MÖSSBAUER STUDY OF HYDROGEN DIFFUSION AND INTERSTITIAL SITE OCCUPANCY IN 57Co : PdHx AND 57Fe : PdHx

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MÖSSBAUER STUDY OF HYDROGEN DIFFUSION AND INTERSTITIAL SITE OCCUPANCY IN 57Co :

PdHx AND 57Fe : PdHx

F. Pröbst, F. Wagner, M. Karger, G. Wortmann

To cite this version:

F. Pröbst, F. Wagner, M. Karger, G. Wortmann. MÖSSBAUER STUDY OF HYDROGEN DIFFU-

SION AND INTERSTITIAL SITE OCCUPANCY IN 57Co : PdHx AND 57Fe : PdHx. Journal de

Physique Colloques, 1979, 40 (C2), pp.C2-635-C2-638. �10.1051/jphyscol:19792222�. �jpa-00218603�

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JOURNAL DE PHYSIQUE Colloque C2, supplPment au n o 3, Tome 40, mars 1979, page C2-635

M ~ S S B A U E R STUDY O F HYDROGEN D l FFUSI ON AND INTERSTITIAL S I T E OCCU3ANCY I N 7Co :?dHx A N D 5 7 ~ e : ? d ~ x *

F. Prijbst, F.E. Wagner, M. Karger and G. Wortmann

Physics Department, TechnicaZ University of Mmich, D-8046 Garching, Germany

Resume.- En utilisant des sources de 5 7 ~ o dans PdHx ainsi que des absorbeurs de 5 7 ~ e dilu6 dans PdHx, on a &tudi6 le palladium charge en hydrogsne par spectroscopie Mijssbauer 1 concentrations d'hydrogsne allant jusq'l x = 0,89 et 1 temperatures variant entre 4,2 K et 300 K. Les rgsultats fournissent des renseignements sur l'environnement local des atomes Mijssbauer par l'hydrogsne, sur le temps moyen de rgsidence des protons entre sauts de diffusion en fonction de la tempGrature, et sur les changements de structure electronique provoques par l'hydrogsne.

Abstract.- Hydrogenated palladium has been studied by MEssbauer spectroscopy with sources of 5 7 ~ o in PdHx and with absorbers of dilute 5 7 ~ e in PdH, for hydrogen concentrations up to x = 0.89 and in the temperature range between 4.2 K and 300 K. The results are interpreted in terms of the local hydrogen distribution around the substitutional Gssbauer atoms, the temperature dependent hydrogen jump rate, and hydrogen-induced changes in the electronic structure.

1 . Introduction.- The importance of diffusion phe- nomena / I ,2/ and of the occupancy of the interstitial sites in the vicinity of substitutional probe atoms /3-5/ in Gssbauer studies of metal-hydrogen systems have been recognized by various authors.

In a recent study of the PdHx system with the 77keV resonance of '9?4~ information on the local hydrogen environment of the Miissbauer atom could be gained from a comparison of source and absorber Mijssbauer spectra /3/. To get more insight into this matter, and also to study the influence of jump diffusion at higher temperatures than those accessible to

'

"AU Mijssbauer spectroscopy, we have now studied the Mgssbauer spectra of sources of 5 7 ~ o as well as those of absorbers of 5 7 ~ e in PdH at temperatures

X

between 4.2 K and 300 K. Hydrogenated Pd has alrea- dy been studied by the Mijssbauer technique with a variety of probe isotopes (cf. /6/ for a review of this work), but so far all 5 7 ~ e studies of this system except the pioneering but inconclusive work Bemski et al. /7/ have been performed on absorbers only.

2. Experiments and Results.- In order to ensure comparable conditions as to the hydrogen content and homogeneity of hydrogenation in the comparison of source and absorber experiments, most data were taken with a hydrogenated sample that could alter- natively be used as the source or the absorber in a ~Gssbauer experiment. To this end, a Pd foil of

- -

%ork supported by the Bundesministerium fiir Fors- chung und Technologic of the Federal Republic of Germany.

