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VACANCY-CATION DISTRIBUTION IN NON STOICHIOMETRIC LAYERED IRON-TITANIUM
CHALCOGENIDES
G. Fatséas, J. Dormann, M. Danot
To cite this version:
G. Fatséas, J. Dormann, M. Danot. VACANCY-CATION DISTRIBUTION IN NON STOICHIO-
METRIC LAYERED IRON-TITANIUM CHALCOGENIDES. Journal de Physique Colloques, 1979,
40 (C2), pp.C2-367-C2-372. �10.1051/jphyscol:19792130�. �jpa-00218502�
JOURNAL DE PHYSIQUE Co//oque
C2,
suppiement aun
O3,
Tome 40, mars 1979, page C2-367VACANCY-CATION DISTRIBUTION IN NON S T O I C H I O M E T R I C L A Y E R E D IRON-TITANIUM C H A L C O G E N I D E S
+*
++
G.A. FatsZas
,
J.L. ~ormann' and M. Danot+:~aboratoire de Magndtisme, C.N.R.S., 1, Place A. Briand, 92190 !feudon, France
Laboratoire de Chimie miniraze A
-
Chimie des SoZides, U.E.R. de C h i d e , 2, rue de l a Houssinidre, 44072 Nantes Cedes, FranceR6sum6.- Sept chalcog6nures lamellaires de fer et de titane, non stoechiom6triques, ont 6t6 6tu- digs par spectroscopie Mijssbauer, en relation avec des calculs de distribution statistique des cations et des lacunes. Les r6sultats obtenus, nous ont permis de dgterminer la distribution des cations et des lacunes dans les differents plans, et d'identifier les diffgrents sites de fer.
Abstract.-In an extension of a previous study of layered iron titanium chalcogenides / I / , we have studied ~sssbauer spectra of seven non-stoichiometric compounds, in the vicitity of the FeTiaS4 composition, in relation with statistical distribution calculations. The results, enabled us, mainly, to obtain informations on the cations-and-vacancy distribution, and to identify the diffe- rent detected iron-sites.
1. Introduction.- In a previous paper /I/ we have reported X-ray and Mossbauer spectroscopy results on the crystallographic and magnetic ordering of stoi- chiometric (x = o) and non-stoichiometric (x
#
o) iron titanium layered chalcogenides Fe I+X Ti z(l+~)~s.Although those results /I/ contributed to a better understanding of some structural and magnetic pro- perties, they showed the necessity of further expe- rimental work for the determination of the cations- and-vacancy distribution in the non-stoichiometric compounds.
This is due to the fact that the ternary non-stoi- chiometric compounds of general formula M3+*X, ( - I ,< x 6 0.5, M : Fe + Ti, X : S, Se) show f requen- tly, and according to previous known studies, dif- ferent structural modifications, with formation of different types of superstructures, such as M5Xa, M2X3, M3& and M7Xe. These superstructures are re- lated to different situations of more or less orde- red occupation of the octahedral positions and depend on the composition (x) /2/ and on the thermal and mechanical treatments /2,3/.
Tbe unambiguous determination of the cations-and- vacancy distribution in such compounds is, thus, a difficult problem and asks systematic experimental work on several compositions and types of non-stoi- chiometry with well defined preparation conditions.
The present work is an attempt for a better unders- tanding of this problem by obtaining MEssbauer spectroscopy results on seven non-stoichiometric compounds belonging to four different types of
? J e w address :
++ .
non-stoichiometry in the vicinity of the FeTi2S+
composition : namely,
N O 1 x = 0.052, sample a
Fel-x T i 2 + ~ 4' x = 0.104, sample b N o 2 F e ~ + x Ss x = 0.052, sample c
N O 3 Fe Ti x = 0.030, sample e
i+x 2(1+x) 4' x = 0.052, sample f
N O 4 x = 0.052, sample d
Fel+x Til S4 x = 0.400, sample g
These types of non-stoichiometry were chosen to cover different regions of interest in order to obtain a reasonably unambiguous approach to the pro- bLem of the cations-and-vacancy ordering. The re- gions of interest are : deficiency (No I) or excess
(NO 2,3.4) in Fe atoms,relative to the stoichiome- tric composition FeTizSz, and, also, perturbed (No 3, 4) or not-perturbed (NO 1 , 2) MsSr formula (M : Fe + Ti). It is, in fact, interesting to see what happens when the relative amounts of Fe and Ti atoms are changed with or without perturbation of the stoichiometric MsSk composition.
