Correlation-effects, charge-transfer energies and covalency in nickel compounds as determined by x-ray absorption-spectroscopy

~ Solid State Communications, Vol.56,No.8, pp.673-676, |985. 0038-I098/85 $3.00 + .00

Printed in Great Britain. Pergamon Press Ltd.

CORRELATION EFFECTS, CHARGE-TRANSFER ENERGIES AND COVALENCY IN NICKEL COMPOUNDS AS DETERMINED BY X-RAY ABSORPTION SPECTROSCOPY

G. van der Laan, J. Zaanen and G.A. Sawatzky Institute for Physical Chemistry, Materials Science Center

University of Groningen, Nljenborgh 16, 9747 AG GRONINGEN, The Netherlands R. Karnatak and J.-M. Esteva

Laboratoire pour l'Utilization du Rayonnement Electromagnetique (LURE) Universite Paris-Sud, Batiment 209c, 91405 Orsay CEDEX, France

(Received 21 June 1985 by B. M[lhlschlegel)

High resolution L9 ~ near edge absorption spectra of Ni compounds are compared to a~'fmpurity model calculation including the d-d correlation effects. Covalent mixing in the final state gives a change in the observed structure which can be used as an

analytical tool to study the ground state properties. From the analysis we obtain the d-d Coulomb interactions, the charge transfer energy and the covalency in the ground state.

Recent developments in the theory and understanding of core and valence electron spectroscoples in highly correlated materials provide the basis for obtaining surprisingly detailed information concerninglt~e ground state electronic structure of solids. "'~ Most of the effort however has been concentrated on rare earth systems and on core and^valence

photoe¼ectron spectroscopies, j Recently we have shown that x-ray absorption spectroscopy (XAS) has several advantages over other core

spectroscopies for two reasons. I) A suitable core state can be selected such that the electron is excited into a screening orbital in which case the perturbation on the ground state electrons is that of a screened core hole as opposed to XPS in which the perturbation is that of the unscreened core hole.

2) The dipole selection rules allow only a relatively small number of final state

multiplets to be accessible as opposed to XPS in which generally the whole manifold of multiplets is accessible resulting in rather broad

structureless lines. The relative intensities of the multiplets reached are then a signature for the local symmetry of the atom in the ground state of the solid.

In this Communication we present new high resolution XAS data for divalent Ni compounds. Using an impurity like many body theory we show that the spectra can be understood in detail including the observed multiplet and satellite structure. Simulation of the spectra provides information concerning the charge transfer energies, d-d Coulomb interaction and the degree of covalency in the ground state.

Ni L9 ~ x-ray absorption spectra were obtained 0~fng the synchrotron radiation emitted by the Anneau de Collisions d'Orsay (ACO), and a double crystal (beryl) monochromat~r resulting in an energy resolution of 0.3 eV. The spectra were recorded by the electron yield method. Samples of NiF 2, NIO, NiCI 2, NiBr 2 and Nil 2 in

powder form ant sublimed on AI in high vacuum were studied.

The L~ ~ spectra obtained are shown in Fig. I e x h i b i t i ~ O a mu~h higher resolution than those reported earlier.- Qualitatively the spectra show two narrow peaks close to the 2pR/2 and 2p..^ threshold due to multiplet struCtUre. Th~s is followed by a broad satellite structure clearly visible in the LR region and a step like continuum contribution b~tween the L 2 and L. edges. The step like contribution is due t ~ transitions to a 4s like band w~ich is outside the scope of the present study.- The near threshold multiplet structure for the most ionic compounds has the shape and the relative

intensities corresponding to transitions ~rom a 3d v like ~A^ (e ~) ground state to 2p~3d ~ li~e final state'gas ~iscussed by Yamaguchi et al. Of most interest here are the decrease in the multiplet splitting and changes in the broad satellite structure with decreasing

electronegativity of the anion. We now proceed to show that this is a result of configuration interaction and can be used to obtain parameters for determining the covalent mixing in the ground state.

