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DETERMINATION OF THE ALUMINUM CONTENT OF NATURAL AND SYNTHETIC
ALUMOGOETHITES USING MÖSSBAUER SPECTROSCOPY
J. Fleisch, R. Grimm, J. Grübler, P. Gütlich
To cite this version:
J. Fleisch, R. Grimm, J. Grübler, P. Gütlich. DETERMINATION OF THE ALU- MINUM CONTENT OF NATURAL AND SYNTHETIC ALUMOGOETHITES USING MÖSS- BAUER SPECTROSCOPY. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-169-C1-170.
�10.1051/jphyscol:1980146�. �jpa-00219715�
JOURNAL DE PHYSIQUE
Colloque Cl , suppl&ment au n
O1, Tome 41,
janvier1980, page C1-169
DETERMINATION OF THE ALUMINUM CONTENT OF NATURAL
AM)SYNTHETIC
AUIMOGOETHITESUSING &SSBAUER SPECTROSCOPY
J. F l e i s c h , R. G r i m , J. Griibler and P. Gfitlich
I n s t i t u t
fUr~ n o r g d s c h e und AnaZytische Chemie, Johannes- ~utenberg- UniversittZt,
0-6500Mainz.
1. I n t r o d u c t i o n . - The aluminum o r e s c o n t a i n a con- r e l a t i o n AH = f (x) can o n l y b e v e r y rough because s i d e r a b l e amount of ~ l - s u b s t i t u t e d g o e t h i t e . I n of t h e r e l a t i v e l y l a r g e e r r o r s . But t h e r e i s a se- i r o n - r i c h b a u x i t e s t h i s t e c h n i c a l l y i n e x t r a c t a b l e
aluminum c a n b e a s i g n i f i c a n t p a r t o f t h e t o t a l a h .minum c o n t e n t determined by c o n v e n t i o n a l chemical
a n a l y s i s . The isomorphic s u b s t i t u t i o n of aluminum i n t h e system a-Fe A 1 OOH ( d x d 0 . 3 3 ) r e s u l t s i n a
1-x x
l a t t i c e c o n t r a c t i o n because of t h e s m a l l e r i o n i c r a d i u s of ~ 1 ~ ' (rA13+ = 0.57
g)
w i t h r e s p e c t t o l?e3+ (rFe3+ = 0.678 ) .
T h i s h a s been e s t a b l i s h e d by x-ray d i f f r a c t i o n s t u d i e s on s y n t h e t i c g o e t h i t e s /I/.The r e s u l t s d e r i v e d by t h e x-ray d i f f r a c t i o n method, however, s t r o n g l y depend on t h e p r e p a r a t i o n o f t h e compounds, e s p e c i a l l y on a g i n g o f t h e Fel-xA1xOOH c r y s t a l l i t e s . The x-ray d i f f r a c t i o n method i s t h e e
cond v e r y s i g n i f i c a n t i n f l u e n c e of t h e A 1 c o n t e n t on t h e Mbssbauer-spectra, v i z . a l a r g e d e c r e a s e of t h e o r d e r i n g temperature with i n c r e a s i n g x. T h i s c o n c e n t r a t i o n dependence of t h e o r d e r i n g tempera- t u r e a p p e a r s t o b e more promising a s an a n a l y s i s method of t h e alumo-goethites.
The temperature dependent Mbssbauer-spectra were analyzed by a l e a s t s q u a r e s f i t t i n g procedure u s i n g a t h e o r y of e l e c t r i c and magnetic h y p e r f i n e i n t e r - a c t i o n . T h i s could n o t b e a p p l i e d t o t h e temperature r e g i o n where r e l a x a t i o n t a k e s p l a c e , h e r e we o n l y e v a l u a t e d t h e magnitude of t h e i n t e r n a l f i e l d from t h e peak p o s i t i o n s . The problem becomes complicated f o r e n o t s u i t a b l e f o r t h e d e t e r m i n a t i o n o f t h e alu- by t h e f a c t t h a t e l e c t r o n microscope i n v e s t i g a t i o n s minum c o n t e n t o f t h e n a t u r a l o r e s .
2. Experimental.- For t h e Mossbauer-effect s t u d i e s f i f t e e n alumo-goethite samples with t h e Al-concen- t r a t i o n d x g 0 . 3 2 have been prepared a s d e s c r i b e d b y T h i e l / I / . The a g i n g p e r i o d v a r i e d from 12 t o 48 hours.
Powder x-ray d i f f r a c t i o n a n a l y s i s shows t h e w e l l known l a t t i c e c o n t r a c t i o n . The v a l u e s f o r t h e d
111- d i s t a n c e s d e c r e a s e from 2.439 t o 2.410
8
f o r x=O and x=0.32, r e s p e c t i v e l y . The x-ray d a t a a l s o r e - v e a l t h e p r e p a r a t i o n dependent s c a t t e r i n g o f t h edill-values
t o b e f i v e t i m e s l a r g e r t h a n t h e expec- t e d e f f e c t of Ad (x) =0.123a-x.111
We have t a k e n Mbssbauer-spectra i n t h e temperature range ~ O K ~ T C ~ C O K . The s p e c t r a show a temperature dependent change from a paramagnetic quadrupole &b- l e t t o a m a g n e t i c a l l y s p l i t s e x t e t . From t h e high- t e m p e r a t u r e s p e c t r a we cannot deduce any dependence o f t h e i s o m e r i c s h i f t and t h e quadrupole s p l i t t i n g on t h e A 1 c o n c e n t r a t i o n of t h e g o e t h i t e s . The spec- t r a t a k e n a t 20 K show o n l y a weak dependence o n the i n t e r n a l magnetic f i e l d . The magnitude of t h e i n - t e r n a l f i e l d d e c r e a s e s from 510+5 kG f o r x=O t o 470+8 kG f o r x=0.23, t h e f i r s t v a l u e being i n good agreement w i t h t h e v a l u e f o r pure g o e t h i t e
(50425 kG) g i v e n i n t h e l i t e r a t u r e /2/. An estima- t i o n o f t h e A 1 c o n t e n t according t o an e m p i r i c a l
/1/ i n d i c a t e a d i s t r i b u t i o n of t h e p a r t i c l e s i z e about some thousand f o r x=O and s e v e r a l hundred
i?