2. Experimental and samples preparation.- High statistics spectra were obtained with a 57Fe (Rh) source and a ~Essbauer spectrometer, using a symne- trical saw-tooth vibrator and a high counting-rate system 141.
All samples contain
-
10 mg natural iron per cm2, corresponding to a "thin K&ssbauer absorber". The least squares fitting-routine uses Lorentzian line shapes and equal k6ssbauer absorption coefficients for all iron sites.The Mijssbauer samples are slowly cooled polycrys- talline materials prepared from direct reaction of
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792130
C2-368
JOURNAL DE PHYSIQUEh i g h p u r i t y i r o n , t i t a n i u m and s u l p h u r . The p r e p a r a - w i t h some a s s u m p t i o n s o f p r e f e r e n t i a l o r random d i s - t i o n and h e a t t r e a t m e n t were d e s c r i b e d i n r e f e r e n c e
111.
3. R e s u l t s and d i s c u s s i o n . - The MGssbauer s p e c t r a were o b t a i n e d a t 4 . 2 K / 5 / and a r e shown i n f i g u r e
1 f o r t h e F e - d e f i c i e n t compounds and i n 2 and 3 f o r t h e Fe-excedent compounds.
F!g. I : Computer-analysed s p e c t r a o f t h e F e - d e f i - c l e n t compounds Fel-xTi2+xS,, a t 4.2K.
( a ) : x = 0.052 ( b ) : x = 0.104
The z e r o v e l o c i t y i s t h e c e n t r e o f g r a v i t y o f a me- t a l l i c i r o n s p e c t r u m a t 300K.
The d i s t i n c t i o n o f two d i f f e r e n t g r o u p s c a n b e j u s - t i f i e d by t h e o b s e r v a t i o n o f t h e s p e c i f i c f e a t u r e ( a l r e a d y r e p o r t e d i n o u r p r e v i o u s s t u d y / I / ) o f p r e s e n c e o f a s t r o n g - f i e l d F e - s i t e i n a l l t h e Fe- e x c e d e n t c o m p o s i t i o n s ( f i g u r e s 2 , 3 ) , t h i s f i e l d b e i n g a b s e n t i n t h e Fe- d e f i c i e n t o n e s ( f i g u r e I ) ,
( s e e a z d i t i o n a l p e a k s o u t s i d e t h e c e n t r a l p a r t o f t h e s p e c t r a i n f i g u r e s 2 and 3 ) . The s p e c t r a were computer-analysed t o s e v e r a l o v e r l a p e d p a t t e r n s ( F e - s i t e s ) ' o f Zeeman ( 6 - l i n e p a t t e r n ) and combined e l e c t r i c and m a g n e t i c ( 8 - l i n e p a t t e r n ) h y p e r f i n e i n t e r a c t i o n s .
The t a b l e I summarizes t h e d e t e c t e d F e - s i t e s , t h e i r r e l a t i v e i n t e n s i t i e s and t h e h y p e r f i n e p a r a m e t e r s ; f i e l d (H), q u a d r u p o l e s p l i t t i n g (E) and i s o m e r s h i f t ( 6 ) .
The i d e n t i f i c a t i o n o f t h e d i f f e r e n t F e - s i t e s was o b t a i n e d by comparing t h e e x p e r i m e n t a l r e l a t i v e i n - t e n s i t i e s w i t h t h e t h e o r e t i c a l o n e s , c a l c u l a t e d
t r i b u t i o n o f t h e e x c e d e n t Fe o r ( a n d ) T i atoms i n t h e cations-and-vacancy p o s i t i o n s .
