Henry Krumb School of Mines, Columbia University, New York ... - HAL

ANELASTIC AND DIELECTRIC RELAXATION OF SCANDIA-DOPED CERIA W.-K. Lee, R. Gerhardt, A. Nowick

To cite this version: W.-K. Lee, R. Gerhardt, A. Nowick. ANELASTIC AND DIELECTRIC RELAXATION OF SCANDIA-DOPED CERIA. Journal de Physique Colloques, 1987, 48 (C8), pp.C8-251-C2-256. .

HAL Id: jpa-00227139 https://hal.archives-ouvertes.fr/jpa-00227139 Submitted on 1 Jan 1987

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JOURNAL DE PHYSIQUE Colloque C8, supplément au n°12. Tome 48, décembre 1987

C8-251

ANELASTIC ftND DIELECTRIC RELAXATION OF SCANDIA-DOPED CERIA

W.-K.

LEE,

R.

GERHARDT ( 1 )

Henry Krumb School NY 10027, U.S.A.

and A.S.

of Mines,

NOWICK

Columbia University,

New York,

Résumé - Les résultats obtenus avec 1'oxide de cérium (CeC^ contenant des ions Se (pour des concentrations jusque 1 mole % SC2O3) illustrent l'utilisation simultanée des mesures de relaxation anélastique et de relaxation diélectrique pour l'étude des défauts ponctuels dans les cristaux isolants. On observe les deux types de pics de relaxation dans deux domaines de température. Les relaxations à "basses températures" (100-150 K) sont dues aux ions Se isolés mais en configuration non-centrée. Dans le domaine des "hautes températures" (165-450 K) , on observe plusieurs pics de relaxation, dont l'un est dû à la paire Sc-V (V représentant une lacune d'oxygène ionisé), et les autres sont attribués à des "clusters" plus importants d'ions Sc +3 "

A b s t r a c t - The combined use of a n e l a s t i c and d i e l e c t r i c r e l a x a t i o n t e c h n i q u e s t o s t u d y p o i n t d e f e c t s i n i n s u l a t i n g c r y s t a l s i s i l l u s t r a t e d by a s t u d y of Sc -doped c e r i a (Ce0 ? ) f o r c o n c e n t r a t i o n s up t o 1 mole % SCgO,. Relaxation peaks of both k i n d s a r e found i n two r a n g e s . I n t h e " l o w - t e m p e r a t u r e " r e g i o n (100-150 K) observed r e l a x a t i o n s a r e due t o an i s o l a t e d S c - i o n i n an off-center configuration. In t h e " h i g h - t e m p e r a t u r e " r e g i o n (165-450 K) a s e r i e s of r e l a x a t i o n peaks a r e found, one of which i s due t o Sc-V p a i r s (where V r e p r e s e n t s an oxygen-ion v a c a n c y ) , w h i l e t h e o t h e r s a r e due t o h i g h e r S c - i o n clusters.

INTRODUCTION A r e c e n t paper a t t h e c o n f e r e n c e ICIFUAS-8 / 1 / d e a l t w i t h t h e a d v a n t a g e s of measuring s i m u l t a n e o u s l y both d i e l e c t r i c and a n e l a s t i c r e l a x a t i o n i n t h e s t u d y of p o i n t d e f e c t r e l a x a t i o n phenomena i n i o n i c m a t e r i a l s . That a r t i c l e a l s o reviewed a s e r i e s of c a s e s f o r which such s t u d i e s have been made. I n c l u d e d among them was a l o w - t e m p e r a t u r e r e l a x a t i o n i n Sc -doped e e r i a (CeO ) . The p r e s e n t paper d e a l s w i t h t h i s system i n g r e a t e r d e t a i l w i t h s p e c i a l emphasis on t h e h i g h e r t e m p e r a t u r e r e l a x a t i o n s (between 165 and 150 K) where a complex s e r i e s of peaks i s o b s e r v e d . We hope t o show what kind of i n f o r m a t i o n can be o b t a i n e d by t h e combined u s e of b o t h d i e l e c t r i c and a n e l a s t i c r e l a x a t i o n , and how much more i n f o r m a t i v e t h i s i s t h a n t h a t which could be o b t a i n e d from i n t e r n a l f r i c t i o n a l o n e .

