV).

A

mixing of t h e l o c a l i z e d we c a l l i n t e r a t o m i c i n t e r m e d i a t e valence systems. Where t h i s h y p o t h e s i s w i l l be v e r i f i e d t h e 4e3 s a t e l l i t e i n t h e M4-XANES can e a s s o c i a t e d with t h e f i n a l P

s t a t e 3d I , 5f and t h e main l i n e w i t h the

&

5f

I

f i n a l s t a t e . The l a r g e

i n t e n s i t y of t h e s a t e l l i t e i n t h e XANES can be explained by t h e importance of t h e mixing of 5f0 and 5 f 1 L c o n f i g u r a t i o n s i n t h e ground s t a t e i n agreement

w i t h t h e expected l a r g e covalence of t h e uranium-oxygen bond i n u r a n y l U02++ groups.

The uranium

L

-XANES s p e c t r a of

"29 "2 66 an2 u r a n y l n i t r a t e

a r e s h o d I n f i g . 3 a . The s p e c t r a show a broad maximum which i s determined by t h e enhancement of t h e t o t a l a b s o r p t i o n c r o s s s e c t i o n f o r t r a n s i t i o n s t o t h e lowest d - l i k e conduction band by t h e 2 ~ - - ( 6 , c ) d resonance i n t h e atomic c r o s s s e c t i o n . W e have s u b t r a c t e d i n each spectrum an a r c t a n curve which s i m u l a t e s t h e atomic a b s o r p t i o n jump.

The r e s u l t i n g s i g n a l s a r e r e p o r t e d i n f i g . 3 b . M3 s p e c t r a have been recorded and a r e very s i m i l a r s i n c e t h e s m a l l e r experimental broadening i s counterbalanced by t h e l a r g e c o r e - l e v e l width (though s m a l l e r t h a n t h e p r e d i c t e d v a l u e ) .

A

s i n g l e l i n e i s observed i n U02. The L (and

M,) s p e c t r a of u r a n i u i i n t h e u;anyl n i t r a t e and i n U O ~ show a . ~ ~

m-

Uranium L3-edge XANES of UOp, U308 and the uranyl nitrate

broad muliple

scattering

resonance

hexahydrate. a-The arctan simulate the atomic absorption jump.

b-Differences between the measured bv sDectra and the arctan

(MSR) at

loeV

and 15,5eV above the

curves (dashed lines). ~ i t t i n g by ~o;en;zian curves (full lines).

a b s o r p t i o n maximum r e s p e c t i v e l y .

The energy positions of these MSR features have been determined by fitting with a second broad lorentzian curve. In the case of uranyl compounds it has been shown by angular-resolved measurements that the MSR is determined by multiple scattering in the direction of the linear U-0-U group (11). In U02 66 the MSR seems characteristic of the pentagonal bipyramid group. The ptesence of these MSR in spectra of glasses bring direct information about the uranium sites.

REFERENCES

1- J.R.NAEGELE, J.GHIJSEN and L.MANES in Structure and Bonding 59,60 , Springer (1985) p. 197.

2- J.M.LAWRENCE, M.L.den BOER and R.D.PARKS and J.L.SMITH, Phys.Rev.

B 2 , 568 (1984).

3- V-HEERA, G.SEIFERT and P.ZIESCHE, Phys.Status Solid b 118 ,K107 (1983).

4- V.A.GUBANOV, A-ROSEN and D.E.ELLPS, Solid State Comm. 2 , 219 (1977).

5- J-SHOENES, Physics Reports 3 , 301 (1981).

6- C.BONNELLE and G.LACHERE, J. Phys. 2 ,295 (1974); and ibidem. 5 ,

C5-15 (1980).

7- G.C.ALLEN and P.A. TEMPEST. J.Cem.Soc.Dalton Trans. 1982 , ^2169;

ibidem 1983 ^2673.

8- Yu.A.TETERIN, V.M.KULAKOV, A.S.BOER, N.B.NEVZOROV, I.V.MELNIKOV,

L.V.MASHIROV, D.N.SUGLOBOV and A.G.ZELENKOV, Phys.Chem.Miner. 2 ,151 (1981).

9- D.PETIT-MAIRE, J.PETIAU, G.CALAS and N.JACQUET-FRANCILLON (This volume).

10- A.BIANCON1, J.GARCIA, A.MARCELL1, M.BENFATT0, C.R.NATOL1 and I.DAVOLI.

J.Phys. 46 , C9-101 (1985).

11- D.H.T=PLETON and L.K.TEMPLETON, Acta Cryst. A38 ,62 (1982).