SYMMETRY EFFECTS IN THE H-INDUCED W(100) SURFACE RECONSTRUCTION

HAL Id: jpa-00224134

https://hal.archives-ouvertes.fr/jpa-00224134

Submitted on 1 Jan 1984

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.

SYMMETRY EFFECTS IN THE H-INDUCED W(100) SURFACE RECONSTRUCTION

E. Didham, M. Foster, B. Hinch, R. Willis

To cite this version:

E. Didham, M. Foster, B. Hinch, R. Willis. SYMMETRY EFFECTS IN THE H-INDUCED W(100) SURFACE RECONSTRUCTION. Journal de Physique Colloques, 1984, 45 (C5), pp.C5-113-C5-117.

�10.1051/jphyscol:1984516�. �jpa-00224134�

SOURNAL DE PHYSIQUE

Colloque C 5 , suppl6rnent a u n04, Tome 45, avril 1984 page C5-113

SYMMETRY EFFECTS I N THE H-INDUCED ~ ( 1 0 0 ) SURFACE RECONSTRUCTION

E.F.J. Didham, M.S. F o s t e r , B . J . Hinch and R.F. W i l l i s

Cavendish Laboratory, Department of P h y s i c s , U n i v e r s i t y of Cambridge, Cambridge CB3 OHE, U.K.

Rdsum6

-

Nous avons u t i l i s 6 l a s p e c t r o s c o p i e d e p e r t e d ' s n e r g i e d f 6 1 e c t r o n s p a r r s f l e x i o n (REELS) pour s u i v r e l e s t r a n s f o r m a t i o n s d e p h a s e s l o r s d e l ' a d s o r p t i o n d e l ' h y d r o g s n e s u r l a s u r f a c e (100) du t u n g s t h n e . Nous proposons d e s modsles pour l e s d i f f s r e n t e s r e c o n s t r u c t i o n s , e n a c c o r d avec l e s diagrammes LEED. Lorsque l e t a u x de recouvrement e n hydrogzne augmente, l a s y m s t r i e < l o > de l a s u r f a c e e s t b r i s b e , B t r a v e r s l e s d s p l a c e m e n t s atomiques p e r p e n d i c u l a i r e s e t p a r a l l s l e s B l a s u r f a c e . Ceci permet l ' o b s e r v a t i o n d e s modes de v i b r a t i o n q u i n e s o n t p a s norma- lement v u s dans l a d i r e c t i o n s p b c u l a i r e .

A b s t r a c t

-

We have used R e f l e c t i o n E l e c t r o n Energy Loss S p e c t r o s c o p y (REELS) t o m o n i t o r t h e complex phase t r a n s f o r m a t i o n s t h a t o c c u r on t h e hydrogen covered W(100) s u r f a c e , and we p r o p o s e models f o r t h e d i f f e r e n t s u r f a c e r e c o n s t r u c t i o n phases which a r e i n agreement w i t h t h e observed LEED p a t t e r n s . As t h e H-coverage i n c r e a s e s , t h e s u r f a c e <10> symmetry i s b r o k e n , v i a atomic d i s p l a c e m e n t s

p e r p e n d i c u l a r and p a r a l l e l t o t h e s u r f a c e , e n a b l i n g v i b r a t i o n a l modes n o t normally s e e n i n t h e s p e c u l a r beam d i r e c t i o n t o be observed.

1 . INTRODUCTION

A t low coverages of hydrogen (Hydr0gen:Tungsten r a t i o (H:W) ,< 0.35) i t i s found by R e f l e c t i o n E l e c t r o n Energy Loss S p e c t r o s c o p y (REELS) t h a t t h e W atom

Q l o n a (100) s u r f a c e a r e pinched t o g e t h e r , w i t h H atoms occupying b r i d g e s i t e s

.

^{F o r}

a H:W r a t i o a p p r o a c h i n g 2 . 0 , t h e W atoms r e l a x back t o t h e b u l k l a t t i c e s p a c i n g , t h e H atoms remaining i n b r i d g e s i t e s a t a l l c o v e r a g e s . I n t e r e s t i n g e f f e c t s occur a t i n t e r m e d i a t e coverages when t h e W s u r f a c e r e c o n s t r u c t s v i a commensurate- incommensurate d i s o r d e r e d s t r u c t u r a l phase t r a n s f o r m a t i o n s ( F i g . I ) .

"PINCHEDSTATE "TILLTED'' STATE

COMMENSURATE INCOMMENSURATE 1-DDISORDER 2-DDISORDER (1

x

1)

F i g . ] . Schematic LEED p a t t e r n s i n d i c a t e H-adatom induced long-range o r d e r - d i s o r d e r t r a n s f o r m a t i o n phenomena.

