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PRESSURE INDUCED ORBITAL GROUND STATE INVERSION IN Fe(H2O)6, SiF6
U. Volland
To cite this version:
U. Volland. PRESSURE INDUCED ORBITAL GROUND STATE INVERSION IN Fe(H2O)6, SiF6.
Journal de Physique Colloques, 1980, 41 (C1), pp.C1-309-C1-310. �10.1051/jphyscol:19801112�. �jpa-
00219594�
JOURNAL DE PHYSIQUE Colloque
C1,
supplt!ment au n O 1 , Tome41,
janvier1980,
pageCl-309
PRESSURE INDUCED ORBITAL GROUND STATE INVERSION IN F ~ ( H ~ o ) 6 w SiF6
U. Volland
PhysikaZisches I n s t i t u t der Universitiit ErZangen-Nttmberg GZUekstraBe 10, 8520 ErZmgen, W. Germany
1
.
I n t r o d u c t i o nOver t h e p a s t y e a r s much work h a s b e e n d o n e i n i n v e s t i g a t i o n s o f p r e s s u r e induced e l e c - t r o n i c changes i n i r o n compounds by means of Mdssbauer r e s o n a n c e measurements. Concer- n i n g o n l y h i g h s p i n i o n i c ferrouscompounds, t h e e f f e c t o f p r e s s u r e t o 200kbar on t h e Mossbauer r e s o n a n c e i n a s e r i e s o f compounds was s t u d i e d 3 , 2 ] . I n g e n e r a l o n l y s m a l l e f f e c t s were o b s e r v e d . The isomer s h i f t s l i g h t l y d e c r e a s e s w i t h i n c r e a s i n g p r e s s u r e and u s u a l l y t h e q u a d r u p o l e s p l i t t i n g i n c r e a - s e s w i t h p r e s s u r e .
T h i s work i s
a
s t u d y of t h e e f f e c t of p r e s s u r e on t h e Mossbauer r e s o n a n c e o f f e r r o u s f l u o s i l i c a t e h e x a h y d r a t e . To t h e a u t h o r ' s knowledge a new t y p e of p r e s s u r e e f f e c t i n f e r r o u s i o n i c compounds i s r e p o r - t e d , which seems t o b e due t o a n o r b i t a l l e v e l r e v e r s i o n c a u s e d by a change o f t h e c r y s t a l f i e l d .2 . Experimental and R e s u l t s
The p r e s s u r e c e l l used c o n s i s t s o f a s t e e l c y l i n d e r and a screw b o t h w i t h l o n g i t u d i n a l h o l e s f o r t r a n s m i s s i o n of t h e r - r a y s . The a b s o r b e r samples, k e p t between two boron- c a r b i d e p l a t e s (B4C), were embedded i n t a l c powder, which h a s good homogeneity i n t r a n s - m i t t i n g p r e s s u r e . The a p p l i e d p r e s s u r e c a n r o u g h l y be e s t i m a t e d by means of t h e t o r q u e t o t h e screw w i t h which t h e p r e s s u r e i s g e n e r a t e d . S i n g l e c r y s t a l s andpowder samples were measured a t room t e m p e r a t u r e .
The s i n g l e c r y s t a l s p e c t r a o b t a i n e d a t r o o m t e m p e r a t u r e a r e shown i n f i g . 1 . The&-direc- t i o n was chosen p e r p e n t i c u l a r t o t h e t h r e e - f o l d c r y s t a l a x i s which n e a r l y c o i n c i d e s w i t h t h e p r i n c i p a l a x i s V Z Z o f t h e E F G . T h e two s p e c t r a i n t h e middle p a r t o f f i g . 1 were r e c o r d e d a t a p r e s s u r e of 7kbar and
p-7 kbar
F i g . 1 : S p e c t r a o f a Fe(B20)6.SiF6 s i n g l e c r y s t a l w i t h o u t and w i t h p r e s s u r e
(8LvZz .,
T=295K). From t o p t o bottom: a t athmosphe- r i c p r e s s u r e (p=O) w i t h p r e s s u r e p a 7 k b a r ; p a l 5kbar; a f t e r r e l e a s i n g p r e s s u r e (p=O)
.
