MÖSSBAUER STUDY OF AMORPHOUS FeIII FLUORIDES

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MÖSSBAUER STUDY OF AMORPHOUS FeIII FLUORIDES

M. Henry, F. Varret, J. Teillet, G. Ferey, O. Massenet, J. Coey

To cite this version:

M. Henry, F. Varret, J. Teillet, G. Ferey, O. Massenet, et al.. MÖSSBAUER STUDY OF AMOR- PHOUS FeIII FLUORIDES. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-279-C1-280.

�10.1051/jphyscol:1980198�. �jpa-00219791�

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JOURNAL DE PHYSIQUE Colloque C1, suppl&ment au n ^O1, Tome 41, janvier 1980, page C1-279

M. Henry, F. Varret, J. Teillet, G. Ferey, 0. Massenet and J.M.D. coeyx

+

Universitg du Maine, F 72017 Le Mans, France.

+ Service NationaZ des Champs Intenses, F 38042 GrenobZe, France.

x T r i n i t y CoZZege, DubZin (21, IreZand.

R6sumB.- Nous avons etudie les fluorures amorphes KFeF4 et NaFeF4, dont nous comparons les resultats a ceux de FeF amorphe, dejd publies ( 1,2

1.

Abstract.- Amorphous KFeF4 and NaFeF4 have veen studied ; the results are compared with those of amorphous FeF3, previously published ( ^1,2(

.

1. Introduction.- Using the vapor deposition method (already d e s c r i b ~ a f o r FeF3 / l / ) ; we pre- pared f u r t h e r amorphous ~ e " ' fluorides : KFeF4 and NaFeF4. In addition t o t h e M8ssbauer study we performed magnetic measurements t o be presen- ted elsewhere (FeF3 / 2 / , KFeF4, NaFeF4 / 3 / ) . Because of the s t r i k i n g s i m i l a r i t y of these three compounds, we systematically compare t h e i r data which we give in t a b l e 1.

2 . Results and discussion : quadrupole data

( f i g . 1).

-

The non-lorentzian 1 ine broadening has

been expressed interms of a quadrupole s p l i t t i n g ( A ) d i s t r i b u t i o n , determined a s an histogram.

(In the case of FeF3, we could not s a f e l y f i t a A-distribution, because of the presence of a small ferrous contribution i n t h e spectrum).

The actual shape of the A-distributions f i t s ra- t h e r well the model of a random i s o t r o p i c d i s t r i - bution of the principal components of the EFG, developed by COEY

,

with the simplest d i s t r i - bution law P(A) = (1/02) A exp (-A2/202). The o-values compare well t o the average

a

^values,

obtained from a single doublet f i t t i n g of the spectrum.

A t the moment, no d e f i n i t e sign has been assigned t o the EFG. This i s consistent with an i s o t r o p i c random model, which generates i n equal amounts EFGs with both signs. I t i s worth noting t h a t the values of FeF3 and AFeF4 a r e close t o each other (although they are very d i f f e r e n t in

the c r y s t a l l i n e s t a t e ) . The introduction of the A F molecules in the FeF3 amorphous packing (assuming the l a t t e r remains unchanged) seems t o induce no s i g n i f i c a n t contribution t o t h e EFG.

( s i m i l a r l y the s i z e of the A+ cation seems t o have neql i g i b l e influence on the EFG.)

Fig. 1. Lorentzian ( a ) and A-distribution ( b ) f i t s

.

histogram and random i s o t r o p i c P(A) i c ) .