The LEED p a t t e r n shows h a l f - o r d e r s p o t s p l i t t i n g i n t h e incommensurate phase ( 2 i n d i c a t i v e of a p e r i o d i c l a t t i c e d i s t o r t i o n e x t e n d i n g o v e r many u n i t c e l l s . REELS d a t a on t h e H v i b r a t i o n a l c r o s s s e c t i o n s s u g g e s t t h a t t h e incommensurate phase i n v o l v e s t i l t i n g of p a i r s of W atoms o u t of t h e s u r f a c e , i e a l o c a l i s e d

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984516

C5-114 J O U R N A L DE PHYSIQUE

l a t t i c e d i s t o r t i o n w i t h W atom d i s p l a c e m e n t s p e r p e n d i c u l a r and p a r a l l e l t o t h e s u r f a c e ( 3 . T h i s b e h a v i o u r i s c o m p a t i b l e w i t h a phonon s o f t mode mechanism d i c t a t e d by s t r i c t symmetry d e p e n d e n t r e q u i r e m e n t s ( 4 . F o r H:W > 0 . 5 one d i m e n s i o n a l d i s o r d e r i s o b s e r v e d f o l l o w e d by complex 2-D d i s o r d e r a t h i g h e r c o v e r a g e s w i t h H:W > 0 . 8 . The ( I x I ) LEED l a t t i c e i s r e c o v e r e d a t s a t u r a t i o n c o v e r a g e , i e H:W = 2 . 0 ( F i g . I ) .

2 . PHONON SOFT MODE MECHANISM OF COIMENSURATE-INCOMMENSURATE PHASE TRANSFORMATION The r e d u c e d B r i l l o u i n zone f o r t h e s u r f a c e i s shown i n F i g . 2 . At

M

two phonon modes a r e p r e s e n t : M modes w i t h W atom d i s p l a c e m e n t s n o r m a l t o t h e s u r f a c e , and M modes w i t h motion p a r a l l e l t o t h e s u r f a c e ( 5 . 1 The e f f e c t of t h e hydrogen i s

5

FHGI\!GN MODES AT iC] POINT AT W E HAVE

(I) OM, MODES

0 0

(Dl SPLACEMENTS I SURFACE

Ff

^{(i3 AND M5 MODES}

^{0 0 0-}

(Dl SPL ACEMENTS

/I SURFACE)

t---O

Dl SPLACEMENZ ALONG

<

¹ I

>

AS FOR CLEAN W (COMM.) AND CLEAN Mo(100) (INCOMM)

THESE MODES CAN COUPLE (Ref. AFasohno, G.%toro & E Tosatti PRL 44(1960)1664>

Modes

soften ONE OF M,MODES

kb'

I I

r

Q A R R

a*

F i g . 2 . Phonon S o f t Mode Mechanism of Commensurate-Incommensurate P h a s e T r a n s f o r m a t i o n .

( a ) t o c a u s e p i n c h i n g of p a i r s of W atoms a l o n g < l o > d i r e c t i o n s , a n d (b) t o l o w e r t h e e n e r g y o f t h e M1 mode.

T h i s l i f t s t h e d e g e n e r a c y of one o f t h e M5 modes which can t h e n c o u p l e w i t h t h e M mode a l o n g t h e A symmetry d i r e c t i o n away from t h e h i g h symmetry

9

p o i n t ( 6

1 k

i . e a l o n g t h e < l o > d i r e c t i o n , w i t h t h e mixed mode e v e n t u a l l y g o i n g s o f t a t Q

,

a d i s c r e t e incommensurate wave v e c t o r . T h i s c o u p l i n g o f modes t h u s p r o d u c e s a p e r i o d i c l a t t i c e d i s t o r t i o n (PLD) wave c o n s i s t i n g o f two component waves i n q u a d r a t u r e w i t h W a t o m i c d i s p l a c e m e n t s p e r p e n d i c u l a r and p a r a l l e l t o t h e s u r f a c e , r e s u l t i n g i n a t i l t i n g o f W d i m e r s o u t o f t h e s u r f a c e p l a n e ( 7 . 3 . SYMMETRY ASPECTS

T h e p h a s e r e l a t i o n s h i p between t h e ~ e r ~ e n d i c u l a r and p a r a l l e l motions o f t h e s u r f a c e W atoms i s shown i n F i g . 3 . The waves may t a k e any form: t h e extreme c a s e o f a s q u a r e wave d i s p l a c e m e n t r e p r e s e n t s a s u r f a c e c o n s i s t i n g of a n t i p h a s e domains w i t h h i g h l y l o c a l i s e d boundary r e g i o n s ( w a l l s ) o f m a i n l y M mode n a t u r e

( d i s p l a c e m e n t s p e r p e n d i c u l a r t o t h e s u r f a c e ) . The domain w a l l s o r i e r w i t h a

SITE OCCUPATION PROBAslLlTV

-

--

x /

F i g . 3 . ( a ) Phase r e l a t i o n between l a t e r a l d i s p l a c e m e n t s Y(UlI) and v e r t i c a l d i s p l a c e m e n t s @(UL) a t domain w a l l .