(1)

Present address : Center for Ceramics Research, Rutgers University. Piscataway. NJ 08854. U.S.A.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987835

^-I&---- --

JOURNAL DE PHYSIQUE

Fig. 1 . Diagram of one-eighth of t h e u n i t c e l l f t h e f l u o r i t e s t r u c t u r e , showing a s u b s t i t u t i o n a l M3' c a t i o n and an oxygen vacancy a s a n e a r e s t neighbor.

0Oxygen--

/'

vacancy

/

r

@

~ o p o n lt M'+)

BACKGROUND

Cerium dioxide possesses t h e cubic f l u o r i t e s g u c t u r e which can be i o n a t t h e c o r n e r s , and regarded a s made up of small cu&s, each with an 0 every a l t e r n a t e one having a Ce a t i t s c e n t e r . The u n i t ( f c c ) c e l l is then made up of 5 i g h t of t h e s e small cubes. When t h e c r y s t a l is doped with a trivalent M dopant (MOT Y, Gd, La o r S c ) , t h e dopant e n t e r s t h e l a t t i c e s u b s t i t u t i o n a l l y f o r Ce thus r e q u i r i n g charge compensation. This occurs through t h e f rmation of one oxygen vacancy ( V ) f o r every two dopant ions. I n t h e case of Y9+ doping, which has been most e x t e n s i v e l y s t u d i e d , we have found t h a t , r a t h e r than forming t h e n e u t r a l Y2V t r i p l e t d e f e c t , t h e Y ions remain randomly dispersed. Thus, t h e r e is present i n t h e c r y s t a l an a r r a y c o n s i s t i n g of equal number of Y-V p a i r s , which c a r r y an e f f e c t i v e charge of + e , and i s o l a t e d Y i o n s c a r r y i n g t h e charge -e. The reason f o r t h i s d e f e c t s t r u c t u r e is t h a t c a t i o n mobility is n e g l i g i b l e below 1000°C, s o t h a t a random Y d i s t r i b u t i o n is f r o z e n i n ; on t h e o t h e r hand, oxygerl vacancies remain mobile down t o much lower temperatures. /2/ The Y-V p a i r c o n s t i t u t e s both an e l e c t r i c and an e l a s t i c d i p o l e and t h u s g i v e s r i s e t o both d i e l e c t r i c and a n e l a s t i c r e l a x a t i o n peaks. The d e f e c t is represented by t h e 8 - p o ~ i t i o n ~ g o d e l shown i n Fig. 1 , i . e . , t h e vacancy can r e o r i e n t among e i g h t equivalent 0 s i t e s about t h e dopant ion. It has been shown /3/ t h a t d i f f e r e n t r e l a x a t i o f modes a r e involved i n t h e two'cases, such t h a t t h e two r e l a x a t i o n r a t e s , T , a r e r e l a t e d by:

-

This factor-of-two

r a t i o has been well demonstrated f o r Y-doped c e r i a /4/.

The case of sc3+ doping, because of t h e smaller i o n i c r a d i u s , (0.73 A a s vs. 0.89 A f o r y3+) seems t o be q u i t e d i f f e r e n t , however. F i r s t , i t was shown by e l e c t r i c a l measurements /5/ t h a t t h e Sc-V p a i r has a much higher a s s o c i a t i o n energy than t h e Y-V p a i r (0.67 a s a g a i n s t 0.20 eV). Secondly, while t h e Y-doped c a s e shows n e a r l y s i n g l e Debye r e l a x a t i o n s up t o 1 mole % Y 0 , doping with 1 mole % Sc20 g i v e s much more complex r e l a x a t i o n behavior. I$ ?s t h e purpose of t h e presen2 paper t o analyze t h i s behavior. A f u l l e r treatment w i l l be found elswhere. /6/ METHODS Samples were prepared by mixing and s i n t e r i n g of t h e sample oxides. I n t e r n a l f r i c t i o n was measured f o r f l e x u r a l v i b r a t i o n of samples a t a f i x e d frequency using e l e c t r o s t a t i c e x c i t a t i o n and d e t e c t i o n . D i e l e c t r i c l o s s ( t a n 6 ) was measured a t f r e q u e n c i e s of 1 and 5 kHz with a conventional capacitance bridge. For f u r t h e r d e t a i l s about methods s e e r e f e r e n c e /6/.

