ASYMMETRICAL QUADRUPOLE SPECTRA OF Fe(H2O)2+6 IN CRYSTALS AND FROZEN AQUEOUS SOLUTIONS

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ASYMMETRICAL QUADRUPOLE SPECTRA OF Fe(H2O)2+6 IN CRYSTALS AND FROZEN

AQUEOUS SOLUTIONS

A. Cérèze, F. Varret

To cite this version:

A. Cérèze, F. Varret. ASYMMETRICAL QUADRUPOLE SPECTRA OF Fe(H2O)2+6 IN CRYS-

TALS AND FROZEN AQUEOUS SOLUTIONS. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-

229-C1-230. �10.1051/jphyscol:1980174�. �jpa-00219743�

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JOURNAL DE PHYSIQUE Colloque C l , supplkment au n ^O1, Tome 41, janvier 1980, page C1-229

A. C6reze and F. Varret

FacuZte' des Sciences, Laboratoire de Spectrome'trie Massbauer, C.N.R.S., Route de LavaZ, Ze Mans, 7201 7 LE MANS-CEDEX.

I. Introduction.- The asymmetry of the quadrupolar spectrum of Fe2+ in some compounds i s a subject of recent i n t e r e s t . The thermal variation of the asymmetry i s an essential information t o understand the mechanism responsible f o r such a phenomenon : t h e observation of a symmetrical spectrum a t low temperature r u l e s out any s t a t i c explanation. In t h a t case ( t h a t of Fe(H20)? in several f l u o s i l i c a t e s / I / ) a dynamical explanation such a s a s5ow elec- t r o n i c relaxation (spi n - l a t t i c e ) i s favoured.

In order t o investigate the influence of t h e l a t t i c e on the F~(H,o);+ spectra, we prepared aqueous solutions of ferrous f l u o s i l i c a t e (70 mg f l u o s i l i c a t e , 370 mg H20, deposed on blotting-paper of n cm2) ; the thermal treatment was the following:

( i ) samples ( i ) : f a s t quenching i n t o l i q u i d nitvo- gen, giving F ~ ( H ~ O ) ? diluted in glassy host.

( i i ) samples ( i i ) : aging a t 240 K f o r several hours ( t y p i c a l l y 6 hours) of samples ( i ) , giving Fe(H20);' diluted in a s t a b l e microcryrtalline host.

( i ) and ( i i ) are thought to have e u t e c t i c compo- s i tion /2/..

( i i i ) samples ( i i i ) : slow cooling of t h e solution giving a polycrystalline mixture of ferrous f l u o s i - 1 ic a t e , e u t e c t i c and ice.

( i v ) a s a reference, we report the data of c r y s t a l - l i n e ferrous f l u o s i l i c a t e (sample ( i v ) ) .

To t e s t the s e n s i t i v i t y of the quadrupole spectra t o the local environment of Fe2+, we a1 so prepared samples of ferrous f l u o s i l i c a t e , obtained by f a s t evaporation of the water solution (under

vacuum) : Asymmetrical spectra were obtained a t any temperature, requiring a spread of the isomer s h i f t indicating a departure of t h e local environment ^, from the hexaaqueous coordination Fe(H20)6 2+

.

^On^the

contrary, a l l the reported samples gave a symmetrical spectrum a t low temperature, indicating t h a t the local environment of Fe2+ remains hexaaqueous F ~ ( H , O ) ? .

2. Results and discussion (Figs. 1,2).

Sample ( i ) (glassy host) : symmetrical spec- t r a a t any temperature, sharp 1 ines (0.25-0.29mm/s).

The thermal variation of the quadrupole s p l i t t i n g compares r a t h e r well with t h a t of the pure ferrous f l u o s i l i c a t e ( i v ) , by including a s h i f t of about 0.2 mm/s which might r e s u l t from the removal of the l a t t i c e term. Therefore, the local symmetry around

~ e i s l i k e l y t o be the same ~ + b u t hight f i e l d expe- riments are needed t o check this point (see /2/).

When the quench,ing was hot f a s t enough, asymmetrical spectra were observed a s i n the case of crys- t a l l i n e samples ( ( i i ) t o ( i v ) ) . The magnitude of the asymmetry was sometimes very large but hardly reproducible, therefore, we focused our i n t e r e s t on the thermally s t a b l e phase ( i i ).