To explain the spectra we use an impurity model to calculate the valence band structure and band gaps in the presence of strong

correlations. The basic assumption is that the translational symmetry of the transition metal ions can be neglected because the dlspersi~nal width of the d bands is only about 0.5 eV. ~ In this case the Ni ions can be treated as

impurities hybridizing with an anion p valence band. For the anion p band we tare into account the translational symmetry s~nce its band width is known to be about 3-4 eV.

It is well known that in a purely ionic configuration and in O h symmetry the crystal field together with th~ d-d Co~lomb interactions cauae ~he ground state of a 3d ion to be ~A^

L lu ~ ~g

(eg3). This state can mix with states d

674

CHARGE-TRANSFER ENERGIES AND COVALENCY IN NICKEL COMPOUNDS Voi. 56, No. 8 I I r,:, o ~ -, ; ' NI F 2 \ ' ~ . ~ N, o . ^ J I

850

N I C I 2 ~ / _ NI B r 2 • "~ NI I~ I , I , I

860

870

880

EXCITATION ENERGY {eV)

Figure

1.

The L~ = absorption spectra of Ni compounds. T h e - ~ ( L ~ ) edge is found at about

852 (870) eV. ~ "

of A^ symmetry which in turn can mix with state~gdi°~ ~', where k denotes a ligand hole with wave vector k. T~e ground state, including the covalent mixln~, is then the lowest energy two-hole state of A2_ syr:metry whose energy and wave function can easily belobtained using Greens functlon techniques. The basic

parameters involved are shown in Fig. 2a with A equal to the charge transfer energy, U the d-d Coulomb interactlon, and W the width of the semlelllptical ligand band. Not shown in Fig. 2 are the transfer integrals (T) mixing the various states. These, being one-e~ectron q matrix1~lements, mix only d- with d-k and d-k with d k ~' and are taken to be k independent.

The final states are of the form c d and c d I0 k, where c denotes a core hole. In addition to the interactions in the initial states we must take into account I) The spln-orbit coupling of the 2p hole which splits the total spectrum into an L 2 and L~ region, 2) The average core hole-~ el~ct~on ~ t e r a c t i o n (Q) whlch modifies the ~ d- - ~ d-- k splitting to A' = A + U - Q and since Q is generally larger than U, A' < A, and 3) The higher multipole core

1, f

_ 12 > A÷U

2.~P 3/23d9"

- - ~ - - - . ~

÷ U -Q

O) INITIAL

STATE

CONFIGURATIONS

b}

FINAL

STATE

CONFIGURATIONS

Figure 2. Schematic representation of the energy

of the d i f f e r e n t configurations for the initial

state (a) and final state (b) shifted by an

arbitrary energy in the Lq absorption for the c a s e o f NiCl~. C o n f i g u r a t i o n s w i t h a l i g a n d h o l e (k) h a ~ a band width W.

T h e 2_~/~3d ~ c o n f i g u r a t i o n i s m u l t i p l e t s p l i t . N o t d r ~ k ~ i s t h e m i x i n g b e t w e e n t h e

c o n f i g u r a t i o n s by a t r a n s f e r l n t e E r a l T.

cause a further splitting within the 2P~l 2 and states in the presence of a d h o l ~ 2Pl/2The various states considered for the calculation are shown schematically in Fig. ~b. Note t h ~ since A' < A the mixing of the c d J and ~ 8 d-- k ~tates will be larger than that for the d and d" k states in the initial states. For a purely i~nic comp~un~ (i.e. A and A' >> T) the final states are 2p 3d- which span the irreducible representations Ap, two E2, two T I and three T 2 for the 2PI/p pa~ent and-two A 2 , three E., fZve T. and five T~ for the

2Pq/2 parent. T~e splitting~ of these states ar~-obtained using atomic Coulomb and exchange integrals and a1~rystal field splitting 10 Dq = 1.5 eV This calculation should correspond, after including the optical

selection rules, closely to the spectrum of the highly ionic compound NiFp as demonstrated in Fig. 3 and8agrees well wi~h the calculation of Yamaguchi.

