High oxygen pressures and recent developments in the field of high oxidation states

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High oxygen pressures and recent developments in the field of high oxidation states

Gérard Demazeau, Michel Pouchard, B. Buffat, Paul Hagenmuller

To cite this version:

Gérard Demazeau, Michel Pouchard, B. Buffat, Paul Hagenmuller. High oxygen pressures and recent

developments in the field of high oxidation states. Journal de Physique Colloques, 1984, 45 (C8),

pp.C8-345-C8-351. �10.1051/jphyscol:1984862�. �jpa-00224364�

JOURNAL DE PHYSIQUE

Colloque C8, supplément au n ° l l , Tome W, novembre 198* page C8-3W

HIGH OXYGEN PRESSURES AND RECENT DEVELOPMENTS IN THE FIELD OF HIGH OXIDATION STATES

G. Deraazeau, M. P o u c h a r d , B. B u f f a t and P . Hagenmuller

Laboratoire de Chimie du Solide du CNRS, Université de Bordeaux I, 351 Cours de la Libération, 33405 Talenee Cedex, France

Résumé - Les h a u t e s p r e s s i o n s d'oxygène s o n t t r è s i m p o r t a n t e s dans l a s t a b i l i s a t i o n de h a u t s d e g r é s d ' o x y d a t i o n des é l é m e n t s de t r a n s i t i o n . En u t i l i s a n t des f a c t e u r s s t r u c t u r a u x e t de l i a i s o n chimique c a r a c t é r i s a n t l ' e n v i r o n n e m e n t a n i o n i q u e des c o n f i g u r a t i o n s é l e c t r o n i q u e s i n u s u e l l e s peuvent ê t r e o b t e n u e s . Le f e r (V) en s i t e OH d ' u n e p a r t e t l e f e r (IV) à s p i n f o r t ou l e c o b a l t ( I I I ) de s p i n i n t e r m é d i a i r e en s i t e D4H o n t é t é c h o i s i s pour i l l u s t r e r c e s d i v e r s e s p o s s i b i l i t é s .

Abstract - High oxygen pressures are very suitable for the preparation of high oxidation states of transition elements.

Using, in addition, appropriate structural and chemical

bonding factors characterizing the anionic surrounding unusual electronic configurations can be obtained. The stabilization of Fe (V) in an OH site and high spin Fe (IV) or intermediate spin Co (III) in a D4H symmetry have been selected to illustrate these possibilities.

During the last thirty years great progress has been obtained in the stabilization of unusual oxidation states of transition elements using high oxygen pressures.

Three main methods can be developped to generate high oxygen pressures:

- compressed oxygen gaz (1),

- oxydizing solutions such as diluted solutions of NH.CIO, in an hydrothermal device (I),

- the thermal decomposition in solid state of unstable oxides such as Cr0

(2) or KC10, (3) in a "belt" apparatus.

The oxidation state of an ion of a transition metal element is a formal number reflecting the average num- ber of d electrons involved in chemical bonding. Its stabili- zation closely depends on that of the corresponding electronic configuration, in particular on (i) structural factors (size and symmetry of the cationic site), (ii) nature of the catio- nic neighbours (covalence and influence of the competing bonds).

Using a simple model based both on the Tanabe-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984862

C8-346 JOURNAL DE PHYSIQUE

S u g a n o ' s d i a g r a m ( 1 ) and o n t h e K r i s h n a m r t h y and S c h a a p ' s c a l c u l a t i o n o f t h e e n e r g y of d o r b i t a l s i n a D q h symmetry (51, w e h a v e u n d e r l i n e d t h e i n f l u e n c e of t h e s t r u c t u r a l e l o n g a t i o n of t h e M 0 6 o c t a h e d r o n o n t h e s t a b i l i z a t i o n o f e l e c t r o n i c c o n - f i g u r a t i o n s ( 6 ) .

