USE OF LIII XANES FOR THE DETERMINATION OF ATOMIC RELAXATION IN MIXED VALENT RARE EARTH SYSTEMS

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USE OF LIII XANES FOR THE DETERMINATION OF ATOMIC RELAXATION IN MIXED VALENT

RARE EARTH SYSTEMS

E. Beaurepaire, J. Kappler, G. Krill, F. Gautier

To cite this version:

E. Beaurepaire, J. Kappler, G. Krill, F. Gautier. USE OF LIII XANES FOR THE DETERMINATION

OF ATOMIC RELAXATION IN MIXED VALENT RARE EARTH SYSTEMS. Journal de Physique

Colloques, 1986, 47 (C8), pp.C8-971-C8-974. �10.1051/jphyscol:19868186�. �jpa-00226092�

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JOURNAL D E PHYSIQUE

Colloque C8, supplgment au n o 12, Tome 4 7 , dgcembre 1986

USE OF Lm XANES FOR THE DETERMINATION OF ATOMIC RELAXATION I N MIXED VALENT RARE EARTH SYSTEMS

E. BEAUREPAIRE, J.P. KAPPLER, G. KRILL* and F. GAUTIER

L . M . S . E . S . ( u . A . C . N . R . S . 3 0 6 ) ,

u n i v e r s i t e

L o u i s P a s t e u r , 4 , rue B l a i s e P a s c a l , F - 6 7 0 7 0 S t r a s b o u r g C e d e x , F r a n c e

" L a b o r a t o i r e d e P h y s i q u e d u S o l i d e , U n i v e r s i t e d e N a n c y I , BP 2 3 9 , F - 5 4 5 0 6 V a n d o e u v r e - l & - N a n c y C e d e x , F r a n c e

Resume

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Nous prgsentons des mesures XANES dans le domaine dVCnergie 20-50 eV aprGs le seuil L111 de Sm dans quelques alliages B base de SmS. Nous montrons que dans les systemes

B

valence mixte les resonances XANES reproduisent le double seuil L111 avec un poids relatif en accord avec la valence de Sm et que les relaxations atomiques peuvent ltre estim6es.

Abstract

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We present XANES results in the energy range 20-50 eV above the LII1 edge of Sm in some SmS alloys. We show that in mixed valent systems the XANES resonances replicate the splitting of the L111 edge with a relative weight in agreement with Sm valence, and that atomic relaxation can be esti- mated.

I

-

INTRODUCTION

In its sim lest picture, mixed valence phenomenon implies fluctuations

(rf + 10-l5 S) betweenoREn and

RE"^^

ions in a solid. Due to the large sire change of these ions ^(%0.15 A), strong anomalies in the phonon spectra (particularly in the soft modes) are expected, and effectively observed /l, 201. In fact, even the existence of such large size changes in mixed valent. systems is controversial since the effect of hybridization has been shown to be important in this problem 12, 31.

EXAFS a pears then to be a helpful technique because its characteritic time scale

(% 10-lg S) is shorter than rf and the determination of "instantaneous" interatomic distances is in principle possible. Unfortunately, and because this analysis is complicated, the "first generation" EXAFS studies 14-61 did not properly take into account the presence of two energy thresholds in mixed valent systems and led to ambiguous conclusions (one average distance in agreement with lattice constant data). However, a recent: reexamination of EXAFS analysis /7, 8/ has shown that the importance of relaxations can be obtained in homogeneous or inhomogeneous mixed valent systems.

Concurrently, we have shown in a recent paper 191 that an analysis of after edge structures for energies 20-60 eV above the threshold (thereafter labelled XANES

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X ray Absorption Near Edge Structures) is possible. In this energy range, the modu- lations of the absorption coefficient are due to multiple scattering of the photo- electron on neighbouring shells, in opposition to EXAFS for which single scattering is dominant. We showed in that paper that XANES resonance replicates the edge splitting of mixed valent compounds (SmB6, YbA12, CePd3

...

^{) ,}concluded that two energy thresholds should be taken into account and outlined a method to obtain informations on relaxation effects.

The aim of this paper is to draw out the ideas developed in 191 by a study of nearly divalent SmS and inhomogeneous mixed valent (IMV) or homogeneous mixed valent

(HMV) pseudo binary alloys.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868186

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JOURNAL DE PHYSIQUE

I1

-

EXPERIMENTAL

X-ray absorption on the Sm L111 edge has been carried out on an "inlab" spectrome- ter /l01 using a Si 14001 monochromator with an energy resolution better than 2 ev.