F i g . 2 : Computer a n a l y s e d s p e c t r a o f t h e Fe-exce- d e n t compounds ( c )
,
( d ) and ( e ) a t 4.2K.( c ) : Fel+xTi2-,S, ( x = 0.052)
( d ) : Fel+,Ti2S4 (x = 0.052)
( e ) : Fel+xTi,(,+,)S, ( x = 0.030) T h i s was done a s f o l l o w s :
We u s e d , a s s t a r t i n g p o i n t o f o u r r e a s o n i n g , t h e s t o i c h i o m e t r i c FeTi2Sr compound, i n which f u l l T i - l a y e r s a l t e r n a t e w i t h l a y e r s c o n t a i n i n g o r d e r e d Fe atoms and v a c a n c i e s
,
t h a t i s , (Fe + ) Ti2S4 ( I ) / I / . I n t h i s s t r u c t u r e , o f C r 3 S 4 - t y p e ( s p a c e g r o u p I 2/m), t h e Fe atom i s s u r r o u n d e d by 2 Fe and 4 0 w i t h i n t h e (Fe + D ) l a y e r / I / . The q u e s t i o n i s , now, t o d e t e r m i n e t h e p o p u l a t i o n s o f t h e s e s i x n e i g h b o u r s - p o s i t i o n s and a l s o t h e popula- t i o n s o f t h e T i - l a y e r s , i n t h e n o n - s t o i c h i o m e t r i c compounds, by t e s t i n g d i f f e r e n t p o s s i b l e c a t i o n s - and-vacancy d i s t r i b u t i o n s c o m p a t i b l e w i t h t h e expe- r i m e n t a l i n t e n s i t i e s .To do t h i s , we d i t i n g u i s h , a s above, t h e Fe-defi- c i e n t and t h e Fe-excedent compounds.
3.1. F e - d e f i c i e n r - c g g p g g g i s ( a and b).-Both com- pounds, ( a ) and ( b )
,
h a v e , f o r any x, t h e M 3 X k f o r - mula and. t h u s . a c o n s t a n t number of v a c a n c i e s' l ) ~ h r o u g h o u t t h i s a r t i c l e t h e t e r m s T i - l a y e r and ( F e + a ) - l a y e r r e f e r t o t h e s e two d i s t i n c t l a y e r s e v e n i f t h e y c o n t a i n d i f f e r e n t a d d i t i o n a l atoms and v a c a n c i e s .
( 1 vacancy p e r f o r m u l a ) .
Fe
-
s i t eand neighbours
situated on the (Fe + a ) - l a y e r
INTENSITIES ( e r r o r . t 0.02)
Fe
-
s i t e s H cCompounds surrounded by : kOe mnl s
a b c d e f 9
1 or zero
Ll
+2 Fe + 3 Ti 75 i 5 0.07
*
0.04-
(0.06) 0.07 (0.12) 0.10Fe
-
s i t e n e i ~ b o u r 1 Fe near 125*
15 0 .*
0.10-
0.05 0.05 0.05 0.315 0.65on both
( F ~ +a)-and- along the C-axis (0.05) (0.05) (0.05) ( 0 . a ) (0.65)
linewidth
> -
2.20 1.80 2.52 2.44 2.25 2.90 3.00T a b l e I : D e t e c t e d F e - s i t e s (and t h e i r n e i g h b o u r s i n Fe, T i and v a c a n c i e s ) , h y p e r f i n e f i e l d (H), quadrupole s p l i t t i n g (E), l i n e w i d t h and e x p e r i m e n t a l and t h e o r e t i c a l i n t e n s i t i e s (...) f o r t h e d i f f e r e n t compounds.
The q u a d r u p o l e s p l i t t i n g E i s e q u a l t o 114 e 2 q ~ f o r b o t h t h e 8 - l i n e and t h e 6 - l i n e p a t t e r n s ( s u p p o s i n g f o r t h e second-one,
0
and q e q u a l t o z e r o , q, asymmetry p a r a m e t e r , 0 t h e known p o l a r a n g l e ) .For r e a s o n s o f l i m i t e d s p a c e , o n l y t h e mean H-value i s g i v e n f o r e a c h s i t e . F o r ex. H = 25
+
4 kOe means t h a t t h i s s i t e was d e t e c t e d , i n a l l t h e compounds, w i t h v a l u e s l y i n g between 21 and 29 kOe. (+ 4 i s n o t , t h u s , t h e e r r o r ! ) . Same remark f o r t h e € - v a l u e s .When t h e t h e o r e t i c a l i n t e n s i t y (...) i s g i v e n a l o n e , w i t h o u t e x p e r i m e n t a l v a l u e , i t means t h a t t h i s s i t e was n o t d e t e c t e d i n t h e s p e c t r u m . Only t h e F e - s i t e w i t h ( 4 0 + 2 Fe) c o n f i g u r a t i o n i s a n 8 - l i n e p a t t e r n
( n
= 0 . 3 5 0 . 1 , O = 30"+
5, I$ = 0 ) . The o t h e r s i t e s a r e 6 - l i n e p a t t e r n s b e c a u s e o f t h e v e r y s m a l l qua- d r u p o l e s p l i t t i n g .The l i n e w i d t h s a r e g i v e n i n t h e form o f a c o e f f i c i e n t . With t h e same c o e f f i c i e n t t h e e x p e r i m e n t a l l i n e - w i d t h o f a t h i n n a t u r a l m e t a l l i c i r o n a b s o r b e r a t 4.2K i s e q u a l t o 1.80.