RESULTS AND DISCUSSION Relaxation behavior is observed i n two temperature r e g i o n s , a s shown i n Fig. 2, which is a p l o t of both i n t e r n a l f r i c t i o n and d i e l e c t r i c l o s s a s a f u n c t i o n of 1 / T f o r t h e sample with 0.3 mole % Sc 0 [Henceforth, % w i l l always mean mole %.I The region between 100 and 1803K is h e r e r e f e r e d t o a s t h e lTlow-temperature r e g i o n n , while t h a t from 165 t o 450 K i s t h e "high-temperature regionv. I t is c l e a r from Fig. 2 t h a t both types of measurem5nt.s give r i s e t o r e l a x a t i o n i n each of t h e s e r e g i o n s . The Low-Temperature Region A well defined peak was found near 130 K f o r both a n e l a s t i c and d i e l e c t r i c measurements. (See Fig.2.) The peak is s u f f i c i e n t l y c l o s e t o a s i n g l e Debye peak (about 20 t o 30% broader) t o suggest t h a t i t must have r e s u l t e d from a simple and well defined d e f e c t centejy. Th' peak absent i n Gd" o r La3'. Further samples doped with l a r g e r t r i v a l e n t c a t i o n s , e.g. s t u d i e s showed t h a t t h e peak is due t o i s o l a t e d Scrt i o n s i n an o f f - c e n t e r c o n f i g u r a t i o n /6/. This is c o n s i s t e n t with t h e f a c t t h a t t h e a c t i v a t i o n energy f o r r e l a x a t i o n is low (- 0.20eV) and t h a t t h e estimated d i p o l e s e p a r a t i o n is l e s s than 1 A ( i . e . t h e motion involved is l e s s than one atomic d i s t a n c e ) . I n a d d i t i o n , t h e d i f f e r e n c e of t h e peak l o c a t i o n s between t h e d i e l e c t r i c and a n e l a s t i c c a s e s , a f t e r t a k i n g i n t o account t h e Crequency s h i f t of t h e peaks, s u g g e s t s t h a t they involve d i f f e r e n t r e l a x a t i o n a l modes c o n t r o l l e d by d i f f e r e n t atomic jumps. /1,3/ This is again c o n s i s t e n t with a low symmetry o f f - c e n t e r c o n f i g u r a t i o n . I n a c t computer s i m u l a t i o n calculation^ /7/ show t h a t i t is not t h e ScS*ion, but t h e cage of e i g h t surrounding oxygen ions t h a t is displaced from t h e r e g u l a r s i t e s , thereby c r e a t i n g a d e f e c t c o n f i g u r a t i o n of very low symmetry ( i . e . t r i c l i n i c ) .

400 L

I

300 I

20 0 1

150

125

I

I

Coo2 : 0.3% Sc203

0

DIELECTRIC ANELASTIC

Fig. 2. Variation of d i e l e c t r i c l o s s , t a n 6 , and i n t e r n a l f r i c t i o n , Q-', with r e c i p r o c a l a b s o l u t e temperature f o r an 0.3% Sc 0 sample showing t h e two temperat u r e regions. D i e l e c t r i c d a t a a r e a t 1 kHz an5 ? n t e r n a l f r i c t i o n a t 8.5 kHz. (A constant background of 1 x has been s u b s t r a c t e d from i n t e r n a l f r i c t i o n data).

JOURNAL DE PHYSIQUE

The High-Temperature Region Fig. 3 shows t h e d i e l e c t r i c l o s s d a t a f o r samples with 0.05, 0.3 and 1.0 mole% Sc20 from 165 t o 450 K . The dashed l i n e f o r t h e 0.05% sample i l l u s t r a t e s t h e temperJture s h i f t of t h e spectrum with a change i n frequency ( 1 kHz and 5 kHz). The peaks a r e l a b e l e d A , B and C i n o r d e r of i n c r e a s i n g temperature. While peaks A and C a r e q u i t e prominent, peak B, which is not d e t e c t a b l e f o r 0.05%, appears only a s a shoulder on t h e high-temperature s i d e of peak A. In a d d i t i o n , t h e main peak ( A ) is c l o s e t o a Debye peak f o r t h e 0.05% composition and is only s l i g h t l y broader f o r t h e 0 . 3 and 1%samples a f t e r peak B is s u b t r a c t e d out. Table 1 summarizes t h e d a t a f o r t h e k i n e t i c s and Table 2 g i v e s t h e magnitudes of t h e s e t h r e e peaks. R e s u l t s from a n e l a s t i c r e l a x a t i o n measurements i n t h e high-temperature A nearly region f o r 0.05, 0.3 and 1 mole % Sc 0 samples a r e shown i n Fig. 4. s i n g l e Debye peak was observed f o r tge30.05% Sc 0 sample. However, t h e peaks f o r t h e two h i g h e r c o n c e n t r a t i o n s a r e very broag 2nd t h e i r s t r u c t u r e s a r e more complicated than t h e corresponding d i e l e c t r i c peaks. In f a c t , i t is c l e a r t h a t t h e peak temperature of t h e s e broad peaks i n c r e a s e s with dopant concentration. Thus, tt38 higher temperature components of t h e s e peaks must grow non-linearly with Sc c o n c e n t r a t i o n i n order t o produce t h e observed s h i f t i n peak temperature.