For the more covalent compounds both the initial and final states are no91onger p u r ~ y

ionic and contain substantial d k and c d k character, respectivel~. For the--final--states each of.the atomic c d- states couples with its _]u own c d k continuum via the transfer integral T. ~his final state covalent mixing causes a change in the multiplet splittings and together with the initial state covalent mixing causes the spectral weight to b~ distributed over a bound, predominantly c d , final s t a ~ and a strongly distorted predominantly ~ d--

V o l . 56, N o . 8 C H A R G E - T R A N S F E R E N E R G I E S A N D C O V A L E N C Y IN N I C K E L C O M P O U N D S 6 7 5 i t, I% 'I • 2

" i'::

:¢r,,

,'[.',.-

...

III ;~k"*,'~..,.,,~...,v_-~,.~ .-'.--~ _,C'- "-~ I" I '>:,__

8">o

E X C I T A T I O N E N E R G Y (eV} F i g u r e 3. C a l c u l a t e d m u ~ t l p l e t ~ t r ~ c t u r e f o r a d i p o l e t r a n s i t i o n NI 3 d - - - > 2 p - 3 d - (O. = 5 . 7 9 , F ~ = 7 . 7 2 , O ~ = 3 , 2 9 a n d 1 0 D q = 1 , 5 eV), the d a s h e d l i n e is ~ c o n v o l u t i o n b y a 0.6 e V FMHH L o r e n t z i a n , F o r c o m p a r i s o n t h e

experimental(dots).

L2, 3 s p e c t r u m o f N I F 2 is s h o w n i n t e r a c t i o n s , four a d d i t i o n a l p a r a m e t e r s T, A, Q and U. As s h o w n in a n a n a l y s i s of XPS d a t a of Cu a n d N ~ 1 ~ l h a l l d e s T, Q a n d U are n e a r l y c o n s t a n t . This l e a v e s us w i t h o n l y o n e s t r o n g l y c o m p o u n d d e p e n d e n t p a r a m e t e r A. The

c a l c u l a t e d s p e c t r a for N ~ C I p a n d N i I _ for the p a r a m e t e r s l i s t e d in T a b l e I a r e s h o ~ n in F~g. 4. T h e s p e c t r a are e x t r e m e l y well d e s c r i b e d by the t h e o r y a s ~ d ~ f r o m the not i n c l u d e d 4s

c o n t i n u u m edge. We see the d e c r e a s e in

m u l t l p l e t s p l i t t i n g w l t h d e c r e a s i n g a n i o n e l e c t r o n e g a t i v i t y b e c a u s e of c o v a l e n t m i x i n g in A(eV) 3d 8 3d 9 3d I0 NiI 2 1.5 0.47 0.44 0 . 0 9 N ~ B r 2 2.6 0.61 0.32 0.07 N ~ C 1 2 3.6 0.71 0.23 0.06 NiO 4.6 0.73 0.21 0.06 N~F 2 7 1.0 - 0 - 0

T a ~ 6 e I. The v a l u e s for A and the 3d 8, 3d 9 and

3d c h a r a c t e r s in the g r o u n d s t a t e as o b t a i n e d

from a best fit to an i m p u r l t y model. F u r t h e r

c o n s t a n t for all c o m p o u n d s : Q = 7,U = 5 (4,5 :n N i I ~ ) , . T = 1.5 (I~75 zn N I O ) ~ W = 3, 10 Dq = 1.5~ G ~ = 5.79, F- = 7.72, G" = 3.29 eV, the e f f e c t l v e s p l n - o r b l t s p l i t t i n g is 17 eV, the I n t r l n s l c llne w l d t h ls a 0.3 eV L o r e n t z l a n , the e x p e r i m e n t a l r e s o l u t i o n is a 6 = 0.3 eV G a u s s i a n .

the flnal state. We a l s o o b t a l n the h i g h e r

e n e r g y r a t h e r b r o a d l o w i n t e n s l t y ~ 8 o u l d e r w h i c h

Is p r i m a r i l y d u e to a c c e s s i b l e e d k s t a t e s

r e s u l t i n g f r o m c o v a l e n t m l x l n g ? n b o t h the

i n l t l a l and flnal states• Also we n o t e that the

> - z w _{I -} z

0

N - J .< n- o z

i:

i'