The i s o m e r s h i f t ( 6 ) i s e q u a l t o 0.94
+
0.02 m/s a t 4.2K f o r a l l s i t e s and a l l compounds ( r e l a t i v e t o t h e m e t a l l i c i r o n a t 300K) and e q u a l t o 0 . 7 8+
0 . 0 2 a t 300K.The most s i m p l e s i t u a t i o n i s t o s u p p o s e , t h a t t h e T i - l a y e r s remain unchanged and t h a t t h e e x c e d e n t T i atoms, e i t h e r r e p l a c e t h e d e f i c i e n t Fe o r a r e ran- domly d i s t r i b u t e d i n t h e v a c a n c y - p o s i t i o n s ( b o t h t h e o r i g i n a l v a c a n c i e s o f t h e (Fe + ) Ti2Ss compound and t h e v a c a n c i e s coming from t h e i r o n de- f i c i e n c y a s w e l l ) .
The s i t u a t i o n o f random d i s t r i b u t i o n h a s g i v e n 1 6 1 t h e b e s t r e s u l t ( b e s t goodness o f f i t ) f o r b o t h compounds ( a ) and ( b ) b u t i t i s p a r t i c u l a r l y s u i t a - b l e f o r ( a ) , a s c a n be s e e n from t h e agreement b e t- ween t h e f i t t e d e x p e r i m e n t a l i n t e n s i t i e s (numbers w i t h o u t p a r e n t h e s i s ) and t h e t h e o r e t i c a l o n e s ( n u n r b e r s i n p a r e n t h e s i s ) . T h i s l e a d s t o t h e f o l l o w i n g cations-and-vacancy d i s t r i b u t i o n f o r m u l a :
(Feo.st,s T i 0 . 0 5 2
0 1 )
(Ti21 S4 f o r ( a )where t h e f i r s t p a r e n t h e s i s r e p r e s e n t s t h e ( ~ e + a ) - l a y e r and t h e second-one t h e T i - l a y e r .
I n t h e more F e - d e f i c i e n t compound ( b ) , t h e a g r e e - ment i s l e s s good, p a r t i c u l a r l y f o r t h e ( 2 3
+
2 Fe + 2 T i ) c o n f i g u r a t i o n , i n which t h e experimen- t a l i n t e n s i t y ( 0 . 1 6 ) i s much l a r g e r t h a n t h e theo- r e t i c a l ( 0 . 0 5 ) . T h i n g s a r e h a p p e n i n g a s i f t h e i n - c r e a s e of t h e amount o f t h e T i atoms i n ( b ) r e l a t i v e t o ( a ) , b r i n g s o u t t h e p u r e s t a t i s t i c a l d i s t r i b u - t i o n , ( e x i s t i n g i n ( a ) ) , by f a v o u r i n g t h e f o r m a t i o n of t h e F e - s i t e s u r r o u n d e d by T i - p a i r s , a t t h e e x p e n s e o f t h e t h r e e o t h e r s i t e s .
The f u l l s t a t i s t i c a l c a l c u l a t i o n and t h e comparison w i t h t h e e x p e r i m e n t a l i n t e n s i t i e s show, t h a t t h e
c2-370
JOURNAL DE PHYSIQUE b e s t r e s u l t c o r r e s p o n d s t o a d i s t r i b u t i o n where t h eT i - l a y e r s a r e s l i g h t l y p e r t u r b e d by t h e p r e s e n c e o f a s m a l l amount o f v a c a n c i e s , a c c o r d i n g t o t h e f o r -
mula :
(Feo.es6 T i o . l o ~ + ~ 0 1 - y ) ( T i z - y m y ) S4 w i t h y = 0 . 0 3
+
0.01F i g . 3 : Same, c a p t i o n a s f i g u r e 2, f o r t h e com- pounds ( f ) and ( g )
.
