HAL Id: jpa-00226102
https://hal.archives-ouvertes.fr/jpa-00226102
Submitted on 1 Jan 1986
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
INTERACTION OF HYDROGEN WITH
IMPURITIES IN DILUTE PALLADIUM ALLOYS
B. Lengeler
To cite this version:
B. Lengeler. INTERACTION OF HYDROGEN WITH IMPURITIES IN DILUTE PAL- LADIUM ALLOYS. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-1015-C8-1018.
�10.1051/jphyscol:19868195�. �jpa-00226102�
INTERACTION OF HYDROGEN WITH IMPURITIES IN DILUTE PALLADIUM ALLOYS
B. L E N G E L E R
Kernforschungsanlage Julich, I F F , ~ o s t f a c h 1913, 0-5170 Julich, F.R.G.
Resume
L ' i n t e r a c t i ~ n HydrogOne-impuretG d a u s l e s a l l i a g e s d i l u % s de P a l l a d i u m a 6t'e ' e t u d i ' e e p a r EXAFS. A l a t e m p ' e r a t u r e a m b i a n t e e t d 7 7 K , 1 Hydrog6ne n ' e s t p a s piGgG p a r l e s impuretGs s u b s t i t u t i o n e l l e s d a n s l e P a l l a d i u m . A 77 K , l ' i n t e r a c t i o n Y y d r o g e n e - i m p u r e t 6 e s t msme r 8 p u l s i v e : 1 ' Hydrogsne en s o l u t i o n e v i t e l e s s i t e s i n t e r s t i t i e l s v o i s i n s d e l ' i m p u r e t ' e .
A b s t r a c t
The h y d r o g e n - i m p u r i t y i n t e r a c t i o n i n d i l u t e Pd a l l o y s was i n v e s t i g a t e d by EXAFS m e a s u r e m e n t s . Hydrogen i s n o t t r a p p e d a t room t e m p e r a t u r e and a t 7 7 K by s u b s t i t u t i o n a l i m p u r i t i e s i n P d . A t 7 7 K t h e
H - i m p u r i t y i n t e r a c t i o n i s even r e p u l s i v e , i . e . , t h e d i s s o l v e d hydrogen a v o i d s t h e i n t e r s t i c e s i n t h e v i c i n i t y of t h e i m p u r i t i e s .
T h i s p a p e r r e p o r t s on an EXAFS i n v e s t i g a t i o n of t h e hydrogen- i m p u r i t y i n t e r a c t i o n i n a number of d i l u t e p a l l a d i u m a l l o y s . P a l l a d i u m i s known t o a b s o r b s u b s t a n t i a l amounts of h y d r o g e n . T h i s l e a d s t o an i n t e r s t i t i a l s o l u t i o n PdH . The maximum H/Pd r a t i o i n x = 1, r e s u l t i n g i n t h e compound f d ~ w i t h NaCl s t r u c t u r e . Hydrogen o c c u p i e s t h e o c t a h e d r a l i n t e r s t i c e s i n t h e f c c l a t t i c e of Pd / I / . I n t e r s t i t i a l hydrogen i n m e t a l s always expands t h e l a t t i c e . I n Pd t h e l a t t i c e e x p a n s i o n i s 6 a / a = 0.0567, a s deduced f r o m l a t t i c e p a r a m e t e r d a t a /1/ and from t h e p r e s e n t EXAFS
m e a s u r e m e n t s . The q u e s t i o n now a r i s e s i f s u b s t i t u t i o n a l i m p u r i t i e s i n t h e Pd m a t r i x a t t r a c t o r r e p e l t h e hydrogen d i s s o l v e d i n t h e l a t t i c e . I n p a r t i c u l a r , t h e f o l l o w i n g q u e s t i o n s a r e of i n t e r e s t . What i s t h e i n f l u e n c e of t h e c h e m i c a l n a t u r e of t h e i m p u r i t i e s and of t h e l o c a l l a t t i c e d i s t o r t i o n t h e y i n d u c e i n t h e l a t t i c e ? What i s t h e i n f l u e n c e of t h e t e m p e r a t u r e and of t h e hydrogen c o n c e n t r a t i o n ? I f t h e r e i s a n a t t r a c t i v e i m p u r i t y - h y d r o g e n i n t e r a c t i o n d e t r a p p i n g may o c c u r when t h e t h e r m a l e n e r g y
e x c e e d s t h e b i n d i n g e n e r g y . O n t h e o t h e r hand, a t low t e m p e r a t u r e s t h e t e n d e n c y o f t h e hydrogen t o p r e c i p i t a t e i n a h y d r i d e p h a s e may a l s o l e a d t o ( p a r t i a l ) d e t r a p p i n g of t h e h y d r o g e n .