(b) Atomic d i s p l a c e m e n t s i n t h e v i c i n i t y o f a domain w a l l produced by c o u p l i n g between v e r t i c a l d i s p l a c e m e n t s (+) of M I symmetry and l a t e r a l d i s p l a c e m e n t s (+) of M5 symmetry.

( c ) Domain w a l l a t o m i c d i s p l a c e m e n t s i n r e l a t i o n t o s u b l a t t i c e p e r i o d i c p o t e n t i a l , Va, and "pinched-dimer" s i t e o c c u p a t i o n p r o b a b i l i t y . p e r i o d i c i t y r e l a t e d t o Q* p r o d u c i n g a s t r u c t u r e s i m i l a r t o t h e " s t r i p e d ~ h a s e s "

observed i n o t h e r incommensurate s y s t e m s ( 8 ( F i g . 4 . ) .

I

,

^{- +}

-- - - -

^{I - D}

I h X + x C 6 X ^I

1

Fig. 4. (a) H:W < 0.5, w i t h a n t i p h a s e domain w a l l s , W.

(b) H:W > 0 . 5 , w i t h l i n e s of discommensuration, D.

C5-116 JOURNAL DE _PHYSIQUE

It has been deduced from t h e symmetry of t h e observed r e a t i v e i n t e n s i t i e s of t h e h a l f - o r d e r s p l i t s p o t s on s i n l e domain stepped s u r f a c e s i 9 , and LEED

8

k i n e m a t i c a l s t u d i e s ( 7 , t h a t Q i s p a r a l l e l t o U,,. ( F i g . 5.) The LEED s i m u l a t i o n s t u d i e s a l s o show t h a t a co-ordinated motion of t h e second l a y e r W atoms occurs which a f f e c t s t h e d i s t r i b u t i o n of t h e s p o t i n t e n s i t i e s .

For H:W >, 0.5 t h e s u r f a c e d i s o r d e r s i n one dimension, demonstrated by a l i m i t e d degree of h a l f - o r d e r s p o t s p l i t t i n g i n LEED r e s t r i c t e d t o < l o > d i r e c t i o n s . This s u g g e s t s t h e c r e a t i o n of l i n e s of discommensuration ( D i n Pig. 4.) p e r p e n d i c u l a r t o t h e d i r e c t i o n of t h e incommensuration wave v e c t o r Q*. As more hydrogen

goes down, more discommensurations between t h e w a l l s a r e produced, up t o H:W 0.8.

T h e r e a f t e r , a d d i t i o n a l H atoms a r e f o r c e d t o occupy s i t e s w i t h i n a n t i p h a s e domain r e g i o n s , thus d i s r u p t i n g t h e (J2 x ~ ' 2 ) ~ ~ ordered a r r a y of pinched l a t t i c e s i t e s , c r e a t i n g complex 2-D d i s o r d e r e f f e c t s I n LEED, i e two dimensional s t r e a k i n g of t h e h a l f - o r d e r LEED s p o t s o c c u r s away from t h e A symmetry l i n e (Fig. 1 . ) . The l o c a l W atom displacements now occur away from A and towards C symmetry d i r e c t i o n s , i e < l o > t o < I l > pinching.

Fig. 5 . h i r r o r p l a n e symmetry argument f o r Q* p a r a l l e l t o UIl

.

4. REELS DATA

The t i l t i n g of WHW u n i t s along A ( < l o > ) which occurs a t t h e o n s e t of t h e incomm- e n s u r a t e phase i s d e t e c t e d i n R E E L S ( ~ ; normally only one v i b r a t i o n i s seen i n t h e s p e c u l a r d i r e c t i o n ( u t h e symmetric s t r e t c h ) . When t i l t i n g o c c u r s , t h e

asymmetric s t r e t c h u a i Y ( i n t h e s u r f a c e plane) g a i n s a component p e r p e n d i c u l a r t o t h e s u r f a c e , and as r e d i c t e d by t h e d i p o l e s e l e c t i o n r u l e , i s then seen i n t h e s p e c u l a r

direction('^

(Fig. 6 . ) . Since Q* remains along t h e < l o >

symmetry d i r e c t i o n (A), only the us and u as v i b r a t i o n a l modes a r e contained i n t h e e l e c t r o n s c a t t e r i n g m i r r o r p l a n e .

I I

Fig. 6.

ENERGY LOSS (md)

I - ^__I -

Schematic REELS s p e c t r a f o r H:W(100) a s a f u n c t i o n of H coverage.

The arrow marks the peak corresponding t o uw.