I I p thickness containing 0.5% of enriched 5 7 ~ e in addition to 5 mCi of 5 7 ~ o was prepared by repeated induction melting and subsequent rolling. This sample served either as the source in an emission experiment with a single-line ~ d o . 9 9 5 ~ ~ F e ~ . 0 0 5 stan- dard absorber, or as the absorber in a transmission experiment with a single-line source of "CO in Rh.

The foil was loaded with hydrogen in an electrolytic cell that permitted the simultaneous recording of the emission and absorption spectrum during the electrolysis. Different hydrogen contents were achieved by varying the cell current or, for low concentrations, by permitting the sample to slowly loose hydrogen starting from higher hydrogen contents.

Concentrations above x = 0.85 were obtained by recirculating a refrigerated mixture of sulfuric acid and methanol through the electrolytic cell.

For measurements at temperatures below 200 K the hydrogenated sample was quickly transferred into a cryostat. The hydrogen content of the samples was determined volumetrically by outgasing after the individual Mijssbauer measurements.

At room temperature all spectra are single lines with a nearly Lorentzian shape. There is no significant difference between the source and absorber isomer shift data. Only the latter are therefore shown in figure 1. For the dependence of the isomer shift S on x one can distinguish three regions, which can roughly be approximated by straight lines with slopes of dS/dx = 0.056 and 0.66 mm/s for x 5 0.52 and 0.52 5 x 5 0.83,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792222

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JOURNAL DE PHYSIQUE

respectively. Above x = 0.83 the data are yet scarce but one clearly sees that the S versus x curve becc- mes still steeper, with a slope that may be as high

Fig. 1 : Hydrogen induced changes of the isomer shift in (Pdo.g9sFeo.oos)Hx absorbers at 105 K and 3000 K. The hydrogen concentration x is known with an accuracy of about 0.03. The errors of the AS va- lues are 0.005 mm/s or less. The two high temperature data points with the highest hydrogen concentrations have been obtained at 263 K and corrected for the second-order Doppler shift in order to be comparable with the 300 K data.

At 105 K the absorption patterns were found to remain single lines up to the highest concentrations studied (Fig. 2), albeit with a slight concentration-dependent broadening.

I- a

095 -

-1 0 r l

VELOCITY (mmls)

Fig. 2 : Mijssbauer spectra of ( ~ d o . 9 s 5 ~ ~ ~ e o . 0 0 ~ ) ~ ~ absorbers with different hydrogen contents x taken

at 105 K with a source of 5 7 ~ o in Rh kept at room temperature. Single Lorentzian lines have been fitted to the data points.

The same is true for the source emission spectra up to x = 0.74 (Fig. 3). At x = 0.85, however, one observes a shoulder on the far side of the main emission line, near -1.1 mm/s, and a less pronounced

one on the near side of the main peak, close to zero velocity.

L

+I 0 -1

VELOCITY (mmls)

Fig. 3 : Gssbauer emission spectra of sources of

5 7 Co in ( ~ d o . g g 5 ~ ~ ~ e o . o o s ) ~ ~ at 105 K taken with a single-line absorber of Pdo. 9 9 5 5 7 ~ e ~ . o 0s kept at room temperature. The spectra for x>0.8 were fitted with superpositions of Lorentzian lines as described

in the text. In order to facilitate the comparison with the corresponding absorber spectra (Fig. 2),

the direction of the velocity scale has been rever- sed.

The fit of the spectrum shown in figure 3 has been based on the assumption that these shoulderre-nn a symmetrical quadrupole doublet. For x>0.86 an additional single line is seen to grow near -0.62 mmls.

At x = 0.89 this line has become comparable to the main peak at -0.43 mm/s that is predominant at

lower hydrogen contents. The position of this "main"

peak also shifts with increasing hydrogen content.