Fig. 3.

Fig. 4.

.

Fig. 3. D i e l e c t r i c l o s s i n t h e high-temperature r e g i o n f o r c e r i a samples showing t h r e e peaks l a b e l e d A, B and C . c o n t a i n i n g 0.05, 0.3 and 1.0% Sc 0 A l l d a t a a r e f o r a frequency of $HZ, except f o r t h e dashed l i n e showing some of t h e d a t a a t 5 kHz f o r t h e 0.05% sample t o i l l u s t r a t e t h e peak s h i f t with frequency. Fig. 4. I n t e r n a l f r i c t i o n of c e r i a doped with 0.05, 0.3 and 1.0% Sc20 3 i n t h e 6 kHz f o r 0.05% and 8.5 kHz f o r t h e high-temperature region. Frequency

?

-

Table 1 . Activation e n t h a l p i e s , H R , and preexponentials, vO1 f o r t h e t h r e e d i e l e c t r i c r e l a x a t i o n peaks i n t h e high-temperature range,

Peak Peak Peak

A B C

(ev) 0.41 0.50 0.60

(1013 s e c - l ) 1.3 2.0 0.13

Table 2. Summary of h e i g h t s of t h e t h r e e d i e l e c t r i c r e l a x a t i o n peaks i n t h e high-temperature range.

Mole $ Sc-0,

Peak A

Peak B

Peak C

I n Y- doped c e r i a , both a n e l a s t i c and d i e l e c t r i c measurements showed almost a s i n g l e Debye r e l a x a t i o n peak, which was a t t r i b u t e d t o Y-V p a i r s , f o r dopant c o n c e n t r a t i o n s < 1 mole 8 . /4,8/ By analogy, t h e r e should be a d i e l e c t r i c r e l a x a t i o n peak due t o Sc-V p a i r s i n Sc-doped samples. Since t h e low temperature peak was a t t r i b u t e d t o i s o l a t e d s c 3 + , t h e r e l a t i v e l y clean r e s u l t of d i e l e c t r i c peak A f o r 0.05% Sc20 suggests t h a t i t i s t h e only reasonable candidate f o r t h e Sc-V p a i r r e l z x a t i o n . The idea is a l s o c o n s i s t e n t with t h e observation t h a t t h e height of peak A i n double-doped samples (Sc 0 + Y 0 ) is g r e a t e r than t h a t with Sc 0 a l o n e / 6 / . This is explained by t g e 3 sca6eJging e f f e c t of Sc i o n s f o r oxy$eJ vacancies from o t h e r dopants ( d u e t o t h e e x c e p t i o n a l l y high Sc-V a s s o c i a t i o n energy), a s f i r s t demonstrated by c o n d u c t i v i t y experiments./5/ Another s t r o n g evidence f o r assigning peak A t o Sc-V p a i r s i s obtained by comparing t h e r e l a t i v e p o s i t i o n s of both t h e d i e l e c t r i c and a n e l a s t i c peaks. Using 0.41 eV f o r t h e a c t i v a t i o n energy of peak A (Table 1 ) t o g e t h e r with Eq. ( I ) , t h e l o c a t i o n of the corresponding a n e l a s t i c peak (measured a t 6.2 kHz) is p r e d i c t e d t o be a t 1000/T = 4.12 (243 K). This i s very c l o s e t o where t h e a n e l a s t i c peak f o r 0.05% Sc 0 a c t u a l l y f a l l s (Fig 4) and t h u s f u r t h e r confirms t h a t peak A i s indeed dug Sc-V pairs.

