NMR STUDIES OF HYDROGEN DIFFUSION IN AMORPHOUS Pd-Cu-Si-H

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NMR STUDIES OF HYDROGEN DIFFUSION IN

AMORPHOUS Pd-Cu-Si-H

H. Schone, E. Seymour, G. Styles

To cite this version:

JOURNAL

DE PHYSIQUE

Colloque C8, supplkment a u n012, Tome 46, d6cembt-e 1985 page c8-675

NMR

STUDIES

O F H Y D R O G E N

DIFFUSION

I N

AMORPHOUS Pd-Cu-Si-H H.E. Schone, E.F.W. seymour+ and G.A. Styles'

CoZZege o f WiZZiam

and

Mary, W i Z Z i a m s b u r g , VA 23185, U.S.A. + u n i v e r s i t y o f W a r w i c k , C o v e n t r y CV4 7AL, U.K.

Rksumk - Le temps de r e l a x a t i o n T1(T) de 11hydrog8ne dans l'amorphe Pd-Cu-Si a k t 6 mesurk p a r RMN s u r l e p r o t o n . L ' k n e r g i e d ' a c t i v a t i o n pour l a d i f f u s i o n e s t trouvke kgale

3

0 , 2 3 eV. Le nombre des chemins de d i f f u s i o n p o s s i b l e s semble S t r e r k d u i t e t l e s p r o t o n s s o n t kgalement d i s t r i b u k s s u r l e s atomes de c u i v r e e t de palladium.

A b s t r a c t - We have measured T1(T) f o r hydrogen i n amorphous Pd-Cu-Si. We qetermine an a c t i v a t i o n energy f o r d i f f u s i o n of 0.23 eV. We o b t a i n evidence f o r a r e d u c t i o n i n a v a i l a b l e d i f f u s i o n p a t h s , and we f i n d t h a t p r o t o n s approach palladium and copper atoms with e q u a l p r o b a b i l i t y .

We r e p o r t experimental r e s u l t s f o r t h e

NMR

T1 f o r hydrogen absorbed i n amor- phous Pd80Cu7Si13. The t h i r d element, copper, i s o f t e n used i n t h i s a l l o y because it i s found t o a s s i s t i n s t a b i l i z i n g t h e amorphous phase.

NMR s t u d i e s have p r e v i o u s l y been made on a t l e a s t two amorphous m e t a l hydride systems. Bowman and coworkers1 have s t u d i e d t h e aCu-Ti-H and aPd-Zr-H systems f o r

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hydrogen c o n c e n t r a t i o n s of o r d e r 0.5 H / M , and Panissod, e t a 1 have a l s o examined t h e aPd-Zr-H system. Both groups f i n d evidence f o r a range of hydrogen s i t e s with d i f f e r e n t a c t i v a t i o n e n e r g i e s , and, a t l e a s t i n t h e c a s e of Cu-Ti, an i n d i c a t i o n t h a t t h e l o c a l o r d e r i n amorphous and c r y s t a l l i n e phases i s s i m i l a r .

I n t h e p r e s e n t work we f i n d t h a t hydrogen d i f f u s e s i n aPd-Cu-Si w i t h a low a c t i v a t i o n energy. T h i s system, i n c o n t r a s t t o t h o s e s t u d i e d by Bowman, i s of t h e metal-metalloid t y p e , which has not p r e v i o u s l y been examined by NMR t e c h n i q u e s . We have measured t h e s p i n l a t t i c e r e l a x a t i o n a s a f u n c t i o n of temperature and have o b t a i n e d d i f f u s i o n parameters.

Berry and P r i t c h e t have performed i n t e r n a l f r i c t i o n measurements on a s e r i e s of m e t a l l i c g l a s s e s which have r e c e i v e d e l e c t r o l y t i c hydrogenation t r e a t m e n t s , and have a s s o c i a t e d i n t e r n a l f r i c t i o n peaks with t h e presence of hydrogen i n many systems, i n c l u d i n g Pd-Si. Such a peak c o n s t i t u t e s evidence t h a t t h e hydrogen atom r e s i d e s i n an asymmetric p o s i t i o n from which r e o r i e n t a t i o n can o c c u r .