, r i i i i L ' ~. _{~ , J I} j! :; , _,. N, CI 2 L m i I i I

,I

;L I I I J I

i,

, ~ ~, NIl 2 8 5 0 860 870 880

EXCITATION ENERGY (eV) F i g u r e 4. I m p u r i t y m o d e l c a l c u l a t i o n (dashed line) c o m p a r e d to the e x p e r i m e n t a l L 2 ~ x-ray a b s o r p t i o n s p e c t r a of NiCl 2 a n d N i I p [dots). T h e v a l u e s o f the p a r a m e t e r s a r e g i v e n in T a b l e I a n d its c a p t i o n . v a l d e s of A found f o l l o w n i c e l y the e x p e c t e d e l e c t r o n e g a t i v l t y trend. The o b s e r v e d c h a n g e s in m u l t l p l e t s p l i t t i n g c o u l d a l s o be s i m u l a t e d by an a t o m i c c a l c u l a t i o n wlth c o m p o u n d d e p e n d e n t S l a t e r integrals. Dolng

thls h o w e v e r one m i s s e s the p h y s i c a l r e a s o n for the c o m p o u n d d e p e n d e n c e and one c a n n o t e x p l a i n

the s h o u l d e r in the s p e c t r a • ~iso o n e can

e a s i l y be m i s l e d to c o n c l u d e that the c o m p o u n d d e p e n d e n t r e d u c t i o n f a c t o r s are a m e a s u r e of the c o v a l e n e y in the ;nit!al state w h l c h as we a r g u e d a b o v e is c o n s i d e r a b l y less than that ~n the final state•

The initza~ s t a t e c o v a l ~ c y is r e l a t e d to

the a m o u n t of d-, d- k a n d d - k k' c h a r a c t e r in

the g r o u n d states• The v a l u e s o b t a i n e d from the

p r e s e n t a n a l y s l s are g i v e n in T a b l e I and a g a i n s h o w the e x p e c t e d t r e n d b a s e d on

e ! e e t r o n e g a t l v l t y a r g u e m e n t s .

A C K N O W L E D G E M E N T - We are g r a t e f u l to the LURE t e c h n l c a l s t a f f for thelr v a l u a o l e ald and to Tneo T h o l e for d o l n g ~ e l a t l v l s t l c H a r t r e e Fock

c a l e J l a t l o n s . T h i s work w~s s u p p o r t e d by the

676 CHARGE-TRANSFER ENERGIES AND COVALENCY IN NICKEL COMPOUNDS Vol. 56, No. 8

R e f e r e n c e s

I. O. Gunnarsson and K. Schonhammer, Phys.

Rev.

S

2._.88,

4315 (1983).

2. A. FuJimorl, Phys. Rev. B 28, 4489 (1983); A. Fujimori and F. Minami, Phys. Rev. B 30, 957 (1984).

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4. B.T. Thole, R.D. Cowan, G.A. Sawatzky, J. Fink and J.C. FUEEIe, Phys. Rev. B 31, 6856 (1985).

5. M. Lemonnler, O. C o l l e t , C. Depautex, J.-M. Esteva and D. Raoux, Nucl. Instrum. Methods 152, 109 (1978).

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P h y s i q u e 1964, 826 (1964); C. Bonnelle, E.

Belin and C. Sinimaud, Jap. J. Appl. Phys.

1 7 , 125 (1978).

7. A detailed description of this will be given in a subsequent paper, G. van der Laan, J. Zaanen, G.A. Sawatzky, R Karnatak and J.-M. Esteva, to be published.

8. T. Yamaguohi, S. Shibuya, S. Suga and S. Shin, J. Phys. C15, 2641 (1982).

9. S. Antoci and L. Mihich, Phys. Rev. B 18,

5768 ( 1 9 7 8 ) ; P h y s . Rev. B 2_j.1, 3383 (19EO);

K. T e r a k u r a , A.R. W i l l i a m s , T. Oguchi and

J. Kubler, Phys. Rev. Lett. 52, 1830 (1984).

10. C . J . Ballhausen, "Introduction to l l g a n d

field theory", McGraw-Hill, New York (1962).

11. The 10 Dq value is that appropriate for the final state and includes the contribution from covalency. The ground state 10 Dq

v a l u e is estimated to be about 1 eV (see Ref. I0).