( f ) : Fel+xTi2(l+x)S,, ( x = 0.052) ( g ) : Fe,+,Ti2Ss ( X = 0 . 4 )
3 . 2 . Fe-excedent c o m p m ~ < ~ ( c , d , e , f , g)
.-
Herea g a i n we have t r i e d t o f i t t h e s p e c t r a w i t h t h e o r e - t i c a l i n t e n s i t i e s c o r r e s p o n d i n g t o d i f f e r e n t p o s s i - b l e cations-and-vacancy d i s t r i b u t i o n models.
The s p e c t r a of t h e s e compounds ( f i g u r e s 2, 3) show a d d i t i o n a l p e a k s , o u t s i d e t h e c e n t r a l p a r t o f t h e spectrum, c o r r e s p o n d i n g t o a s t r o n g - f i e l d F e - s i t e , n o t e x i s t i n g i n t h e F e - d e f i c i e n t compounds.
The o r i g i n of t h i s s i t e remained a p u z z l e i n o u r p r e v i o u s work / I / , p a r t i c u l a r l y t h e f a c t t h a t 5 % a d d i t i o n a l Fe atoms i n t h e ( f ) compound ( r e l a t i v e t o t h e Fe atoms i n t h e s t o i c h i o m e t r i c FeTi2Sk) c o u l d g i v e a new s i t e o f 2 34 % i n t e n s i t y .
S i m i l a r l y t o t h e F e - d e f i c i e n t compounds and a c c o r - d i n g , a l s o , t o t h e d i s c u s s i o n o f o u r p r e v i o u s s t u d y / I / , we s t a r t e d from t h e most s i m p l e s i t u a t i o n , w h i c h i s t o suppose t h a t t h e T i - l a y e r s remain unchanged, t h e a d d i t i o n a l Fe-atoms (x-value) g o i n g , s t a t i s t i - c a l l y , t o t h e vacancy p o s i t i o n s of t h e (Fe +U) l a y e r s . T h i s model h a s g i v e n r e s u l t s ( n o t i n c l u d e d i n t a b l e I) i n complete d i s a g r e e m e n t w i t h t h e e x p e r i m e n t a l i n t e n s i t i e s and l e d t o t h e c o n c l u s i o n t h a t t h e s t r o n g - f i e l d d o e s n o t b e l o n g t o Fe atoms h a v i n g o n l y T i n e i g h b o u r s i n t h e T i - l a y e r s , and t h a t i t i s ne- c e s s a r y t o c o n s i d e r d i s t r i b u t i o n s i n which Fe atoms a r e a l s o p r e s e n t i n t h e T i - l a y e r s . The f u l l s t a t i s -
t i c a l c a l c u l a t i o n on t h i s d i s t r i b u t i o n and t h e com- p a r i s o n w i t h t h e e x p e r i m e n t a l i n t e n s i t i e s l e d t o t h e f o l l o w i n g d i s t r i b u t i o n - f o r m u l a e f o r t h e Fe- e x c e d e n t compounds :
( F e l . 0 2 6 T i y 0 0 . 9 7 ~ * - y ) (Fe0.026 T i 1 . 9 4 8 - ~ 0 . 0 2 6 + ~ ) f o r (c) w i t h y = 0.05 0.03 (Fel.oz6 T i o . 0 2 6 + ~
a
0 . 9 s e - ~ ) (Feo.026 Tii.974-yY) f o r ( d )
w i t h y = 0.05
+
0.03 (FelalTio.os+y Do.so-y) (Feo.02 T i l * g 8 - yO Y ) w i t h y = 0.01
+
0.01f o r ( e ) (Feo.88 T i ~ . n a + ~ n0.84-y) (Fe0.17 T i l , ~ s - ~ my)
f o r ( f ) (Feo.90 T i o . ~ o + ~ 0 0 . 6 0 - y ) (Feo.50 T i ~ . s o - ~
f o r (g) w i t h y 2 0 and e r r o r 5 0.04 f o r a l l numbers o f b o t h ( f ) and ( g ) The c o r r e s p o n d i n g , t o t h e above d i s t r i b u t i o n ; Fe- s i t e s , i n t e n s i t i e s , and h y p e r f i n e p a r a m e t e r s , a r e g i v e n i n t h e t a b l e I.