They have a l s o s t u d i e d t h e Gorsky e f f e c t f o r aPd-Si samples. The Gorsky e f f e c t depends on motion of d e f e c t s (hydrogen atoms) from o r t o r e g i o n s of high s t r a i n , and t h u s r e f l e c t s d i f f u s i o n over macroscopic d i s t a n c e s . These experiments

C8-676

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show an a c t i v a t i o n energy f o r d i f f u s i o n of H i n aPd-Si of o r d e r 0.25 eV (dependent on hydrogen c o n c e n t r a t i o n ) . The two t e c h n i q u e s g i v e e s s e n t i a l l y t h e same energy.

S t r u c t u r a l models f o r m e t g l a s s e s of t h e Pd-Si t y p e a r e u s u a l l y based on random c l o s e packing; t h e second ( s m a l l e r ) c o n s t i t u e n t may e i t h e r be included i n t h e pack- i n g model o r may simply be assumed t o occupy t h e l a r g e h o l e s which remain i n a monatomic packing model. The random packing of palladium atoms should c r e a t e a l a r g e number of h o l e s , some perhaps occupied by s i l i c o n atoms, but i n c l u d i n g many h o l e s which a r e v e r y s i m i l a r t o t h e 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 occupied by hydro- gen i n c r y s t a l l i n e palladium h y d r i d e . This l a t t e r , of c o u r s e , has cubic symmetry. The s i m p l e s t e x p l a n a t i o n of t h e aPd-Si-H r e s u l t s , t h e n , i s t h a t t h e s i l i c o n atom h a s t h e e f f e c t of d i s t o r t i n g t h e o c t a h e d r a l h o l e occupied by t h e hydrogen atom, which i s t h e n involved i n both r e o r i e n t a t i o n and d i f f u s i o n p r o c e s s e s . On t h e o t h e r hand t h e two experiments could be l o o k i n g a t d i f f e r e n t p o p u l a t i o n s of hydrogen atoms; t h o s e p a r t i c i p a t i n g i n t h e i n t e r n a l f r i c t i o n p r o c e s s e s may remain l o c a l i z e d .

The s o l u b i l i t y f o r H i n aPd-Cu-Si i s found t o be about 0.005 a t S T P ~ ( f o r PdHx t h e v a l u e o f H/M r a n g e s up t o 0 . 7 ) .

The samples used i n t h i s work were prepared by Johnson-Matthey, who k i n d l y s u p p l i e d them t o u s . They were made by t h e melt-spinning t e c y n i q u e i n t h e form of r i b b o n s of t h i c k n e s s 50 microns and width 2 cm. For t h e

NMR experiments t h e y were

r o l l e d i n t o a c o i l w i t h hydrogen-free t e f l o n p r o v i d i n g i n s u l a t i o n between l a y e r s . The sample was loaded w i t h hydrogen a t room t e m p e r a t u r e a t a p r e s s u r e of 25 atmos- p h e r e s , and was s e a l e d i n a pyrex t u b e .

T measurements were made on a Polaron spectrometer a t 45 MHz over t h e tem- 1

p e r a t u r e range 150 K t o 400 K . The t e m p e r a t u r e was c o n t r o l l e d a u t o m a t i c a l l y u s i n g a l i q u i d n i t r o g e n bubble pump ( f o r low t e m p e r a t u r e s ) and a simple r e s i s t i v e h e a t e r ( f o r high t e m p e r a t u r e s ) .

The experimental v a l u e s f o r T ( T ) a r e shown i n F i g . ₁ 1.

I n o r d e r t o a n a l y z e t h e r e l a x a t i o n r e s u l t s , we assume l/T1 = l/Tle + l/Tld where Tle i s t h e Korringa (conduction e l e c t r o n ) r e l a x a t i o n t i m e . I f we assume T T = 100 secK and s u b t r a c t t h e l/Tle term we g e t t h e Tld r e s u l t s shown. We t a k e

l e t h e form

where r c = roeEaikT, and we f i t t h e r e s u l t s by assuming an a c t i v a t i o n energy of 0.23 eV. T h i s i s t h e s l o p e of t h e high t e m p e r a t u r e p a r t of t h e curve and i s i n r e a s o n a b l e agreement w i t h t h e Gorsky e f f e c t r e s u l t s o f Berry and P r i t c h e t t . We determine t h e v a l u e of