I - STABILIZATION OF A VERY H I G H OXIDATION STATE: SIX- COORDINATED FetV I N AN OXYGEN-LATTICE

Fe+V was unknown i n o c t a h e d r a l c o o r d i n a t i o n . I t s i s o t r o p i c e l e c t r o n i c c o n f i g u r a t i o n (ts

f i t s v e r y w e l l w i t h a p u r e o c t a h e d r a l s i t e (Oh s y m m e t r y ) . To s t a b i l i z e Fe+V 2g i t i s n e c e s s a r y , i n a d d i t i o n o f t h e u s e o f h i g h oxygen p r e s - s u r e s , t o s e l e c t s t r u c t u r a l a n d c h e m i c a l b o n d i n g f a c t o r s a b l e t o f a v o r i z e : ( i ) a n Oh s i t e , (ii) t h e c o v a l e n c y o f t h e Fe-O b o n d s .

With t h i s s e l e c t e d s t o i c h i o m e t r y , La2LiFe06, h a s b e e n p r e p a r e d u n d e r h i g h oxygen p r e s s u r e ( 7 0 k b a r ) . Chemi- c a l and p h y s i c a l c h a r a c t e r i z a t i o n s c o n f i r m t h e s t a b i l i z a t i o n

- f o r t h e f i r s t t i m e - o f p u r e s i x c o o r d i n a t e d F e w i n a n o x i - d e ( T a b l e I ) ( 7 ) .

2 - STABILIZATION OF AN UNUSUAL ELECTRONIC CONFIGURATION:

H I G H SPIN Fe+IV I N AN OCTAHEDRAL S I T E

Owing t o t h e e v a l u a t e d Dq/B v a l u e f o r s i x - c o o r - d i n a t e d Fe+IV i n a n oxygen l a t t i c e (Dq/B

4 ) , i n t h e Fe+IV o x i d e s s o f a r p r e p a r e d - m o s t l y w i t h t h e p e r o v s k i t e s t r u c t u r e -

Fe+IV seems t o have a low s p i n c o n f i g u r a t i o n

S r F e 0 3 ) ( 8 - 1 1 ) r e l a t e d t o some e l e c t r o n i c d e l o c a l i z a t i o n (CaFeOj) (1-12-15).

U s i n g t h e p r e v i o u s model, a s t r o n g e l o n g a t i o n of t h e Fe06 o c t a h e d r o n seems t o b e a p p r o p r i a t e t o s t a b i l i z e t h e h i g h s p i n s t a t e ( t 3 e 1 ) ( F i g . 1 ) .

2g g

C h o o s i n g a R2NiF4 l a y e r s t r u c t u r e s t a c k i n g o f

r o c k s a l t and p e r o v s k i t e l a y e r ( F i g . 2 ) and f o u r weak L i - O b o n d s

a s c o m p e t i n g bonds i n t h e p e r o v s k i t e l a y e r , s u c h a d i s t o r t i o n

c a n b e i n d u c e d . The c o r r e s p o n d i n g o x i d e s

A O . 5Lal 5 L i 0 . 5Fe (IV) ^{( A} = Ca, S r , B a ) have b e e n s e l e c t e d and p r e p a r e d u n d e r oxygen p r e s s u r e .

C h e m i c a l and p h y s i c a l c h a r a c t e r i z a t i o n s a r e i n a g r e e m e n t w i t h Fe+IV w i t h f o u r u n p a i r e d e l e c t r o n s ( h i g h s p i n s t a t e )

T a b l e I

T a b l e I1

isom shift 300 K (rnssbauer)

mn. s-l

q x d m p o l e s p l i t t i n g

mn.s -1

0 oxidation

state

I

g value (R P E)

2,0135 1 mlar W i e

Constant

I

quadruplle s p l i t t i n g

mn. _s ^-1 isomer shdft

300 K (rnssbauer)

mn. s -1 oxidation

state o b e r v e d

values

t h e o r e k i c a l , 7 5

Curie Constant

0 5 , 0 2

probably 1 , 9 3

2 , 0 1 3 1 values

- 0 , 4 1

unknm, but

3.06

1

ObSenr*

values low spin Fe (W

- 0.19

0.0 (in SrFe03) very negati-

ve value

high spin Fe (rV)

