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PHASE TRANSITIONS IN Si, Ge AND Sn UNDER PRESSURE
H. Olijnyk, W. Holzapfel
To cite this version:
H. Olijnyk, W. Holzapfel. PHASE TRANSITIONS IN Si, Ge AND Sn UNDER PRESSURE. Journal
de Physique Colloques, 1984, 45 (C8), pp.C8-153-C8-156. �10.1051/jphyscol:1984828�. �jpa-00224328�
JOURNAL DE PHYSIQUE
Colloque C 8 , supplement au n ° l l , Tome * 5 , novembre 198* page C8-153
PHASE TRANSITIONS IN S i / Ge AND Sn UNDER PRESSURE
H. O l i j n y k and W.B. H o l z a p f e l
Faehheveich physik, Vnivevsitdt-GH-Paderborn, D-4790 Paderborn, F.R.G.
A b s t r a c t - S i , Ge and Sn have been s t u d i e d by energy d i s p e r s i v e X-ray d i f f r a c t i o n up to 50 GPa. Si shows f i r s t the w e l l known t r a n s i t i o n from diamond to B - t i n s t r u c t u r e around 11 GPa. I t transforms t o a p r i m i t i v e hexa- gonal s t r u c t u r e ( S i - V ) around 15 GPa, to an intermediate phase S i - V I around 35 GPa and f i n a l l y t o hep ( S i - V I I ) around 40 GPa. Ge stays s t a b l e i n the B - t i n s t r u c t u r e up to 54 GPa. The w e l l known b e t high pressure m o d i f i c a t i o n o f Sn a t f i r s t shows a continuous increase i n c/a from 0.91 to 0.94 and around 35 GPa a discontinuous t r a n s i t i o n t o bee.
I t i s w e l l known t h a t the group IV B elements show a change from openly packed covalent t o denser packed m e t a l l i c s t r u c t u r e s w i t h i n c r e a s i n g atomic number: carbon c r y s t a l l i z e s i n the hexagonal layered s t r u c t u r e o f g r a p h i t e w i t h c o o r d i n a t i o n number (CN) 3,as well as i n the metastable cubic diamond (cd) s t r u c t u r e (CN=4), which i s also the s t a b l e form f o r Si and Ge a t ambient pressure. The cd phase o f Sn(a-Sn) transforms to a denser m e t a l l i c phase ( B - t i n s t r u c t u r e ) w i t h 6 nearest neighbours j u s t below room temperature a t normal p r e s s u r e , w h i l e Pb i s known to c r y s t a l l i z e i n the cubic closed packed ( f e e ) s t r u c t u r e .
This tendency to higher coordinated s t r u c t u r e s has also been observed f o r the i n d i - vidual elements under compression. I t was shown by Jamieson / l / t h a t Si and Ge transform t o the m e t a l l i c B - t i n s t r u c t u r e above 10 GPa and Sn i s found i n a body centered tetragonal s t r u c t u r e ( c / a = 0 . 9 1 , CN = 8) above 9.5 GPa / 2 / .
We have extended the s t r u c t u r a l i n v e s t i g a t i o n s of the l a t t e r three elements i n t o the 50 GPa regime by energy d i s p e r s i v e X-ray d i f f r a c t i o n . A conventional X-ray source w i t h a conical s l i t system / 3 / was used and i n the case o f Sn and Si
a d d i t i o n a l work was done i n HASYLAB w i t h synchrotron r a d i a t i o n /A/. The pressure was determined by the ruby fluorescence method w i t h d X/d p = 3.65 GPa/n / 5 / . The 4 : 1 methanol : ethanol m i x t u r e was used as pressure t r a n s m i t t i n g medium.
Si i s s t a b l e i n the B - t i n phase only over a small pressure range and transforms t o a p r i m i t i v e hexagonal (ph) c e l l (CN=8), S i - V , around 15 GPa / 6 , 7 / . Two f u r t h e r phase t r a n s i t i o n s to S i - V I and S i - V I I occur around 35 GPa and 40 GPa, r e s p e c t i v e l y / 6 / . Whf?e S i - V I I c r y s t a l l i z e s i n the hep s t r u c t u r e (CN=12) w i t h c/a = 1.69, we were not able t o a r r i v e a t a d e f i n i t e s t r u c t u r a l assignment f o r S i - V I , because the d i f f r a c t i o n patterns between 35 GPa and 40 GPa obviously c o n s i s t o f a phase mixture of S i - V , S i - V I and S i - V I I .
The high pressure behaviour of Ge i s much s i m p l e r . Ge remains i n the e - t i n s t r u c t u r e up t o the h i g h e s t pressure o f 54 GPa.