These r e s u l t s c a n be commented a s f o l l o w s :
-
Both l a y e r s c o n t a i n F e , T i and v a c a n c i e s , t h e e x c e d e n t Fe b e i n g d i s t r i b u t e d h a l f and h a l f i n t h e two l a y e r s f o r ( c ) and ( d ) ( s e e f o r m u l a abo- ve) and more t h a n h a l f i n t h e T i - l a y e r s f o r ( e ) , ( f ) and ( g ) .-
Small d i s a g r e e m e n t s between e x p e r i m e n t a l and t h e w r e t i c a l i n t e n s i t i e s , r e f l e c t s l i g h t d e v i a t i o n s from t h e i d e a l b i n o m i a l low ( s e e t a b l e I ) . T h i s i s t h e c a s e o f t h e compound ( f ) i n which t h e d i - sagreement between t h e e x p e r i m e n t a l i n t e n s i t y (0.22) and t h e t h e o r e t i c a l ( 0 . 1 5 ) , r e f l e c t s a pre- f e r e n t i a l f o r m a t i o n o f F e - s i t e s s u r r o u n d e d by T i - p a i r s .-
I d e n t i c a l l o w - f i e l d F e - s i t e s w i t h , n e a r l y , e q u a l h y p e r f i n e p a r a m e t e r s , were found f o r b o t h t h e Fe- d e f i c i e n t and Fe-excedent compounds. T h i s "cohe- r e n c e " was a r e a s o n a b l e r e q u i r e m e n t i n t h e a n a l y - s i s of t h e d i f f e r e n t compounds and was u s e d , sometimes, a s a c r i t e r i u m f o r t h e c h o i c e between d i f f e r e n t i n e q u i v a l e n t s o l u t i o n s .-
The e x i s t e n c e o f t h e ( 5 0 + 1 F e ) - c o n f i g u r a t i o n depends, n o t on t h e t o t a l number of v a c a n c i e s , b u t on t h e number o f i r o n - v a c a n c i e s on t h e(Fe + ) - l a y e r . More t h a n one Fe i n tho.
(Fe +
0
) - l a y e r , d e c r e a s e s t h e number o f vacan- c i e s i n t h i s l a y e r and t h e p r o b 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 becomes n e g l i g i b l y s m a l l ( c a s e of( c ) , ( d ) and ( e ) ) . T h i s p r o b a b i l i t y i s no l o n g e r n e g l i g i b l e i f t h e number o f Fe i n t h e ( F e + ) - l a y e r i s s m a l l e r t h a n one ( c a s e o f ( f ) and ( g ) ) .
-
The s i m u l a t i o n of two o r t h r e e s i t e s i n o n l y one s i t e i n ( f ) ( o f 0.22 i n t e n s i t y ) and ( g ) ( o f 0.16 i n t e n s i t y ) , r e s p e c t i v e l y , i s due t o t h e l a r g e in- t e n s i t y o f t h e s t r o n g - f i e l d F e - s i t e coming from t h e l a r g e e x c e s s i n Fe and Ti atoms i n t h e s e two compounds. T h i s f a c t d e c r e a s e s t h e i n t e n s i t i e s of t h e l o w - f i e l d s i t e s , and, some o f them, become s o s m a l l t h a t t h e y can b e f i t t e d w i t h many d i f f e r e n t c o m b i n a t i o n s o f i n t e n s i t i e s , b e i n g f o r t h i s r e a s o n u n d i s t i n g u i s h a b l e . The s p e c t r a were, t h u s , f i t t e d w i t h a r e d u c e number of s i t e s . T h i s i s r e f l e c t e d i n an i n c r e a s e of t h e l i n e w i d t h ( t a b l e I ) .-
The s t r o n g - f i e l d F e - s i t e i n a l l t h e Fe-excedent compounds, i s a s p e c i f i c p r o p e r t y of t h e s e com- pounds ( s e e a l s o r e f e r e n c e / I / ) and c o n c e r n s Fe atoms h a v i n g a n e a r n e i g h b o u r Fe atom a l o n g t h e c - a x i s . T h i s c o n f i g u r a t i o n becomes p o s s i b l e w i t h t h e p r e s e n c e o f Fe atoms i n t h e T i - l a y e r s , and e x i s t s i n FesSes o f s i m i l a r s t r u c t u r e 181.Although t h i s s i t e i s o b s e r v e d o n l y i n t h e com- pounds w i t h e x c e d e n t Fe, i t s i n t e n s i t y does n o t depend on t h e amount of t h e excedent-Fe, b u t on t h e t o t a l number o f v a c a n c i e s i n t h e two l a y e r s . I t seems, t h a t more t h e number of v a c a n c i e s de- v i a t e s from one vacancy p e r f o r m u l a ( t h e number o f t h e s t o i c h i o m e t r i c FeTizSt,), h i g h e r i s t h e i n - t e n s i t y of t h e s i t e . T h i s i s t h e r e a s o n why t h i s s i t e h a s a 31 7, i n t e n s i t y i n ( f ) , and 5 % i n ( d ) , w h i l e ( f ) and (d) h a v e t h e same amount o f e x c e d e n t i r o n . T h i s e x p l a i n s a l s o t h e l a r g e i n t e n s i t y o f t h i s s i t e i n ( g ) .