At h i g h e r H c o v e r a g e s , when d i s o r d e r b e g i n s t o o c c u r , o u r e x p e r i m e n t a l r e s u l t s show t h a t a t h i r d v i b r a t i o n a l mode of t h e H:W(100) system, u

,

which i s a n o u t of p l a n e wag mode, a l s o b e g i n s t o a p p e a r i n t h e s p e c u l a r X i r e c t i o n . T h i s i s a c l e a r i n d i c a t i o n of a b r e a k i n g of t h e s u r f a c e m i r r o r p l a n e A symmetry i n v o l v i n g a l o c a l d i s t o r t i o n of t h e WHW u n i t s . T h i s o c c u r s a t a H coverage which t h e LEED measurements show t o be a s s o c i a t e d w i t h a breakdown i n 1-D d i s o r d e r i n g a l o n g

< l o > d i r e c t i o n s t o one i n v o l v i n g complex 2-D d i s o r d e r w i t h d i s p l a c e m e n t s a l o n g i l I > a s w e l l a s < l o > d i r e c t i o n s .

5. CONCLUSION

The REELS measurements demonstrate t h a t LOCAL d i s t o r t i o n s o f t h e t u n g s t e n s u r f a c e , due t o d i m e r i s a t i o n of W p a i r s , o c c u r which c o r r e l a t e w e l l w i t h t h e behaviour e x p e c t e d of a NON-LOCAL s o f t phonon mode mechanism f o r t h e H-induced s t r u c t u r a l 2 phase t r a n s f o r m a t i o n s r e p o r t e d by LEED o b s e r v a t i o n s . These l o c a l d i s t o r t i o n s a r e a consequence of p e r i o d i c l a t t i c e d i s t o r t i o n s which i n v o l v e W a t o m i c d i s p l a c e - ments b o t h w i t h i n and normal t o t h e s u r f a c e l a t t i c e p l a n e i n t h e incommensurate p h a s e . The incommensurate wave v e c t o r , Q*, i s a measure o? domain w a l l

p e r i o d i c i t i e s on a s c a l e n o t s m a l l e r t h a n 8 u n i t c e l l s p a c i n g s , a s i n d i c a t e d by s h a r p 4 t h o r d e r s p o t s p l i t t i n g o f t h e h a l f - o r d e r ( J 2 x 42) LEED beams.

The t r a n s f o r m a t i o n s i n t o t h e d i s o r d e r e d phases a p p e a r s t o b e c o n t r o l l e d by t h e d e n s i t y and b e h a v i o u r o f t h e s e domain w a l l s , d r i v e n by t h e need f o r t h e hydrogen t o occupy W2 pinched l a t t i c e s i t e s . The b e h a v i o u r would s u g g e s t some s i m i l a r i t y t o mechanisms i n v o l v i n g domain w a l l dynamics i n o t h e r incommensurate " o v e r l a y e r "

s y s t e m s ( 8 . However, t o what e x t e n t t h e s c a l i n g arguments used i n t h e s e systems apply t o r e c o n s t r u c t e d m e t a l s u r f a c e s h a s y e t t o be e s t a b l i s h e d .

Acknowledgments

The a u t h o r s wish t o thank t h e Royal S o c i e t y and SERC f o r f i n a n c i a l s u p p o r t i n t h i s work. EFJD w i s h e s t o thank t h e I s l e of Man Board of E d u c a t i o n f o r a s t u d e n t g r a n t .

REFERENCES

1 . Ho W , W i l l i s R F , and Plummer E W , Phys. Rev. L e t t e r s ,

40,

(1978) 1463.

2. Debe M K, and King D A, S u r f . S c i . ,

81,

(1979) 193.

3. W i l l i s R F, S u r f . S c i . ,

89,

(1979) 451.

4. Heine V , and McConnell J D C, Phys. Rev. L e t t e r s ,

g,

(1981) 1092.

5. F a s o l i n o A , S a n t o r o G , and T o s a t t i E , Phys. Rev. L e t t e r s ,

44,

(1980) 1684.

6 . Heine V , P a p e r p r e s e n t e d a t ' S o l i d S t a t e P h y s i c s C o n f e r e n c e ' , Bedford C o l l e g e , London, December 1982.

7. Hinch B J , F o s t e r M S , J e n n i n g s G , and W i l l i s R F, P r o c . ICSS V I , Warwick, UK, A p r i l 1983.

8. Kortan A R, E r b i l A , Birgeneau R J , and D r e s s e l h a u s M S, Phys. Rev. L e t t e r s , 49, (1982), 1427.

-

9. B a r k e r R A , and E s t r u p P J , J. Chem. Phys.,

74,

(1981) 1442.

10. Didham E F J , A l l i s o n W , and W i l l i s R F, S u r f . S c i . ,

126,

(1983) 219.