f o r x=0.32, r e s p e c t i v e l y . For l a r g e v a l u e s o f x t h e resonance l i n e s a r e i n c r e a s i n g l y broadened due t o superparamagnetism, which i s s t i l l p r e s e n t even a t low temperatures. The r e s u l t s f o r t h e i n t e r n a l magnetic f i e l d a s a f u n c t i o n , o f temperature a r e given i n F i g . 1 f o r Fe0,79A10.2100H. The s o l i d l i n e corresponds t o a l e a s t s q u a r e s f i t of a B r i l l o u i n - f u n c t i o n w i t h S=5/2, Ho=493+5 kG, T =288+5 K. Be-N
c a u s e of t h e d i f f i c u l t i e s i n v o l v e d i n t h e e v a l u a t i o n of t h e Mbssbauer s p e c t r a we have used a n o t h e r method f o r t h e d e t e r m i n a t i o n o f t h e o r d e r i n g temperature.
The v a l u e s of t h e isomeric s h i f t (6=0.257+0.005 mms-1 ) and t h e quadrupole s p l i t t i n g (eQV / 2 =
z z
0.541+_0.005 nuns-') show t h a t one l i n e o f t h e para- magnetic quadrupole d o u b l e t l i e s p r a c t i c a l l y a t zero v e l o c i t y ( t h e s h i f t from v=O b e i n g small compared t o t h e resonance l i n e w i d t h ) . T h i s s u g g e s t s a munt- r a t e measurement a s a f u n c t i o n of temperature w i t h t h e Mbssbauer a b s o r b e r and source k e p t f i x e d . With t h e o n s e t of magnetic o r d e r i n g t h e resonance c r o s s s e c t i o n at v=O d i m i n i s h e s and t h e c o u n t r a t e r i s e s . F i g . 2 shows such a "thermal scan" f o r Fe A 1
0.84 0.16 OOH. For t e m p e r a t u r e s below 210 K we observe a s i g n i f i c a n t i n c r e a s e i n t h e c o u n t r a t e , which appro- a c h e s a s a t u r a t i o n - v a l u e a t low temperatures, when
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980146
C1-170 JOURNAL DE PHYSIQUE
F i g u r e 1: I n t e r n a l magnetic f i e l d o f Fe0.79A10.2100H VS. temperature.
F i g u r e 3 : C o n c e n t r a t i o n dependence of t h e minimum temperature (Tmin) f o r Fei-xA1xOOH
F u r t h e r i n v e s t i g a t i o n i n t h i s range o f t h e aluminum c o n c e n t r a t i o n x a r e i n p r o g r e s s . Magnetic s u s c e p t i - b i l i t y measurements a r e a l s o underway. F i r s t r e - s u l t s from t h e s e i n v e s t i g a t i o n s s u p p o r t t h e d a t a o b t a i n e d from t h e Miissbauer experiments. They a l s o show t h a t t h e superparamagnetic behaviour o f t h e samples w i t h r a t h e r broad p a r t i c l e s i z e d i s t r i b u - t i o n s has t o b e c o n s i d e r e d v e r y c a r e f u l l y .
F i g u r e 2: Countrate a t z e r o v e l o c i t y a s a f u n c t i o n of t e m p e r a t u r e f o r Fe A 1 OOH
0.84 0.16
a l l t h e i n t e n s i t y of t h e d o u b l e t h a s d i s a p p e a r e d a t t h e favour of t h e m a g n e t i c a l l y s p l i t s e x t e t . I n t h e paramagnetic r e g i o n t h e c o u n t r a t e i n c r e a s e s w i t h temperature because o f t h e weak t e m p e r a t u r e dependence o f t h e h y p e r f i n e parameters o f t h e qua- d r u p o l e d o u b l e t . A t Tmin a minimum i n t h e neigh- bourhood of t h e o r d e r i n g t e m p e r a t u r e r e s u l t s from t h e s e two opposing e f f e c t s .
3. R e s u l t s and d i s c u s s i o n . - F i g u r e 3 summarizes t h e r e s u l t s f o r t h e minimum temperature a s a func- t i o n of t h e aluminum c o n t e n t x. For t h e range A 4 . 3 2 t h e minimum temperature changes by n e a r l y 200 A. For ~ ~ 0 . 1 5 t h e change is l i n e a r a c c o r d i n g t o t h e l e a s t s q u a r e s f i t t e d s t r a i g h t l i n e T ( X I =
min 381~-722x, whereas a t h i g h e r Values o f x devi- a t i o n s from l i n e a r i t y a r e observed. These d e v i - a t i o n s cannot a r i s e from t h e i n f l u e n c e o f super- paramagnetism, because, i f s o , one would e x p e c t t h e s e p o i n t s t o l i e below t h e s t r a i g h t l i n e .
Acknowledgement.- We thank t h e V e r e i n i g t e Aluminium- Werke f o r p r e p a r i n g t h e samples and t h e x-ray d i f - f r a c t i o n a n a l y s i s , and t h e Bundesministerium f i i r Forschung und Technologie f o r f i n a n c i a l s u p p o r t .
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