The counting time is typically 4 hours to get spectra of Fig. 1. The energy step for these spectra was 0.4 eV and allows a determination of peaks position with this accuracy. The distances determination with the method outlined below is then

*

^{0.02 A.}

I11

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RESULTS AND DISCUSSION

The compound SrnS in its black phase is known as a semiconductor where Sm ions are divalent. This system has been one of the most studied in the field of mixed valence compounds because of the possibility of driving its valence from 2 to 3 as an insulator-metal transition takes place by pressure or by alloying effect (for more details see /Ill). We report here XANES results on divalent SmS, an inhomogeneous mixed valent alloy : SrnSo. 96AS0.04, mixed valent SmSO. 92P0.08 and S q . 75Yo. 25s. The latter system has the advantage to present strong variations of the valence with temperature between 77 K and 300 K 1121.

300 K 1.5-

SmS 1.0-

0.5 -

1.5

J

1.0-

0.5-

E

2

⁰

I 0 -

Fig. 1 :

XANES after the L111 edge in SmS, SmSo.96Aso.04 and Sm0.75Y0.25S at 300 K. Curve a is the expe- riment magnified 8 times. Curve b is a simulation using D2 and D3 values of Table 1 (see text). Curve c is a simulation with D2 = D3 = DL

(see Table 1).

The Fig. 1 shows some typical L111 spectra ex- tending up to 60 eV above the edge. The Sm valence deduced from a fit of the "white line"

(listed in Table 1) are in qualitative agreement with other measurements 112, 131, though SrnS seems to be weakly mixed valent (v E 2.1) rather than divalent, this is of minor importance for the following discussion.

As it is now well-established both theoretically 114, 151 and experimentally 116, 171, some information about the central atom (Sm here) and its neighbours can be obtained from after edge structures (XANES) using an "universal" 'energy scale :

E + (E - E*) (D~ID) (1)

1

^, ^,

i;y ^tl

E is in principle the threshold energy minus a

^*

muffin-tin interstitial potential 1151 and we

-10 0 2 SO 50 replaced it for simplicity by the energy of the

E trv) top of the "white line". D holds for a characte-

ristic interatomic distance (see below) and Do is representative of a reference compound chemically and structurally comparable to the studied sample. We found in our previous work /9/ that light atoms (B, Al, S. ..) contribute more efficiently than the heavy ones to the scattered amplitude in this energy range ; it is therefore reasonable to identify d with the cation-anion distance in the NaCl structure of SmS.

We propose now to deduce Sm2+-X (D2) and s~~+-x(D~) interatomic distances by adding two "reference" XANES spectra rescaled by (1) and separated by the measured

splitting between Sm2+ and sm3+ edges (E 7 eV). SrnS is such a reference compound

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since (i) it is nearly integral valent (ii) in the studied pseudo-binary alloys the first neighbour shell is either unperturbed (Sm0.75Y0.25S) or weakly perturbed by less than 10% substituant atoms. Some examples of simulation are reported on the Fig. 1 for SmSo.g6As0.04 and Sm0.75Y0.25S at 300 K (it should be noted that we did not try to preserve the amplitudes of the resonances in the fitting procedure ; curves b and c in Fig. 1 are set in an arbitrary ordinate scale).

In the particular case of SmS0.96As0.04, the distinction can be easily made between two extreme simulations : either unrelaxed lattice (i.e. D2 = D3) represented by curve c in Fig. 1 or relaxed lattice hypothesis represented by curve

8 .

^{In fact,}

the satisfying simulation is found with D2 = D(SmS) and D2-D3 = 0.16 A, what is comparable to the difference between sm2+ and sm3+ ionic radii. This is not surpri- sing since in such IMV systems strong relaxation effects have also been reported by EXAFS /7/ and XANES 1181 measurements.

The results of our analysis are listed in Table 1. The average Sm-X distance DM =

(3-v)D2+(v-2)D3 compares favourably with the values deduced from lattice constants DL, what is rather encouraging for our analysis (the discrepancies do not exceed a few percent and are within the experimental accuracy due to uncertainties on the determination in the present study of E* in (l)).

