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VNU. JOURNAL OF SCIENCE. M athem atics - Physics, T.xx. N„3AP, 2004

Y T T R IU M IN S E R T IO N IN T O T H E C o S b 3 S K U T T E R Ư D IT E BY

H O T -P R E S S E D M ETH O D

Le V a n Vu

<i>D e p a rtm en t o f Physics, College o f Science, V N U</i>

A bstract: The yttrium fillin g skutterudite compound was prepared by hot-
p re s s e d m e th o d u n d e r p re s s u re o f 3 0 M P a. It w a s fo u n d th a t y ttriu m a to m s cou ld
b e in s e rte d in th e v a c a n t s ite o f th e C o S b j skutteru dite . T h e c rysta l s tru c tu re of
<i>resulting Y,Co4Sb,2 (x = 0 * 0.15) was refined by the Rietveld refinement of the </i>
X -ra y d iffra c tio n d a ta .

1. I n t r o d u c t i o n

S k u tte ru d ite -ty p e s tru c tu re is a cubic stru ctu re w ith th e space group Im-3 composed
of eig h t co rn er-shared MXg octa h ed ra (M=Co, Fe, Ru, Os; X = p , As, Sb). The stru ctu re is
closely re la ted to th e perovskite-type stru ctu re . The linked octahedra produces a void a t the
ce n ter o f (MXn)s — c lu s te r an d th is void (vacant site) occupies a body-centered position of
th e cubic lattice. T h is void is larg e enough to accom m odate large m etal atom s, resulting in
th e form ation of filled- s k u tte ru d ite s tru ctu re s w ith th e general form ula RMịXị., (Sn, Ce,
P r)[l]. R ecently, th ese com pounds a re highlighted a s therm oelectric conversion m aterials
w ith high conversion efficiency. Generally, these com pounds show very high c a rrie r
m obilities an d large therm oelectric power. If lowering the th erm a l conductivity of the
s k u tte ru d ite s com pounds w ith a m inim al reduction in th e electrical properties can be
achieved, high ZT values m ig h t be possible. In the filled s k u tte ru d ite s com pounds, loosely
bound R atom s h av e u n u su ally large therm al vibration am plitudes. Consequently,
re m a rk a b le reduce in th e rm a l conductivity is expected by strong phonon s ca tte rin g [2],

In previous stu d y [3], we have synthesized th e filling s k u tte ru d ite YxCo4Sb12
com pounds by inductive m elting followed by sintering in vacuum an d have investigated the
solubility lim it of Y on th e s e com pounds. In this study, y ttrium atom insertion in th e CoSb;,
lattice has been perform ed u n d e r hot-pressed conditions. We have investigated the
stru c tu ra l of th e gen e ral form YxCo4S b12 (x=0-0.15). T he Rietveld refinem ent m ethod was
used for refining s tru c tu re p a ra m e te rs an d for exam ining y ttriu m distrib u tio n in CoSb3
stru ctu re .

2. E x p e r i m e n t a l

Sb (99.9999% pure),Y (99.9% pure) shots an d Co (99.99% pure) pow der w ere used as
sta rtin g m aterials. P olycrystalline CoSb3 ingots w ere prepared a s follows. Stoichiom etric
m ixture of cobalt a n d an tim o n y (Co:Sb = 1:3) w as loaded into q u a rtz am poules. These
am poules w ere sealed u n d e r vacuum 10'5 mmHg. They w ere hea ted in a furnace a t 600"C
for 24 h an d quenched to room tem p eratu re.

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<i>Yttrium insertion into the CoSb3 skutterudite by...</i>

153

Y t t r i u m a t o m i n s e r t i o n i n t h e C o S b3 h o s t l a t t i c e w a s d o n e b y t w o s t e p s : t h e f i r s t -
p r e p a r i n g Y3C o , p o w d e r b y d ừ e c t r e a c t i o n f r o m t h e c o b a l t a n d y t t r i u m l i q u i d i n i n d u c t i v e
f u r n a c e , t h e s e c o n d s t e p - r e a c t i o n f r o m t h e C o S b3 a n d Y ;,C o , p o w d e r b y h o t - p r e s s e d m e t h o d
( F i g . l a ) . P o w d e r o f Y2C o 3 w a s m i x e d w i t h C o S b 3 p o w d e r i n t h e m o l a r r a t i o c o r r e s p o n d i n g to
t h e s t o i c h i o m e t r i c f i l l e d s k u t t e r u d i t e c o m p o s i t i o n Y „ C o .|S b|2 ( x = 0 , 0 .0 2 5 , 0 .0 5 , 0 .0 7 5 , 0 . 10 ,
0 .1 5 ) . T h e m i x e d p o w d e r w a s g r o u n d a n d p u t i n t o a c y l i n d r i c a l g r a p h i t e m o u l d ( F i g . l b ) . T h e
r e a c t i o n s w e r e c a r r i e d o u t a t p r e s s u r e o f 3 0 M P a u s i n g a c o n v e n t i o n a l h o t - p r e s s . T h e
r e a c t i o n t e m p e r a t u r e a n d t h e d u r a t i o n w e r e f i x e d t o 6 5 0 ° c a n d 1 5 m i n .