-
An i n t e r m e d i a t e - f i e l d F e - s i t e was d e t e c t e d i n ( f ) and ( g ) compounds. T h i s s i t e i s due t o t h e l a r g e amounts o f Fe and T i i n t h e (Fe +0
) - l a y e r ( s e e d i s t r i b u t i o n f o r n u l a ) which g i v e s a "measurable"v a l u e t o t h e p r o b a b i l i t y o f t h e ( 1
+
2 Fe + 3 T i ) c o n f i g u r a t i o n i n t h i s l a y e r . The t a b l e I shows, t h a t t h i s s i t e seems b e l o n g t o t h e group o f t h e l o w - f i e l d s i t e s from b o t h c o n f i g u r a t i o n and H-value p o i n t of vue. One c a n s e e , i n f a c t , t h a t a l l t h e l o w - f i e l d s , from 12 kOe t o 75 kOe, l i e i n a s t r a i g h t l i n e ; H v a l u e v e r s u s number of v a c a n c i e s , w i t h a s l o p e e q u a l t o-
12 kOe p e r va- cancy ( f i g u r e 4 ) . T h i s r e s u l t i s v e r y s i m i l a r t o t h e 11.5 kOe p e r vacancy o b s e r v e d , i n a p r e v i o u s work 1 9 1 f o r t h e o c t a h e d r a l s i t e s ( E e l - s i t e s ) of Fe2-xGe, of r e l a t e d s t r u c t u r e ( B a 2 ) ( s e e r e f e r e n c e191.
F i g . (N) (Fe
, 4 : H y p e r f i n e f i e l d - v a l u e s v e r s u s t h e number of v a c a n c i e s s u r r o u n d i n g t h e i r o n s i t e s i n t h e +
4. Conclusion.- I n c o n c l u s i o n , from computer-analy- s i s o f Mzssbauer s p e c t r a and s t a t i s t i c a l d i s t r i b u - t i o n c a l c u l a t i o n s on s e v e n n o n - s t o i c h i o m e t r i c i r o n - t i t a n i u m l a y e r e d c h a l c o g e n i d e s , we have o b t a i n e d t h e f o l l o w i n g r e s u l t s :
-
We have proposed f o r m u l a e g i v i n g t h e c a t i o n s - a n d - vacancy d i s t r i b u t i o n s i n b o t h t h e (Fe + )-and- T i - l a y e r s ,-
We have d e t e c t e d a p r e f e r e n t i a l f o r m a t i o n of t h e ( 2 + 2 Fe + 2 T i ) - c o n f i g u r a t i o n i n t h e Ti- e x c e d e n t compounds (b) and ( f ) , which shows t h e b e g i n i n g of o c c u r e n c e of a M7Xe-type s u p e r s t r u c - t u r e , c o r r e s p o n d i n g t o an a s s e m b l i n g o f T i p a i r s around some i r o n atoms.-
We have i d e n t i f i e d t h e s t r o n g - f i e l d Fe s i t e ( n o t w e l l u n d e r s t o o d i n o u r p r e v i o u s work), a s t h e Fe- s i t e h a v i n g one Fe n e i g h b o u r a l o n g t h e c - a x i s . T h i s c o n f i g u r a t i o n i s s i m i l a r , a s f a r a s t h e con- f i g u r a t i o n a l o n g t h e c-axis i s concerned, t o b o t h s i t e s of FeaSer ( o f i d e a l C r 3 St,-order) h a v i n g s t r o n g - f i e l d F e - s i t e s/ l o / ,
-