Relaxation effects are also important for Sm0.75Y0.25S at 300 and 77 K (though some- what smaller than in IMV systems) and should be responsible for the phonon anomalies observed in neutron /l/ and Raman scattering 1201. This effect (if it exists) could not be detected within our experimental accuEacy in SmS0.92P0.08. It is therefore conceivable that in this case, D2-D3 0.04 A as it has been found in such systems (this alloy 171, SmS0.9200.08 1191 or TmSe 181) by EXAFS analysis.

Finally, we want to point out that D2-D3 is a parameter explicitely taken into account in mixed valent impurity model, with a numerical application for SmS alloys 121. From this calculation, it appears that the size of D2-D3 is directly related to the balance of elastic energy (favouring a relaxed state) and hybridization energy (favouring the unrelaxed state). Within this framework, our results on (Sm,Y)S and Sm'(S,P) alloys could be an indication of an increase of the hybridization f-d

(let us say A / T 5 to 10 meV) as the Sm valence goes from 2 to 3.

Table 1 :

D2 D3 D~ D~ ^v

SmS 2.98 2.98 2.1

Sm0.75Y0.25S T=300 K 2.88 2.80 2.84 2.83 1121 2.45 T=77 K 2.93 2.83 2.9 '2.89 1121 2.25 SmS0.92P.0.08 2.825 2.825 2.825 2.85 2.7 SmS 0.96~~0.04 2.98 2.82 2.93 2.945 2.3

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Parameters deduced from our simulations (see text). D is the cation- L

anion interatomic distance computed from lattice constant data. v is the Sm valence from a fit of the LIII edge.

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C8-974 JOURNAL DE PHYSIQUE

REFERENCES

/l/ H. Mook, F. Holtzberg, in "Valence Instabilities in Solids", L.M. Falicov, W. Hanke, M.B. Maple eds.

-

North Holland (1981) p. 113.

/2/ W. Kohn, T.K. Lee, Y.R. Liu, Phys. Rev. B

2

(1982) 3557.

/3/ K. SchSnhammer, 0. Gunnarson, Phys. Rev. B &(1984) 3141. l

/4/ H. Launois, M. Rawiso, E. Holland-Moritz, R.Pott, D. Wohlleben, Phys. Rev.

Lett.

44

(1980) 1275.

/5/ G. Krill, J.P. Kappler, J. RShler, EXAFS and Near Edge Structure (ed. by A. Bianconi, L. Incoccia, S. Stipeich) Springer-Verlag (1983) p. 190.

161 J.B. Boyce, R.M. Martin, J.W. Allen, ibid. ref. 5, p. 187.

/7/ G. Krill, J.P. Kappler, M.F. Ravet, C. Godart, J.P. Shateur, J. Mag. Mag. Mat.

47-48 (1985) 190.

181 N. Wetta et al., to be published.

/g/ E. Beaurepaire, J.P. Kappler, G. Krill, Solid State Comm.

57

(1986) 145.

/l01 G.S. Knapp, P. Georgopoulos, in "Laboratories EXAFS Facilities", ed. by E.A. Stern (American Institute of Physics, 1980) p. 2.

/11/ J.M. Lawrence, P.S. Reiseborough, R.D. Parks, Rep. Progr. Phys.

44

(1981) 1. and ref. therein.

/12/ S. Von Molnar, T. Penney, F. Holtzberg, J. de Physique (Paris), suppl. au no 10, Tome 37 (1976) C4-241.

/13/ D.C. Henry,

KJ.

Simon, W.R. Savage, J.W. Schwertzer, E.D. Carter, Phys. Rev.

B

0

(1979) 1985.

1141 G. Materlik, J.E. Miiller, J.W. Wilkins, Phys. Rev. Lett.

50

(1983) 267.

1151 C.R. Natoli, ibid. ref. 5, p. 43.

1161 A.P. Hitchcock, F. Sette, J. StGhr, ibid. ref. 5 , p. 43.

1171 A. Bianconi, E. Fritsch, G. Calas, J. Petiau, Phys. Rev. B

21

(1985) 4292.

1181 D. Malterre, G. Krill et al., to be published.

1191 N. Wetta, private communication (1986).

/20/ N. Stusser, G. Giintherodt, A. Jayaraman, K. Fischer, F. Holtzberg, in "Valence Instabilities", ed. by P. Wachter and H. Boppart, North Holland (1982) p. 69.