MÖSSBAUER STUDY OF Fe(II) DOPED ALKALI CHLORIDE CRYSTALS AND DEFECT STRUCTURE

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MÖSSBAUER STUDY OF Fe(II) DOPED ALKALI CHLORIDE CRYSTALS AND DEFECT STRUCTURE

Y. Takashima, N. Kai, T. Nishida, L. Chandler

To cite this version:

Y. Takashima, N. Kai, T. Nishida, L. Chandler. MÖSSBAUER STUDY OF Fe(II) DOPED ALKALI

CHLORIDE CRYSTALS AND DEFECT STRUCTURE. Journal de Physique Colloques, 1979, 40

(C2), pp.C2-566-C2-567. �10.1051/jphyscol:19792197�. �jpa-00218575�

MOSSBAUER STUDY OF Fe(ll) DOPED ALKALI CHLORIDE CRYSTALS AND DEFECT STRUCTURE

Y. Takashima, N. Kai, T. Nishida and L. Chandler*

Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Fukuoka 812, Japan

Abstract.- By incorporating small amounts of iron(II) in alkali chlorides and using X-ray diffraction measurements and absorption Mossbauer spectroscopy, the behavior of the system especially related to

its defect structure was studied. In the LiCl(Fe) system, only single line which was interpreted as being due to the substitution of F e2 + for Li+ was observed in the Mossbauer spectrum. In the NaCl(Fe) system, one single line and two doublets were observed. One of doublets was assigned to aggregates.

No single line was observed for the other alkali chlorides. The doublets observed for CsCl(Fe) system were assigned to be CS3FeCls and Cs2FeCK .

1. Introduction.- Several studies of iron impurity in alkali halide by using the Mossbauer effect have been done /1-5/. However, almost all of previous studies are associated with Co-doped crystals from which one can observe the after effect of the elec- tron capture decay of Co in solid. The present study is concerned with the behavior of a few atomic percent of iron(II) in alkali chlorides using mainly absorption Mossbauer spectroscopy.

2. Experimental•- A mixture of weighed quantities of alkali chloride and iron(II) chloride were put in a quartz crucible and fused in an electric furnace under a nitrogen atmosphere at 800°C for 2 hours.

The melt was then quenched quickly by compressing it between two copper plates. The samples were always kept in a dry atmosphere. The Mossbauer spectral measurements were carried out with a conventional spectrometer operating in the time mode. The veloci- ty scale was calibrated with a metallic iron foil absorber and the isomer shifts are referred to the centroid of the spectrum. The spectra obtained were fitted using a least squares iteration procedure.

X-ray diffraction measurements were performed with iron filtered Co-K radiations. The concentration of doped iron in alkali chloride was determined by the EDTA titration method. Annealing experiments were also carried out in an electric furnace under a ni- trogen atmosphere.

3. Results and discussion.- In LiCl(Fe) system, only

a single line with the isomer shift characteristic of ionic ferrous iron (<5 = 1.13 ram/s) was obtained for an iron concentration of less than 3.1 at.%

(Fig. la).

A new doublet line which appeared with increa- sing iron concentration up to 3.9 at.% was attribu- ted to the innermost part of Zeeman splitting due to the a-hematite species. This means that excess iron in the sample reacted with a small amount of oxygen.

This single line mentioned previously is inter- preted as arising from a divalent iron ion substi- tuted for a lithium ion located in a perfectly cubic site with no vacancies nearby. This is justified by the fact that there is no change in the X-ray dif- fraction lines between the pure and doped lithium chlorides. Although the spectrum for this system was measured both at liquid nitrogen and helium tempera-

Fig. 1 : Mossbauer spectra of LiCl(Fe) (a) and NaCl(Fe) (b) systems at room temperature.

iPresent address : Radiation Science Department, Rutgers University, New Brunswick, N.J., U.S.A.

JOURNAL DE PHYSIQUE Colloque C2, supplément au n° 3, Tome 40, mars 1979, page C2-566

Résumé.- Une petite quantité de fer(II) a été incorporée dans des chlorures alcalins; le comportement de ces systèmes, en particulier ce qui résulte des défauts de structure, a été étudié par rayons X et spectrométrie Mossbauer. Dans LiCl(Fe), une raie unique a été interprétée comme étant due à la substitution de Li par F e2 +. Dans NaCl(Fe), on observe une raie simple et deux doublets, dont un dû à l'existence d'agrégats. Les autres systèmes ne présentent que des doublets attribués à Cs3FeCls et Cs2FeCU dans CsCl(Fe).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792197

t u r e , l i t t l e change was o b s e r v e d . Moreover, i t seems t h a t f e r r o u s i o n s i n t h e c r y s t a l a r e f a i r l y s t a b l e t o h e a t t r e a t m e n t a t 200°C.

I n NaCl(Fe) s y s t e m c o n t a i n i n g a few a t . % o f i r o n , t h r e e l i n e s were o b s e r v e d ( F i g . I b ) . They a r e s i n g l e l i n e s ( 6 = 1.12 m / s ) , t h e one d o u b l e t w i t h l a r g e s p l i t t i n g ( 6 = 1.13 m / s , A = 2.33 mm/s) and t h e o t h e r d o u b l e t w i t h s m a l l s p l i t t i n g ( 6 = 1.09 mm/s, A = 1.01 mm/s).