F i g . l . T h e c o n v e n tio n a l h o t- p r e s s (a ) a n d th e c y lin d r ic a l g r a p h i t e m o u ld (b)

3 . R e s u l t s a n d d i s c u s s i o n

S i n g l e - p h a s e C o S b3 w i t h t h e c u b ic s k u t t e r u d i t e - t y p e s t r u c t u r e w a s o b t a i n e d . T h e
l a t t i c e p a r a m e t e r w a s d e t e r m i n e d t o b e a = 9 . 0 2 8 A ", w h i c h w a s i n g o o d a g r e e m e n t w i t h
p r e v i o u s s t u d i e s [ 3 ,5 ]. F i g u r e 2 s h o w s t h e s u r f a c e m i c r o g r a p h o f C o S b3 s a m p l e . E D S
a n a l y s i s o f t h i s i m a g e ( F i g .3 - c u r v e l ) s h o w t h a t c o b a l t c o n c e n t r a t i o n o f t h e p a r t i c l e s w a s
n o t c h a n g e d , T h e r e a c t i o n o f c o b a l t p a r t i c l e s a n d a n t i m o n y l i q u i d w a s c o m p l e t e d . T h e c u r v e
2 o n t h e f ig .3 s h o w s t h e E D S s p e c t r u m o f t h e f i l l e d s k u t t e r u d i t e Y xC o4S b 12 ( x = 0 .1 5 ) .

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154

<i>Le Van Vu</i>

T h e r e f i n e m e n t b y R I E T A N 2 0 0 0
p r o g r a m [4] w a s c a r r i e d o u t f o r 5 2 r e f l e c t i o n s .
A n a l y s i s o f t h e c a l c u l a t e d p a t t e r n s s h o w t h a t
f i l l i n g t h e v o id w i t h Y i n t h e <i>2a</i> s i t e l e a d s to
e x t i n c t i o n o f l o w - a n g l e r e f l e c t i o n s (0 1 1 ) , (0 0 2 ),
( 1 1 2 ) . T h u s , i t c a n b e s t a t e d t h a t m o d e l B f i t s
e x p e r i m e n t a l d a t a c o n s i d e r a b l y b e t t e r t h e n
m o d e l A . T h e s t a t i s t i c p a r a m e t e r s s u c h a s a
g o o d n e s s - o f - f i t ( S ) w e r e 2 .4 7 a n d 1 .8 5 f o r t h e
m o d e l A a n d m o d e l B , r e s p e c t i v e l y . I n
c o n c l u s i o n , i t c a n b e a s s u m e d t h a t Y a t o m s
f i l l s t h e v o i d s i n t h e s k u t t e r u d i t e s t r u c t u r e .

4 . C o n c l u s i o n s

P o l y c r y s t a l l i n e s a m p l e s o f C o S b3 s k u t t e r u d i t e c o m p o u n d s w e r e p r e p a r e d b y m e l t i n g
t h e a p p r o p r i a t e q u a n t i t i e s o f c o m p o n e n t e l e m e n t s . Y t t r i u m a t o m i n s e r t i o n i n t h e C o S b3
l a t t i c e h a s b e e n p e r f o r m e d u n d e r h o t - p r e s s e d m e t h o d . W e h a v e i n v e s t i g a t e d t h e s t r u c t u r a l
o f t h e g e n e r a l f o r m Y „C o4S b |9 ( x = 0 - 0 .1 5 ) . S b Y p h a s e w a s n o t f o r m e d u n d e r h o t - p r e s s e d
c o n d i t i o n s . T h e R i e t v e l d r e f i n e m e n t m e t h o d w a s c o n f i r m e d t h a t Y a t o m s f i l l s t h e v o i d s i n

t h e s k u t t e r u d i t e s t r u c t u r e .

T h i s w o r k w a s c o m p l e t e d w i t h f i n a n c i a l s u p p o r t f r o m t h e N a t u r a l S c i e n c e P r o g r a m o f
V i e t n a m N a t i o n a l U n i v e r s i t y , H a n o i ( C o d e Q T - 0 4 - 0 6 ) .

R e f e r e n c e s

1. T . C a i l l a t e t a l., <i>J.Appl.Phys.</i> 8 0 , ( 1 9 9 6 ). p .4 4 4 2 .
2 . H . T a k i z a w a e t a l . , <i>J . Alloys and Compounds.</i> 2 2 2 ( 1 9 9 9 ) p .7 9 .

3 . L e V a n V u , <i>Proceedings o f the 4'h A SE A N Microscopy Conference and the 3th Vietnam </i>
<i>Conference on Electron Microscopy,</i> 2 0 0 4 , H a n o i , V ie n a m . p . 1 8 3 .

4 . F .I z u m i, in R .A Y o u n g ( E d .) . T h e R i e tv e ld M e th o d , c h .1 3 , <i>Oxford University Press, Oxford, </i>

1 9 9 3 .

5 . T .S c h m id t , G .K lic h e , H .D . L u t z , <i>Acta Crystallogr. Sect.</i> C 4 3 , 1 9 8 7 , p . 1 6 7 8 .

1'

I

F i g .4 . X RD p a t t e r n o f t h e s a m p le x = 0 .0 7 5 :
(a) t h e s o lid p o in ts a r e t h e e x p e r im e n ta l d a ta ,

w h ile s o lid lin e r e p r e s e n t s a f itti n g w ith
R ie tv e ld m e th o d e ,( b ,c ) d if f e r e n ti a l lin e fo r

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