Indeed, as can be seen from figure 4, it does so faster than the peak in the absorber spectra. The center of gravity of the total emission pattern shifts even more (Fig. 4), since the additional lines grow mainly on the far side of the main peak.

Further cooling of the samples to 4.2 K does not change their emission or absorption spectra. The transition from the low-temperature behaviour to that observed at room temperature occurs above 200K.

Detailed studies in this temperature range are being continued and will be described elsewhere.

3. Discussion.- The mean time of residence,Tr,between diffusion jumps of the hydrogen atoms in 6-PdHx is known to depend on x and to deviate from an Arrhe- nious type of behaviour at low temperatures 181, but one can roughly say that T, is about 10% and I O - ~ S

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a t 100 K and 300 K, r e s p e c t i v e l y . The mean t i m e e l a p s i n g between t h e decay of "CO and t h e e m i s s i o n o f t h e Mijssbauer quantum from 5 7 ~ e i s e s s e n t i a l l y g i v e n by t h e l i f e t i m e o f t h e 14.4 keV l e v e l ,

m

⁼

1 . 4 ~ 1 0 - ' s . Assuming t h e hydrogen jump r a t e i n t h e v i c i n i t y o f t h e Mgssbauer i m p u r i t y atoms t o b e o f t h e same o r d e r o f magnitude a s i n t h e u n p e r t u r b e d PdH m a t r i x , one t h e r e f o r e e x p e c t s t o f i n d no d i f -

X

f e r e n c e s between t h e s o u r c e and a b s o r b e r s p e c t r a a t 300 K. On t h e o t h e r hand a t 100 K and below, d i f f e r e n c e s i n t h e hydrogen d i s t r i b u t i o n around substitutional Co and Fe atoms s h o u l d p e r s i s t u n t i l t h e e m i s s i o n o f t h e a s s b a u e r q u a n t a . Both e x p e c t a - t i o n s a r e b o r n o u t by o u r d a t a : The r e s u l t s o b t a i - ned a t 105 K show t h a t t h e hydrogen environments o f Co and Fe do, i n d e e d , d i f f e r s u b s t a n t i a l l y . A t room t e m p e r a t u r e t h e s e d i f f e r e n c e s i n t h e ~ G s s b a u e r p a t - t e r n s a r e wiped o u t .

s o u r c e s p e c t r a do n o t , i n d i c a t e s t h a t no hydrogen e n t e r s t h e s i x o c t a h e d r a l i n t e r s t i t i a l s i t e s n e a r e s t t o t h e i r o n atom, whereas t h e s e s i t e s become occu- p i e d around Co i m p u r i t i e s f o r x20.8. The minor l i n e b r o a d e n i n g and a s y m m e t q observed f o r a b s o r b e r s with h i g h hydrogen c o n t e n t s would t h e n r e p r e s e n t s t h e d i s - t r i b u t i o n o f isomer s h i f t s and ( o r ) q u a d r u p o l e s p l i t - t i n g s a r i s i n g from t h e hydrogen d i s t r i b u t i o n i n t h e second and, p o s s i b l y , f a r t h e r neighbour s h e l l s . The r e p u l s i o n between i r o n and hydrogen seems t o e x t e n d a t l e a s t t o t h e second i n t e r s t i t i a l s h e l l , s i n c e t h e "main" peak i n t h e s o u r c e s , i . e . t h e peak t h a t i s c o n s i d e r e d t o c o r r e s p o n d t o no hydrogen i n t h e n e a r e s t n e i g h b o u r s h e l l , s h i f t s more s t r o n g l y f o r x S . 8 t h a n t h e a b s o r b e r l i n e ( F i g . 4 ) . Co a l s o seems t o r e p e l hydrogen, b u t l e s s t h a n i r o n d o e s , s i n c e t h e a d d i t i o n a l l i n e s i n t h e e m i s s i o n s p e c t r a t h a t a r e a t t r i b u t e d t o n e a r e s t hydrogen n e i g h b o u r s a r e found o n l y a t h i g h v a l u e s o f x.