I S k b a r , r e s p e c t i v e l y . Each o f t h e s p e c t r a c o n s i s t s of two w e l l r e s o l v e d d o u b l e t s . T h e p r e s s u r e induced i n n e r d o u b l e t was n e a r l y t h e same isomer s h i f t a s t h e o u t e r d o u b l e t b u t a q u a d r u p o l e s p l i t t i n g which i s r e - duced by more t h a n a f a c t o r o f t w o i n m a g n i - t u d e . The s p l i t t i n g s and isomer s h i f t s a r e m o s t l y p r e s s u r e i n d e p e n d e n t , whereas t h e
i n t e n s i t y r a t i o between t h e i n n e r a n d o u t e r d o u b l e t i n c r e a s e s w i t h p r e s s u r e . A f t e r r e - l e a s i n g p r e s s u r e t h e spectrum c o n s i s t s a g a i n of o n l y o n e d o u b l e t h a v i n g t h e same isomer s h i f t and q u a d r u p o l e s p l i t t i n g as i n t h e u n p r e s s e d c a s e . From comparison of
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801112
C1-310 JOURNAL DE PHYSIQUE
t h e s p e c t r a on t h e t o p and bottom o f f i g . 1 it c a n e a s i l y b e s e e n t h a t t h e i n t e n s i t y r a t i o b e f o r e and a f t e r a p p l y i n g p r e s s u r e h a s d r a s t i c a l l y changed i n magnitude. The same e f f e c t i s a l s o o b s e r v e d f o r powder samples and i s p r o b a b l y due t o t e x t u r e . The Mdssbauer p a r a m e t e r s o b t a i n e d from t h e
I
l e a s t s q u a r e s f i t s a r e summarized i n t a b l e l . T a b l e l : ~ u a d r u p o l e s p l i t t i n g s oEQ and i s o - mer s h i f t s
d
( a g a i n s t m e t a l l i c i r o n ) i n mm/sec o f F e ( H 2 0 ) 6 ' S i F 6 . ( E r r o r s s m a l l e r t h a n 0.005 mm/sec).D i s c u s s i o n
. I n t h e p r e s s u r e induced new phase a d r a s t i c change o f t h e c r y s t a l f i e l d m u s t h a v e o c c u r r e d b e c a u s e o f t h e s t r o n g r e d u c t i o n o f t h e q u a - d r u p o l e s p l i t t i n g . The narrow l i n e s and t h e p r e s s u r e independence o f t h e l i n e p o s i t i o n s a r e a h i n t t o twowelldefinedcrystalfield s i t u a t i o n s c o r r e s p o n d i n g t o t h e p r e s s e d and u n p r e s s e d phase.
T h i s p r e s s u r e induced phase t r a n s i t i o n i s v e r y s i m i l a r t o t h e t e m p e r a t u r e d e p e n d e n t phase t r a n s i t i o n i n t h e c a s e o f Fe ( H 2 0 ) 6 - (C104) which was f i r s t o b s e r v e d by Dgzsi e t a1.2 / 3 / . F o r t h i s compound t h e q u a d r u p o l e s p l i t t i n g EQ amounts t o 3.4mm/sec below 223OK, whereas above 263OK o n l y a v a l u e of 1.4mm/sec was measured. Furthermore w i t h i n a t e m p e r a t u r e r a n g e o f a b o u t 3 0 K b o t h p h a s e s c o e x i s t .
The l i g a n d f i e l d s i t u a t i o n s a r e q u i t e s i m i - l a r i n b o t h compounds. The i r o n ( I 1 ) i o n i s s u r r o u n d e d by a w a t e r o c t a h e d r o n , which i s mainly d i s t o r t e d a l o n g i t s t r i g o n a l a x i s .
I n t h e c a s e of F e ( H 2 0 ) 6 - ( C 1 0 4 ) 2 t h i s was v e r i f i e d by d e t e r m i n i n g t h e s i g n s o f t h e m a i n components of t h e EFGs o f b o t h p h a s e s / 4 / . From t h e s i g n s o f t h e EFG and t h e m a g n i t u d e s of t h e q u a d r u p o l e s p l i t t i n g s it was deduced
t h a t a t low t e m p e r a t u r e s t h e o r b i t a l g r o u n d - s t a t e i s a s i n g l e t (VZZ( 0 ) whereas a t h i g h t e m p e r a t u r e s t h e low q u a d r u p o l e s p l i t t i n g i s due t o an o r b i t a l d o u b l e t ground s t a t e
( V z k 0 ) .
I t i s known t h a t a t r o o m t e m p e r a t u r e f e r r o u s f l u o s i l i c a t e h e x a h y d r a t e h a s n e a r l y t r i g o - n a l symmetry, l e a d i n g t o a n o r b i t a l s i n g l e t ground s t a t e . Because of t h e same symmetry o f t h e i r o n - w a t e r o c t a h e d r o n and t h e co- e x i s t e n c e o f b o t h p h a s e s t he p r e s s u r e e f f e c t c a n b e i n t e r p r e t e d i n a q u i t e a n a l o g o u s manner o n l y by exchanging " t e m p e r a t u r e " by
" p r e s s u r e " i n g o i n g from Fe (H20) (C104) t o Fe (H20) ' S i F 6 .
b e f o r e ( P = 0 )
p s 7kbar p z l 5 k b a r a f t e r
( p = 0 )
Acknowledgement
The a u t h o r acknowledges t h e f i n a n c i a 1 , s u p - p o r t from theDeutscheForschungsgemein- s c h a f t .
R e f e r e n c e s
/ I / A.R. Champion, R.W. Vaughan, and
" p r e s s u r e p h a s e "
H.G. Drickamer, J. Chem.Phys.47, 2583 (1967)
A.R. Champion and H.G. Drickamer J. Chem. Phys.
47,
2591 (1967)/ 2 / R. I n g a l l s , C . J . Coston,G. D e P a s q u a l i and H.G. Drickamer, J. Chem.Phys.
45,
1057 (1966)
/ 3 / I . Dgzsi and L. K e s z t h e l y i , S o l i d s t a t e Commun.
4,
511 (1966)J . M . D . Coey, 1 . D 6 z ~ i ~ P . M . Thomas and P.J. Ouseph, P h y s - L e t t .
G I
125 (1972) P.J. Ouseph, P.M. Thomas and I. Dgzsi J. Phys. Chem. S o l i d s35,
604 (19741, / 4 / W.M. R e i f f , R.B. F r a n k e l , C.R. AbeledoChem.Phys.Lett.
2,
124 (1973)4 EQ
-
1.660 1.715
-
"normal phase"
d
-
1.250 1.254
-
6EQ 3.403 3.458 3.460 3.404
d
1.271 1.265 1.271 1.273