A c c o r d i n g t o t h e X-ray d i f f r a c t i o n p a t t e r n , a weak s h o u l d e r peak was o b s e r v e d o n t h e h i g h e r a n g l e s i d e o f e a c h d i f f r a c t i o n l i n e of t h e doped sodium c h l o r i d e . T h i s means t h a t t h i s sample h a s a d i f f e - r e n t phase h a v i n g t h e pseudo NaCl s t r u c t u r e , i n which t h e l a t t i c e c o n s t a n t i s somewhat s m a l l e r t h a n t h a t o f p u r e NaC1. T h i s f a c t was 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 a wavy e x t i n c t i o n by means o f t h e p o l a r i z i n g m i c r o s c o p i c t e c h n i q u e . T h e r e f o r e , t h e s i n g l e peak i s i n t e r p r e t e d a s t o b e c a u s e d by t h e d i v a l e n t i r o n i o n s s u b s t i t u t e d f o r sodium i o n s j u s t a s i n t h e c a s e of l i t h i u m c h l o r i d e s y s t e m . A d o u b l e t w i t h l a r g e q u a d r u p o l e s p l i t t i n g i s a s s i g n e d t o b e t h e a g g r e g a t e h a v i n g pseudo NaCl s t r u c t u r e i n t h e h o s t m a t r i x , b e c a u s e i t s i n t e n s i t y i s d e p e n d e n t upon t h e i r o n c o n t e n t i n t h e sample.

The o r i g i n of t h e o t h e r d o u b l e t w i t h s m a l l s p l i t t i n g i s n o t c l e a r . But a p o s s i b l e s i t e c o u l d b e i n t e r s t i t i a l i n sodium c h l o r i d e which h a s a t e t r a h e - d r a l symmetry.

I n i s o t h e r m a l a n n e a l i n g f o r NaCl(Fe) s y s t e m , t h e s p e c t r a l c h a n g e s t a k e p l a c e i n t h e v e r y e a r l y s t a g e o f a n n e a l i n g s u c h a s 2 0 0 ' ~ - I h . At f i r s t , t h e weak d o u b l e t d i s a p p e a r e d and a new l i n e due t o t h e t r i v a l e n t i r o n s p e c i e s a p p e a r e d . On c o n t i n u i n g t h e a n n e a l i n g , t h e new l i n e w i t h a l a r g e q u a d r u p o l e s p l i t t i n g ( 6 = 1.21 mm/s, A = 3.01 m l s ) grew up a t t h e e x p e n s e o f t h e o r i g i n a l s i n g l e l i n e and t h e dou- b l e t .

I n t h e l i g h t o f i t s l a r g e q u a d r u p o l e s p l i t t i n g and i n c r e a s e d isomer s h i f t v a l u e s , t h e new l i n e could be e x p l a i n e d by t h e c r e a t i o n o f a new s p e c i e s o f d i v a l e n t i r o n accompanied by n e g a t i v e l y c h a r g e d va- c a n c y . Namely, t h e f o r m a t i o n o f a n iron-vacancy com- b i n a t i o n r e s u l t s i n d i s t o r t i o n o f t h e c u b i c symmetry i n t h e environment o f t h e f e r r o u s i o n s , t h u s c a u s i n g a n e l e c t r i c f i e l d g r a d i e n t . I n a d d i t i o n , t h e c h a r g e d e n s i t y a t t h e n u c l e u s a l s o w i l l b e c h a n g e d , b e c a u s e t h e c a t i o n vacancy h a s a n e f f e c t i v e n e g a t i v e c h a r g e and e l e c t r o n s t e n d t o spend more o f t h e i r t i m e away from t h e v a c a n c y / I

1 .

F i v e h o u r s a n n e a l i n g was needed f o r c o m p l e t e

c o n v e r s i o n o f t h e d i v a l e n t s p e c i e s t o t h e t r i v a l e n t s p e c i e s . T h i s t r i v a l e n t s p e c i e s i s i d e n t i f i e d l a t e r t o a - h e m a t i t e . X-ray d i f f r a c t i o n measurements a l s o were c a r r i e d o u t f o r t h e sodium c h l o r i d e s y s t e m w i t h d i f f e r e n t a n n e a l i n g t i m e s . The r e s u l t s show t h a t t h e s h o u l d e r peak p r e v i o u s l y mentioned d i s a p p e a r e d a s t h e a n n e a l i n g p r o c e e d s . T h i s i s a t t r i b u t e d t o t h e d e c o m p o s i t i o n o f t h e a g g r e g a t e o f t h e m e t a s t a b l e pseudo NaCl s t r u c t u r e .

The marked d i f f e r e n c e i n t h e o t h e r a l k a l i c h l o - r i d e s y s t e m s compared t o l i t h i u m and s o l i u m c h l o r i - d e s i s t h a t a c e n t r a l s i n g l e peak was n o t o b s e r v e d . T h i s f a c t i s u n d e r s t a n d a b l e by t a k i n g i n t o a c c o u n t t h e i o n i c s i z e s o f t h e s e heavy a l k a l i s . I n KCl(Fe) s y s t e m , t h e r e e x i s t s two k i n d s o f f e r r o u s a g g r e g a t e s and a s m a l l amount o f o x i d i z e d s p e c i e s . I n RbCl(Fe) s y s t e m , t h e r e i s o n l y one t y p e o f f e r r o u s a g g r e g a t e and some o x i d i z e d s p e c i e s . Chemical forms of t h e s e o x i d i z e d s p e c i e s have n o t y e t b e e n i d e n t i f i e d .

I n CsCl(Fe) s y s t e m , t h e s p e c t r a of two k i n d s of d o u b l e t s and of t h e a d d i t i o n a l d o u b l e t due t o t h e o x i d i z e d s p e c i e s w e r e o b t a i n e d a t l i q u i d n i t r o g e n t e m p e r a t u r e . The f o r m e r d o u b l e t s a r e a s s i g n e d a s Cs3FeCls and Cs2FeClr r e s p e c t i v e l y from t h e r e s u l t o f X-ray d i f f r a c t i o n measurement.

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