+ ABSORBER DATA ⁰

o SOURCE DATA

(CENTER OF GRAVITY) o

LL SOURCE DATA

(MAIN PEAK) ⁰

0- * .

Although i t i s d i f f i c u l t t o a s s i g n s p e c i f i c hydrogen c o n f i g u r a t i o n s t o t h e s t r u c t u r a l f e a t u r e s i n t h e s o u r c e s p e c t r a , i t i s t e m p t i n g t o assume t h a t t h e q u a d r u p o l e d o u b l e t i s due t o a s i n g l e hy-

,

drogen atom n e x t t o t h e i r o n , w h i l e t h e a d d i t i o n a l

F i g . 4 : Hydrogen i n d u c e d s h i f t s o f t h e c e n t e r o f g r a v i t y o f t h e s o u r c e and a b s o r b e r G s s b a u e r p a t - t e r n s a t 105 K. The s i g n of t h e s h i f t s o b s e r v e d i n t h e s o u r c e e x p e r i m e n t s h a s been r e v e r s e d t o f a c i l i - t a t e t h e comparison w i t h t h e a b s o r b e r d a t a . F o r t h e s o u r c e s and h i g h hydrogen c o n c e n t r a t i o n s ( x > 0 . 8 ) , t h e c e n t e r s h i f t s and t h e s h i f t o f t h e main peak a r e g i v e n s e p a r a t e l y .

W

P

The change from t h e s t a t i c l i m i t t o t h a t of 400

f a s t e n v i r o n m e n t a l r e l a x a t i o n i s e x p e c t e d when T Z n T r . T h i s s h o u l d be t h e c a s e n e a r 200 K 181. P r e l i -

Z

a o r , , , , , , , , , , ,

U 0 0 05 1.0

HlPd RATIO X

minary r e s u l t s i n d i c a t e t h a t above t h i s t e m p e r a t u r e t h e Mijssbauer l i n e s h a p e s indeed b e g i n t o change and t h a t a r e d u c t i o n of t h e f - f a c t o r t a k e s p l a c e . Both e f f e c t s seem t o depend s e n s i t i v e l y o n t h e hydrogen c o n t e n t and must be s t u d i e d i n more d e t a i l b e f o r e d e f i n i t e c o n c l u s i o n s c a n b e drawn. E f f e c t s of t h i s k i n d a r e e x p e c t e d a s a consequence of time-dependent h y p e r f i n e i n t e r a c t i o n s and o f t h e motion o f t h e

~ g s s b a u e r n u c l e i t h a t i s caused by t h e d i f f u s i n g hydrogen 19-111.

The f a c t t h a t t h e low t e m p e r a t u r e a b s o r b e r s p e c t r a s t a y s i n g l e l i n e s up t o x = 0.89 w h i l e t h e

u n s p l i t Line c o u l d r e s u l t from two hydrogen atoms i n a c i s c o n f i g u r a t i o n . The q u a d r u p o l e s p l i t t i n g of a b o u t 1 mm/s a p p e a r s r a t h e r h i g h f o r a s i n g l e hydrogen n e i g h b o u r , b u t i t i s n o t u n r e a s o n a b l y l a r g e

compared w i t h t h e s p l i t t i n g c a u s e d , f o r i n s t a n c e , by c a r b o n i n t e r s t i t i a l 8 i n a u s t e n i t e 1101.