rc from t h e p o s i t i o n of T

Imin. The p r e f a c t o r above i s b a s i - c a l l y t h e d i p o l a r second moment and we f i n d t h a t both M2 and t h e r e l a x a t i o n r a t e a r e dominated by t h e H-Cu i n t e r a c t i o n , assuming random occupation of o c t a h e d r a l s i t e s , because t h e hydrogen c o n c e n t r a t i o n i s low. I n c a l c u l a t i n g t h i s q u a n t i t y we assume t h a t t h e H-Cu d i s t a n c e i s t h e same a s t h e H-Pd d i s t a n c e i n alPdHx

found f o r afPdHx. We assume t h a t t h e amorphous a l l o y has a reduced N(EF) a s s o c i - a t e d with d e n s i t y and s i l i c o n c o n t e n t .

S e v e r a l comments can be made. 1 ) The a c t i v a t i o n energy has been determined i n t h e high t e m p e r a t u r e regime; however, t h e curve i s asymmetric, w i t h a s m a l l e r s l o p e a t low t e m p e r a t u r e . We t h i n k t h i s r e f l e c t s a range o f a c t i v a t i o n e n e r g i e s c o r r e - sponding t o a d i s t r i b u t i o n o f w e l l d e p t h s and s a d d l e p o i n t e n e r g i e s . Berry h a s observed a d i s t r i b u t i o n of width of o r d e r 0 . 2 eV (comparable t o t h e mean a c t i v a t i o n e n e r g y ) . ~ i c h a r d s ~ h a s c o n s t r u c t e d a t h e o r e t i c a l model f o r t h i s d i s t r i b u t i o n by c a l c u l a t i n g t h e d i s t r i b u t i o n of e q u i l i b r i u m and s a d d l e p o i n t s i z e s , based on a Dense Random Packing model f o r t h e amorphous a l l o y . He o b t a i n s a width f o r t h e Ea curve i n r e a s o n a b l e agreement with B e r r y ' s experimental r e s u l t s . 2 ) The p o s i t i o n of t h e minimum i s s h i f t e d t o a higher temperature t h a n t h a t seen i n a a'PdHx, which means t h a t t h e d i f f u s i o n i s slower i n t h i s system. Reducing t h e hydrogen concen- t r a t i o n would be expected t o i n c r e a s e t h e r a t e by t h e blocking f a c t o r ( 1 - x ) . The i n h i b i t i o n of d i f f u s i o n could be blamed on a r e d u c t i o n of t h e number o f 0 t y p e s i t e s o r perhaps on t h e blocking of such s i t e s by s i l i c o n atoms. 3 ) I n some sys- tems, such a s amorphous PdZrH, t h e hydrogen atom i s found t o f a v o r one m e t a l s p e c i e s over t h e o t h e r . I n our c a l c u l a t i o n we assumed t h a t both H-Cu and H-Pd d i s t a n c e s were e q u a l t o t h e H-Pd d i s t a n c e i n a7PdHx. The magnitude of T1 i s pre- d i c t e d c o r r e c t l y , which seems t o v i n d i c a t e t h e assumption t h a t n e i t h e r m e t a l species i s f a v o r e d .

The a u t h o r s wish t o t h a n k Johnson Matthey Research Center f o r p r o v i s i o n of t h e amorphous a l l o y . HES w o ~ l d l i k e t o t h a n k t h e U n i v e r s i t y of Warwick f o r t h e i r h o s p i t a l i t y w h i l e t h e s e experiments were being performed. The work was supported by t h e UK Science and Engineering Research Council.

REFERENCES

[ l ] Bowman, R . C . J r . , Maeland, A.J., and Rhim, W.K., Phys. Rev. B

6,

6362 ( 1 9 8 2 ) . [ 2 ] Panissod, P. and Mizoguchi, T., Proceedings Conference on Rapidly Quenched

Metals, Sendai p . 1605 (1982).

[ 3 ] Berry, B.S. and P r i t c h e t

,

W.L., Phys. Rev. B

&,

2299 ( 1 9 8 1 ) .

[4]

R . G r i e s s e n , p r i v a t e communication.

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FIG.

1 Spin lattice relaxation time for in aPd-Cu-Si (experimental T1 dipolar contribution T