20

I t is worth noting t h a t d i e l e c t r i c peak A i n c r e a s e s more slowly than l i n e a r l y with dopant c o n c e n t r a t i o n (Table 2 ) . This l e a d s t t h e conclusion t h a t even f o r 0.3% dopant, a d d i t i o n a l c l u s t e r s involving Sc3+ a r e p r e s e n t . The o t h e r d i e l e c t r i c a k s , B and C , a r e then a t t r i b u t e d t o higher d e f e c t c l u s t e r s involving scg' ions. The reason f o r t h e d i f f e r e n c e s between t h e a n e l a s t i c and d i e l e c t r i c r e l a x a t i o n s p e c t r a ( F i g s . 3 and 4 ) may be t h a t some of t h e higher c l u s t e r s have no net e l e c t r i c d i p o l e moment and a r e t h e r e f o r e not detected i n d i e l e c t r i c measurement, but they may s t i l l possess n e t e l a s t i c d i p o l e s t r e n g t h and t h u s c o n t r i b u t e t o t h e a n e l a s t i c s p e c t r a . The p r e s e n t s t u d y , however. does not g i v e any c l u e a s t o which higher c l u s t e r s of Sc-ions a r e s t a b l e . Computer modeling may shed l i g h t on t h i s problem. F i n a l l y , t h e reasons f o r t h e d i f f e r e n c e s between y3+ and s c 3 + doped samples is worth d i s c u s s i n g . I t was a l r e a d y mentioned t h a t t h e c a t i o n mobility is n e g l i g i b l y small below 1000°C whil?+oxygen ion vacancies remain mobile doping, t h e c o n d u c t i v i t y and down t o much lower temperatures. For Y r e l a x a t i o n behavior were s u c c e s s f u l l y n t e r p r e t e d under t h e assumption t h a t e s s e n t ' a l l y a random d i s t r i b u t i o n of Y3* ions is frozen i n below 100OoC. /2/ For SC" doping, t h e r e s e n t observations i n d i c a t e t h a t complex c l u s t e r s involving m u l t i p l e Scg+ ions must form above 1000°C, where t h e c a t i o n s a r e s t i l l mobile. These c l u s t e r s a r e then f r o z e n i n t o t h e s t r u c t u r e and thus g i v e r i s e t o t h e observed complex d i e l e c t r i c and a n e l a s t i c r e l a x a t i o n s p e c t r a .

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JOURNAL DE PHYSIQUE

I n conclusion, we have shown a wide v a r i e t y of d i e l e c t r i c and a n e l a s t i c r e l a x a t i o n peaks i n scandia-doped c e r i a . The low-temperature peaks were due t o an i s o l a t e d Sc-ion i n an off-center type c o n f i g u r a t i o n , i n which, p r i m a r i l y , t h e cage of surrounding oxygen i o n s is d i s t o r t e d . The uhigh-temperaturew peaks on t h e o t h e r hand, include t h o s e due t o simple Sc-V p a i r s a s well a s s e v e r a l a d d i t i o n a l r e l a x a t i o n s due t o higher Sc-ion c l u s t e r s t o g e t h e r with oxygen-ion vacancies. ACKNOWLEDGMENTS The authors a r e g r a t e f u l t o t h e U.S. Department of Energy f o r support of t h i s work. REFERENCES / 1 / Nowick, A.S., J. de Physique 5 (1875) C10-507. /2/ Nowick, A.S., i n Diffusion i n C r y s t a l l i n e S o l i d s , ed. Murch, G.E. and Nowick, A.S., Academic Press, Orlando (1984). Chapter 3. /3/ Nowick, A.S. and Berry, B.S., A n e l a s t i c Relaxation i n C r y s t a l l i n e S o l i d s , Academic Press, N.Y. (1972), Chapters 8 and 11. / 4 / Wang, Da Yu and Nowick, A.S., J. Phys. Chem S o l i d s fi (1983) 639. /5/ Gerhardt-Anderson, R. and Nowick, A.S., S o l i d S t a t e I o n i c s 5 (1981) 547. /6/ Gerhardt, R., Lee, W-K and Nowick, A.S., J. Phys. Chem. S o l i d s (1987) 563. /7/ Gerhardt-Anderson, R., Zamani-Noor, F., Nowick, A.S.. Catlow, C.R.A. and Cormack, A.N., S o l i d S t a t e I o n i c s 9/10 (1983) 931. /8/ Anderson, M.P. and Nowick, A.S., J. de Physique 2 (1981) C5-823.

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