For x20.6 t h e s p e c t r a t a k e n a t 300 K e x h i b i t a s u b s t a n t i a l l y l a r g e r hydrogen-induced s h i f t t h a n t h e a b s o r b e r s p e c t r a a t 105 K do ( F i g . 4 ) . The d i f f e r e n - ce i s t o o l a r g e t o b e a t t r i b u t a b l e t o t h e second o r d e r Doppler s h i f t . R a t h e r , one must assume t h a t a t room t e m p e r a t u r e t h e t h e r m a l e n e r g y s u f f i c e s f o r t h e d i f f u s i n g hydrogen atoms t o p a r t i a l l y overcome t h e r e p u l s i v e p o t e n t i a l of t h e Fe i m p u r i t i e s , whence t h e mean number o f hydrogen n e i g h b o u r s i n c r e a s e s w i t h t e m p e r a t u r e .

The hydrogen-induced isomer s h i f t s o b s e r v e d a t room t e m p e r a t u r e ( F i g . 1) a r e l a r g e r t h a n t h o s e found p r e v i o u s l y by Mahnig and Wicke 1121, p e r h a p s b e c a u s e t h e gas-phase l o a d i n g t e c h n i q u e a p p l i e d by t h e s e a u t h o r s y i e l d e d l e s s t h a n t h e assumed e q u i l i - brium hydrogen c o n c e n t r a t i o n . At x = 0.52 t h e S v e r s u s x curve ( F i g . 1) changes s l o p e a b r u p t l y . A

s i m i l a r b e h a v i o u r h a s p r e v i o u s l y b e e n found t o b e q u i t e g e n e r a l a f e a t u r e of Pd-Fe-H and r e l a t e d systems 112,131. I t h a s been d i s c u s s e d i n t e r m s o f t h e e f f e c t i v e number o f e l e c t r o n s , ne, donated i n t o t h e

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C2-638 ^JOURNAL^DE^PHYSIQUE Pd conduction band by the hydrogen interstitials as

well as the substitutional metal atoms, the idea being that the kink quite generally occurs when the host d-band is filled at ne z 0.55 /12,13,6/. From the Pd-Fe-H phase diagram, however, the minimum hy-

(min) drogen concentration in the 6-phase, 76 , f o r

(Pdo.9ssFeo.oos)Hx expected to be x (mln) = 0.57 6

1141. The center shift of the Mgssbauer line in the mixed phase region (x<x (min)

0 ⁾cannot but be a smooth, nearly straight line since it is a weighted mean of the unresolved a and 6 phase components. The different slopes for the dS/dx curve for xS0.52 and

~20.52 thus merely represent the mixed phase and pure 6 phase regions, and do not permit direct conclusions as to band-filling effects to be drawn.

The isomer shift difference between x=O and ~ x ( ~ ~ ~ ) 0

is roughly equal to the shift estimated to result from the volume expansion alone 161. This is concei- vable if local changes in the electronic structure are of minor importance because no hydrogen enters the vicinity of the Fe impurities. The stronger dependence of the isomer shift on x observed for higher hydrogen concentrations should then result not only from band structure and volume changes but also, and perhaps even more importantly, from a dis- proportionately steep increase of the mean number of hydrogens entering the vicinity of the iron atoms.

These arguments demand that hydrogen nei&bours to the iron atoms reduce the electron density at the iron nuclei. This indicates that the shielding of the interstitial protons is effected by a withdrawal mainly of s-electrons from the neighbouring iron atoms. Band structure calculations for the PdH system 1151 indeed show that the density of states below the d-band that represents the Pd-H bonding has mainly d-character inside the Pd sphere.

4. Conclusions.-The present study of the Pd-Fe-H system reveals drastic differences between the source and absorber spectra taken at low temperatures, where the hydrogen distribution is quasistatic.

Although further work will be necessary before these differences can be understood in detail, they show that substitutional Fe and Co in PdHx repel the interstitial hydrogen, the repulsive interaction being stronger for Fe than for Co. The room temperature spectra are typical for the time-average of the hydrogen distribution around the ~gssbauer atom and show that at high hydrogen concentrations the abun- dance of hydrogen in the vicinity of the iron impurities increases with temperature.

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