3.97 112

smll

3

Lmkmwn ht probably im-

lx-~kmwn p r t a n t owing

to t h e Jehn

Teller d i s t a r -

t i o n

JOURNAL DE PHYSIQUE

F i g . 1 - Ground s t a t e r e g i o n s f o r a d 4 c o n f i g u r a t i o n

F i g . 2 - The K2NiF4 s t r u c t u r e

3 - STABILIZATION OF AN INTERMEDIATE ELECTRONIC CONFIGURATION

DURING A SPIN TRANSITION: "MEDIUM SPIN" CO+III

I n a n oxygen l a t t i c e and i n a n o c t a h e d r a l s i t e

1 5

a low s p i n ( A ) - h i g h s p i n ( T ) t r a n s i t i o n o c c u r s w i t h i n -

1 9 2 g

c r e a s i n g t e m p e r a t u r e f o r C o + I I I (Dq/B v a l u e b e i n g c l o s e to t h e c r i t i c a l Dq/B v a l u e ) . Such a phenomenon h a s been o b s e r v e d i n t h e LnCo03 p e r o v s k i t e s (Ln = r a r e - e a r t h ) p r e p a r e d u n d e r oxygen p r e s s u r e ( 1 5 ) ( F i g . 3 ) .

a i u o 410 600 BLX, 1

*

.

F i g . 3 - V a r i a t i o n w i t h t e m p e r a t u r e of t h e

r e c i p r o c a l molar magnetic s u s c e p t i b i l i t y

f o r YCo03 and LuCo03 p h a s e s

U s i n g t h e s i m p l e m d e l d e s c r i b e d e a r l i e r ( F i g . 4 1 ,

1 1 0

a n i n t e r m e d i a t e e l e c t r o n i c c o n f i g u r a t i o n ( d 2 d2zPxydz2dx2-y2

Y Z

S = l ) c a n b e p r e d i c t e d f o r C o + I I I i n a n e l o n g a t e d o c t a h e d r o n ( 0 > 1 , 1 5 ) .

Such a n u n u s u a l c o n f i g u r a t i o n h a s b e e n d e t e c t e d a s i n t e r m e d i a t e s t a t e d u r i n g t h e 'A 5

1g + 5 ~ g ( TZg) t r a n s i t i o n i n t h e two o x i d e s p r e p a r e d u n d e r oxygen p r e s s u r e :

L a 2 L i 0 . 5 C o ( I I I ) 0 . 5 0 4 a n d L a 2 L i 0 . 5 C u ( I I I ) 0 ~ q 5 C o ( I I I ) O ~ 0 5 0 4 (1 6) ( F i g . 5) ( t h e i n c r e a s i n g e l o n g a t i o n of t h e Coo6 o c t a h e d r o n f a v o r i z i n g t h e s t a b i l i t y o f t h i s c o n f i g u r a t i o n w i t h t h e 3

Bag t e r m ) .

F i g . 4 - Ground s t a t e r e g i o n s f o r a d 6 c o n f i g u r a t i o n

F i g . 5 - Comparison o f t h e popu- l a t i o n s f o r t h e d i f f e r e n t t e r m s i n La2Li 0 . 5 ~ ~ 0 . 5 ~ 4 and

La2Li0. 5CU0. 4oC00. 10'4

CONCLUSIONS

High oxygen p r e s s u r e s c a n h e l p t h e s y n t h e s i s of

o x i d e s c o n t a i n i n g u n u s u a l o x i d a t i o n s t a t e s o r e l e c t r o n i c c o n f i -

g u r a t i o n s of t r a n s i t i o n e l e m e n t s . Such m a t e r i a l s l e a d t o a b e t -

t e y u n d e r s t a n d i n g o f t h e c h e m i c a l bond.

JOURNAL DE PHYSIQUE

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