We have found, a l s o , t h a t a l t h o u g h t h e s t r o n g - f i e l d F e - s i t e r e s u l t s from t h e e x i s t e n c e of exce- d e n t i r o n , i t s i n t e n s i t y d o e s n o t depend on t h e amount o f t h i s i r o n b u t on t h e t o t a l amount of v a c a n c i e s t h e i n t e n s i t y i n c r e a s i n g when t h e number o f v a c a n c i e s d e c r e a s e s . T h i s means t h a t t h e c o n d i t i o n f o r h a v i n g a h i g h - i n t e n s i t y s t r o n g - f i e l d s i t e i s n o t t o have, s i m p l y , a l a r g e amountC2-372
JOURNAL DE PHYSIQUE o f Fe b u t t o have l a r g e amount o f Fe i n b o t hl a y e r s s o t h a t t h e p r o b a b i l i t y t o h a v e Fe atoms w i t h one Fe n e i g h b o u r a l o n g t h e c - a x i s become l a r g e ,
We h a v e , f i n a l l y , o b s e r v e d , t h a t t h e r e e x i s t , i n a l l t h e compounds o f t h e p r e s e n t s t u d y , a v e r y s m a l l d i s p e r s i o n o f & - v a l u e s f o r a l l t h e i r o n - s i t e s . T h i s p r o p e r t y o f c l o s e n e s s o f i s o m e r s h i f t s f o r a l l t h e i r o n - s i t e s o f a m u l t i s i t e com- pound was a l r e a d y r e p o r t e d / ] I / a s a common pro- p e r t y o f a l a r g e number o f i n t e r m e t a l l i c i r o n compounds h a v i n g a m e t a l l i c - t y p e c o n d u c t i v i t y , and was o b s e r v e d , a l s o i n NiAs-type compounds 1121.
R e f e r e n c e s
/ 1 / F a t s e a s , G . A . , Dormann, J . L . , and Danot,M., J . P h y s i q u e C o l l o q .
37
(1976) C6-579.121 Danot, M., These DLcembre 1973, U n i v e r s i t 6 de Nantes.
/ 3 / Ahovandjinov, A., "Sur l e s systPmes formds en- t r e l e f e r , l e c o b a l t , l e n i c k e l e t l e s d i s c - l g n i u r e s de t i t a n e e t de zirconium",ThSse 13 J u i l l e t 1977, U n i v e r s i t 6 de Nantes.
/ 4 / Fatseas,G.A., J . Phys. E . S c i . I n s t r u m .
2
(1976) 1066.
/5/ S p e c t r a were a l s o o b t a i n e d a t 77K and 300K.
The same, a s f o r 4.2K, r e s u l t s were o b t a i n e d a t 77K (number o f s i t e s and i n t e n s i t i e s ) , a p a r t a mean 10 Z d e c r e a s e o f H, and a de- c r e a s e o f 6 ( 6 2 0.92
+
0 . 0 2 ) . At 300K, t h e s p e c t r a a r e s u p e r p o s i t i o n o f two d o u b l e t s , s i - m i l a r l y t o t h e r e s u l t s o b t a i n e d i n r e f e r e n c eI l l .
/ 6 / By a p p l i c a t i o n o f t h e known b i n o m i a l f o r m u l a ( s e e , f o r e x . , r e f e r e n c e / 7 / ) .
1 7 1 F a t s e a s , G . A . , Can. J . P h y s . 5 4 (1976) 1850.
1 8 1 T a k a h a s h i , T . , and Yamada,O., J . S o l i d S t a t e Chem.
L
( 1 9 7 3 ) , 25.191 Fatseas,G.A., Dormann,J.L., and B r o s s a r d , L . , J . P h y s i q u e C o l l o q .
32
(1971) C1-785./ l o / Regnard,J.R., Hocque Nghem,J.C., J. Physique C o l l o q .
32
(1971) C1-268./ 1 1 / Fatseas,G.A., Phys. Rev.
8
(1973) 43.1 1 2 1 F a t s e a s , G . A . , Dormann,J.L., D r u i l h e , R . , Bros- s a r d , L . , a n d G i b a r t , P . , P h y s i c a 86-88B (1977) 887.