Résumé - Le silicium, le germanium et l'étain ont été étudiés par rayons X en dispersion d'énergie jusqu'à 50 GPa. Si, le silicium passe de la structure diamant à S -Sn vers 11 GPa, puis à une structure hexagonale primitive (Si-V) vers 15 GPa, à une phase intermédiaire (Si-VI) à 35 GPa et enfin à hexagonal compact (Si-VII) autour de 40 GPa. Le germanium reste stable dans la structu- re g -Sn jusqu'à 54 GPa. La structure tétragonale centrée (bct) bien connue de l'étain, montre d'abord une augmentation continue du rapport c/a de 0,91 à
0,94, puis, vers 35 GPa, une transition brusque vers une structure cubique centrée.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984828
C8-154 JOURNAL DE PHYSIQUE
Sn shows under compression i n i t s h i g h p r e s s u r e b c t phase a t f i r s t a continuous approach towards a bcc s t r u c t u r e as shown i n f i g . l a b y the i n c r e a s e o f c / a from 0.91 t o 0 . 9 5 between 10 and 50 GPa. Howevernew l i n e s develop i n between t h e b c t d o u b l e t s ( f i g . 2 ) above 35 GPa, and w i t h f u r t h e r compression t h e b c t peaks l o s e on i n t e n s i t y . A bcc i n d e x i n g f i t s a l l t h e new l i n e s . Around 56 GPa t h e predominant l i n e s correspond t o t h e bcc phase, b u t some r e m i n i s c e n t b c t l i n e s i n d i c a t e t h a t
F i g . 1
-
Pressure dependence o f a ) t h e c / a r a t i o o f Sn-111 ( b c t ) b ) t h e l a t t i c e parameters o f Sn-111( b c t ) and Sn-IV ( b c c )
c ) t h e reduced volume v / v o Energy
( k e ~ )
F i g . 2
-
X-ray d i f f r a c t i o n p a t t e r n s f o r Sn a t v a r i o u s pressures.The t h i c k l i n e s p o i n t t o d i f f r a c t i o n peaks o f t h e b c c phase w h i l e t h i n n e r r e f e r t o peaks o f t h e b c t phase.
g denotes g a s k e t r e f l e c t i o n s .
even a t t h i s p r e s s u r e t h e t r a n s i t i o n t o bcc has n o t been completed. The p r e s s u r e dependence o f t h e l a t t i c e parameters o f b c t and bcc Sn ( f i g . l b ) c l e a r l y shows t h e f i r s t o r d e r n a t u r e o f t h i s t r a n s i t i o n . F i g . l c reproduces t h e p r e s s u r e dependence o f t h e reduced volume v/vo.
Table 1 compares t h e e x p e r i m e n t a l values f o r t h e t r a n s i t i o n p r e s s u r e s and volumes a t room temperature w i t h t h e o r e t i c a l values f o r 0 K. F o r S i and Ge, t h e agreement between e x p e r i m e n t and t h e o r y i s v e r y good, w h i l e f o r Sn t h e r e i s q u a l i t a t i v e agreement i n t h e l o w e r compression range and an hcp phase comes o u t t o be more s t a b l e a t O K as compared t o t h e observed bcc phase a t room temperature. However, a bcc phase i s commonly observed a t h i g h temperatures and, t h e r e f o r e , we c a n n o t ex- c l u d e t h e e x i s t e n c e o f a hcp phase f o r Sn a t l o w e r temperatures.
A ) REF. 8 B) REF. 9
C ) REF. 10 D) R E F . 11
E ) E S T I M A T E D FROM THE E X P E R I M E N T A L ROOM TEMPERATURE EOS
Table 1 - Comparison o f e x p e r i m e n t a l values f o r t r a n s i t i o n p r e s s u r e s and volumes a t room t e m p e r a t u r e w i t h t h e o r e t i c a l 0 K v a l u e s . The e x p e r i m e n t a l values a r e expressed i n p a r e n t h e s i s .
F o r S i t h e s t r u c t u r a l sequence r e s u l t s i n a s y s t e m a t i c i n c r e a s e i n CN f r o m 4 t o 12 which corresponds t o a c l o s e r p a c k i n g o f t h e atoms and a weakening o f t h e
d i r e c t i o n a l c o v a l e n t bonds. One can assume t h e r e f o r e t h a t S i u l t i m a t e l y has t u r n e d i n t o a s i m p l e metal i n t h e hcp phase. The extended range o f s t a b i l i t y f o r t h e B - t i n phase i n Ge shows on t h e o t h e r hand t h a t one has t o be c a r e f u l w i t h s i m p l e a n a l o g i e s between l i g h t e r and h e a v i e r elements i n one group o f t h e p e r i o d i c t a b l e . A p p a r e n t l y t h e i n n e r d - s h e l l s i n Ge and Sn l e a d t o d i f f e r e n t b e h a v i o u r i n comparison w i t h S i /8,9/.
s t i m u l a t i n g d i s c u s s i o n s w i t h Dr. K. Syassen a r e g r a t e f u l l y acknowledged.
P a r t o f t h i s work was performed a t HASYLAB and s u p p o r t e d by t h e B u n d e s m i n i s t e r i u m F u r Forschung und Technologie.
JOURNAL DE PHYSIQUE
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