The I.S. values of the amorphous and crys- t a l l i n e analogues a r e e s s e n t i a l l y the same

( % 0.54 mm/s versus Fe-metal a t 300 K ) , confir-

ming the expectation (based on s t e r i c arguments) t h a t the octahedral coordination i s obeyed in the amorphous packing,fn addition, the change in covalency a t the Fe-F bonds i s shown t o be small.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980198

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C1-280 JOURNAL DE PHYSIQUE

3. Results and discussion : magnetic data.- The low temperature spectra o f amorphous f l u o r i d e s are b a s i c a l l y s i x - l i n e patterns t y p i c a l f o r a frozen spin structure. The extrapolated satura- t i o n f i e l d s are reduced, as compared t o the crys- t a l l i n e analogues ; t h i s i s thought t o be due t o some enhancement o f the magnons, s i m i l a r t o t h a t observed i n 1 and 2 - d f a n s i o n systems 141. ( A t the moment no d i r e c t evidence has been found i n amorphous i n s u l a t o r s ) . The l i n e broadenings are thought t o extrapol ate t o r a t h e r low values, consistent w i t h a we1 1 defined value o f the l o c a l i z e d moments ( c o n t r a r e l y t o met-gl asses).

The average quadrupole s h i f t o f the magne- t i c l i n e s i s n e g l i g i b l e , i n agreement w i t h the

" i s o t r o p i c " assumpyon o f the absence o f corre- l a t i o n between the

E\FG

and the spin o r i e n t a t i o n .

Table 1.

The magnetic s t r u c t u r e i n the frozen s t a t e

e46' .**

determined by High F i e l d Mb'ssbauer Spectroscopy ^t^.

.

i n the cases o f FeF3 /2/ and NaFeF4 i s speromagnetic (random o r i e n t a t i o n o f the spins /6/).

NaFe

5

-1 0 0 mm/s 10

h

c r y s t .

i

am.

u am.

H~~ c r y s t . H am.

hf

(extrap, kOe)

Tfreezi ng remanence Tfreezing Mossbauer

Fig. 3. P l o t o f the depth o f the paramagnetic 1 ines, reduced scale :

Fig. 2. Speromagnetic NaFeF : experimental and = T/Tfreez. ~ 6 s s b . calculated 1 ineshapg. /5/

where T-freez. M6ssb i s such t h a t F =0.5 P NaFeF4

0.70 0.73 s 0.60

562 s 530

12 K

20 K FeF3

0.1 0.58

-

618 560

29 K

39 K

4. Conclusion.- The f e r r i c f l u o r i d e s i l l u s t r a t e At f i n i t e temperature,as the spin f l u c t u a - the new concept o f speromagnetic order. We wish t i o n frequencies increase, the MSssbauer spectra t o emphasize the high f r u s t r a t i o n o f these mate- d i s p l a y relaxed shapes ; because the Massbauer r i a l s , whose freezing temperature i,s much lower measurement time i s much shorter than t h a t o f a than t h a t o f c r y s t a l l i n e homologues.

magnetic measurement, ma'gnetic components a r e

KFeF4 1.5 0.77

% 0.65

540 s 520

< 4.2 K

12 K

ebserved a t temperatures higher than freezing

/1/ G. Ferey and A1 Sol. Stat. Com. 29 (1979)

temperatures defined by the appearence o f magne-

-

477

-

^480.

t i c remanences /3/. To f o l l o w the spin f l u c t u a -

tions, we measured the depth o f the paramagnetic /2/ G . Ferey, F. Varret, J.FI.D. Coey, J. of 1 in e (denoted Fp) as a f u n c t i o n o f temperature Physics C (to be published).

( f i g . 3): i n a reduced scale, a1 1 three f l u o r i d e s /3/ I n t . Conf. on Magnetism 1979 (Miinchen).

gave the same p l o t ; the existence o f a unique

/4/ GUPTA G.P., J. Physique

2

(1979) C2

-

^304.

model i s t h i s suggested. The freening temperature

i s observed t o depend upon the A+ cation s i z e /5/ Experiments performed i n S.N .C. I. (Grenoble).

( t h e l a r g e r A', the lower T freezing). /6/ J . M . D . Coey, J. Appl

.

^Phys.

9

(1978) 1646-52