Sample ( i i ) (microcrystal 1 ine e u t e c t i c ) gave asjmmetrical spectra ( d i f f e r e n t widths but equal in- tensi t i e s ) ; the magnitude o t the asymmetry i s a t l e a s t four times l a r g e r than t h a t of pure fluo- s i l i c a t e .

The jump in AT a t about 190 K i s a t t r i b u t e d to a

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980174

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(21-230 ^JOURNAL^DE^PHYSIQUE

phase t r a n s i t i o n of t h e c r y s t a l l i n e host. The I " " " " ' " '

v e r y queer thermal v a r i a t i o n o f A E i s n o t i n t e r p r e - t e d a t t h e moment.1n t h e range 20-80 K, t h e decrease

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seems t o be c o r r e l a t e d t o t h e i n c r e a s e o f A r . The jump i n A E a t about 220 K m i g h t be due t o an i r r e v e r s i b l e rearrangment, o b t a i n e d a f t e r aging f o r

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a l o n g t i m e ( a b o u t one day).

Sample ( i i i ) a p r i o r i c o n t a i n s fe2' i n b o t h f l u o s i l i c a t e and e l t e c t i c . The d a t a agree w i t h what i s expected from a m i x t u r e : t h e A r curve e x h i b i t s t h e jump a t 225 K t y p i c a l o f t h e f l u o s i l i c a t e l a t - t i c e as w e l l as a second jump near by 190 K, which i s a t t r i b u t e d t o t h e a i s t o r s i o n o f the e u t e c t i c l a t t i c e r e p o r t e d ab0ve,The A E

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c u r v e shows a depar- t u r e from t h e f l u o s i l i c a t e c u r v e below 225 K. This should be r e l a t e d t o t h e phase change i n t h e f l u o - s i l i c a t e c r y s t a l s c o n s t r a i n e d by t h e e u t e c t i c c r y s t a l s .

We a l s o observed t h a t t h e presence o f Fe3' i m p u r i t i e s i n c r e a s e d ..Ar i n a1 1 samples, excepted t h e g l a s s y ones. Other i m p u r i t i e s , such as zn2' had been r e p o r t e d t o a c t s i m i l a r l y /I/. In a d d i t i o n , we n o t i c e d t h a t t h e c o o l i n g t i m e o f f e r r o u s f l u o s i l i c a - t e , which governs t h e s i z e o f t h e l o w symmetry do- mains, i n f l u e n c e s t h e A r values (quenching t h e sam-

p l e s increased ~ r ) .

F i g . 2 : Thermal v a r i a t i o n o f t h e quadrupole

' s p l i t t i n g AEQ.

3. Conclusion.

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The r e p o r t e d f r o z e n s o l u t i o n s rea- l i z e f u r t h e r examples o f asymmetrical quadrupolar s p e c t r a of Fe2' : v a r i o u s s i t u a t i o n s have been observed which suggest t h a t t h e a c t u a l s t a t e o f t h e c r y s t a l l i n e l a t t i c e would e s s e n t i a l l y govern t h e

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phenomenon. 'However, t h e i n v e s t i g a t i o n o f t h e c r y s - t a l - f i e l d i s d e s i r a b l e ( o n l y m i n o r changes a r e expected b u t t h i s has t o be checked and i s under pro- cess) (see a l s o /2/).

As f a r as t h e c r y s t a l - f i e l d o f F~(H,O);' r e - mains t h e same i n a l l these compounds, i t i s w o r t h n o t i n g t h a t t h e asymmetry depends on t h e c r y s t a l l i - t e size, t h e e x i s t e n c e ( o r absence) o f a p e r i o d i c l a t t i c e , t h e presence o f v a r i o u s i m p u r i t i e s . Such an i n f l u e n c e i s n o t unreasonable i n t h e frame o f a s p i n - l a t t i c e r e 1 a x a t i o n assumption, proposed by /I/.

References

/1/ Chappert, J., Jehanno. G., V a r r e t , F.,-J. Phys.

38 (1977) 411.

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/2/ S p i e r i n g , H., Nagy,, D., t h i s conference.

F i g . 1 : Thermal v a r i a t i o n a o f A r =