I n a r e c e n t p a p e r / 2 / i t was shown how EXAFS c a n be used t o d e t e r m i n e t h e l a t t i c e s i t e o c c u p i e d by hydrogen i n c r y s t a l l i n e m a t e r i a l s . When t h e hydrogen i s l o c a t e d on t h e l i n e j o i n i n g t h e a b s o r b e r and t h e - b a c k s c a t t e r e r ( c o l i n e a r o r s h a d o w i n g c o n f i g u r a t i o n ) t h e p h o t o e l e c t r o n s u f f e r s a c h a n g e i n a m p l i t u d e
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868195
C8-1016 JOURNAL DE PHYSIQUE
and p h a s e when p a s s i n g ( t w i c e ) t h r o u g h t h e p o t e n t i a l g e n e r a t e d by t h e h y d r o g e n . T h i s c h a n g e i n p h a s e and a m p l i t u d e i s v i s i b l e , a l t h o u g h t h e b a c k s c a t t e r i n g s t r e n g t h of t h e hydrogen i s t o o s m a l l t o b e i i v i s i b l e . I n an f c c l a t t i c e ( l i k e Ni o r Pd) we e x p e c t
t h i s l e n s " e f f e c t t o o c c u r f o r t h e s e c o n d n e i g h b o r of a m e t a l atom when t h e H o c c u p i e s o c t a h e d r a l i n t e r s t i c e s . I n NiH i t l e a d s t o a 50 % i n c r e a s e i n t h e a m p l i t u d e of t h e s e c o n d 0 f i 8 ? g h b o r c o n t r i b u t i o n i n t h e EXAFS / 2 / . I f P d c o n t a i n s s u b s t i t u t i o n a l i m p u r i t i e s and i f t h e hydrogen o c c u p i e s t h e n e a r e s t o c t a h e d r a l i n t e r s t i c e s t h e " l e n s " e f f e c t i s e x p e c t e d in t h e s e c o n d s h e l l c o n t r i b u t i o n a r o u n d t h e i m p u r i t i e s . T h i s e f f e c t w i l l be used in t h i s i n v e s t i g a t i o n t o f i n d o u t i f i m p u r i t i e s i n Pd c a n t r a p hydrogen o r n o t .
The s y s t e m s s t u d i e d i n t h i s i n v e s t i g a t i o n a r e l i s t e d i n t a b l e 1.
-
PdYZ r
Nb
T a b l e 1: D i l u t e Pd a l l o y s w i t h i m p u r i t y c o n c e n t r a t i o n c . , h y d r o g e n t o m e t a l r a t i o x and d i s p l a c e m e n t s l of t h e f i S s t Pd s h e l l a r o u n d t h e i m p u r i t i e s i n t h e s a m p l e s w i t h x = o.
The impurities show a great variety in chemical nature (simple metals, t r a n s i t i o n metals and 4f and 5f metals). They a l s o show a great variety in the local l a t t i c e d i s t o r t i o n they induce in the Pd matrix. Table 1 l i s t s the displacements s l in the interatomic.distances r l of the nearest neighbor s h e l l s around the impurities. A Th atom e . g . s h i f t s i t s 1 2 f i r s t Pd n e i g h o r s o u t w a r d s by 0.10 8,
w h e r e a s Nb p u l l s them i n w a r d s by 0.03 h . These d a t a w e r e o b t a i n e d from an EXAFS i n v e s t i g a t i o n of t h e d i l u t e P d a l l o y s p r i o r t o t h e hydrogen l o a d i n g /3/. The a l l o y s w e r e c h a r g e d e l e c t r o l y t i c a l l y and t h e hydrogen c o n c e n t r a t i o n s w e r e d e t e r m i n e d by w e i g h i n g
and h e a t e x t r a c t i o n / I / . I t t u r n e d o u t t h a t t h e hydrogen e s c a p e d c o m p l e t e l y f r o m a l l a l l o y s w i t h i n 1 2 h o u r s a t room t e m p e r a t u r e ( f o i l t h i c k n e s s e s between 8 and 25 pm). T h i s i m p l i e s t h a t
t r a p p i n g a t i m p u r i t i e s i n Pd d o e s n o t o c c u r a t room t e m p e r a t u r e . T h i s made i t a l s o n e c e s s a r y t o s t o r e t h e s a m p l e s i n l i q u i d n i t r o g e n a f t e r l o a d i n g . All EXAFS m e a s u r e m e n t s w e r e made i n t r a n s m i s s i o n a t 7 7 K a t t h e RUM0 beam l i n e o f t h e Hamburger S y n c h r o t r o n s t r a h l u n g s l a b o r . A S i ( 3 1 1 ) d o u b l e c r y s t a l monochrometor was used / I / .
EXAFS x * k 2 a b o v e t h e K e d g e s of Zr and Y and t h e L e d g e of T h i n PdZr, PdY and PdTh w i t h o u t and w i t h hydrogen. ~ a u r
c o o r T i n a t i ' t m s h e l l s a r o u n d t h e i m p u r i t i e s a r e c l e a r l y v i s i b l e .
The s e c o n d n e i g h b o r c o n t r i b u t i o n i s t h e one of s p e c i a l c o n c e r n
h e r e . I t i s o b v i o u s t h a t t h e hydrogen l o a d e d s a m p l e s do n o t
show enhanced s e c o n d n e i g h b o r c o n t r i b u t i o n s . T h i s i m p l i e s t h a t
H i s n o t t r a p p e d by t h e i m p u r i t i e s a t 7 7 K . I n view of t h e h i g h
F i g u r e 1:
F o u r i e r t r a n s f o r m s o f t h e E X A F S p k 2 a b o v e t h e K e d g e s i n Z r a n d Y a n d t h e L 3 e d g e i n Th i n t h e s y s t e m s PdZrH,, PdYHy a n d PdThHx a t 77 K . T h e h y d r o g e n e x p a n d s t h e l a t t i c e . A c o m p a r i s o n o f t h e s e c o n d n e i g h b o r c o n t r i b u t i o n s w i t h o u t a n d w i t h h y d r o g e n shows t h a t t h e h y d r o g e n i s n o t t r a p p e d b y t h e i m p u r i t i e s .
I I I I I ) "
yl
/'