GRAIN BOUNDARY STRUCTURE IN NiO

G R A I N BOUNDARY STRUCTURE IN N ~ O *

K.L. Merkle, J.F. Reddy and C.L. Wiley

MateriaZs Science and Technology Division, Argonne National Laboratory, Argonne,

I L 60439, U.S.

A.

Rhsum6 -

Microscopic

Q l e c t r o n i q u e par t r a n s m i s s i o n e s t u t i l i s g e pour 2 t u d i e r la s t r u c t u r e d e s j o i n t s de g r a i n s dans NiO. En g i n g r a l , on a

t r o u v i que l e s j o i n t s de f l e x i o n <001> s o n t f a c e t t i s . Les p i r i o d i c i t i s s t r u c t u r a l e s dans les f a c e t t e s ont 6 t 6 observees pour l e s j o i n t s a' f a i b l e e t grande d i s o r i e n t a t i o n s . Une d e n s i t i atomique r e ' d u i t e e f f e c t i v e dans une r6gion de 0.5-0.8

nm en l a r g e u r au j o i n t de g r a i n s e s t sugg6r6e par l e

comportement de c o n t r a s t e d i f o c a l i s i des j o i n t s de f l e x i o n e t de t o r s i o n . A b s t r a c t - Transmission e l e c t r o n microscopy is used t o s t u d y t h e s t r u c t u r e of g r a i n boundaries (GBs) i n NiO. <001>

t i l t

GBs i n

N i O

a r e g e n e r a l l y found t o be f a c e t e d . S t r u c t u r a l p e r i o d i c i t i e s w i t h i n f a c e t s have been observed a t low and h i g h m i s o r i e n t a t i o n . An e f f e c t i v e reduced atomic d e n s i t y a t t h e GB w i t h i n a r e g i o n of 0.5-0.8

nm

i n width is suggested by t h e defocus c o n t r a s t behavior of high-angle

t i l t

and t w i s t GBs.

1

- INTRODUCTION

The p r o p e r t i e s of most ceramic m a t e r i a l s s t r o n g l y depend on t h e g r a i n boundary (GB) s t r u c t u r e and p r o p e r t i e s . N e v e r t h e l e s s , o u r knowledge of GBs i n ceramics i s q u i t e l i m i t e d and few s t u d i e s e x i s t of t h e m i c r o s t r u c t u r e of high-angle GBs i n t r a n s i t i o n m e t a l o x i d e s . For t h e most p a r t t h e p r e s e n t i n v e s t i g a t i o n c o n s i d e r s

tilt GBs

i n

N i O

b i c r y s t a l s w i t h a m i s o r i e n t a t i o n a x i s n e a r <001>. Conventional t r a n s m i s s i o n e l e c t r o n microscopy (TEM) _is used t o o b s e r v e t h e s t r u c t u r e of GBs f o r d i f f e r e n t boundary p l a n e s and m i s o r i e n t a t i o n a n g l e s

8.

Q u e s t i o n s of i n t e r e s t concern t h e boundary width, boundary f a c e t i n g and s t r u c t u r a l p e r i o d i c i t i e s w i t h i n t h e f a c e t s . The o u t s t a n d i n g f e a t u r e of a l l our high-angle boundaries observed s o f a r i s t h e i r defocus c o n t r a s t behavior, which i m p l i e s a c o n s i d e r a b l e degree of openness of t h e

GB

s t r u c t u r e a t t h e boundary.

2 - EXPERIMENTAL

NiO s i n g l e and b i c r y s t a l s were grown from high p u r i t y (99.999%) NiO powder i n an a r c image f u r n a c e u s i n g t h e V e r n e u i l technique. An atmosphere of PCO /PCO 200 was used i n o r d e r t o m a i n t a i n growth c o n d i t i o n s r e a s o n a b l y c l o s e t o s8oichiometry.

High a n g l e NiO GBs grown i n a i r o f t e n show macroscopic v o i d s a t t h e GB, presumably r e s u l t i n g from t h e accumulation of n o n s t o i c h i o m e t r i c c a t i o n v a c a n c i e s a t t h e

GB

when a lower d e f e c t c o n c e n t r a t i o n is e s t a b l i s h e d i n t h e m a t r i x a s t h e sample i s c o o l e d . The b i c r y s t a l s were seeded from two o r i e n t e d s i n g l e c r y s t a l s , t h a t were o b t a i n e d by c u t t i n g a s i n g l e c r y s t a l normal t o t h e (001) p l a n e s at

f 812

t o t h e (110) p l a n e s , where

8

i s t h e d e s i r e d m i s o r i e n t a t i o n a n g l e . The o r i e n t a t i o n of t h i s (symmetric) boundary p l a n e between t h e s e e d s was g e n e r a l l y n o t maintained i n t h e r e s u l t i n g b i c r y s t a l s . Macroscopically t h e i n t e r s e c t i o n s of t h e

GB

w i t h (001)

*work supported by t h e

U.S.

Department of Energy.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985409

C4-96

JOURNAL DE PHYSIQUE

planes appeared curved. This feature allowed (in principle) the study of <001>

tilt GBs as a function of the boundary plane for GBs of the same misorientation.

The bicrystals were cut close to (001)1 2 into slices -500 pm thick. These slices were annealed at temperatures between 1200 and 1500°C, held at llOO°C for several hours in C02/c0 = 100 before quenching them to room temperature. An ultrasonic cutting tool was used to cut 3 mu diameter discs from the annealed slices. The discs were ground to a thickness of 100-140 pm. By further grinding and polish- ing, using a dimpler,* it was possible to obtain thicknesses between 10 and 20 pm near the center of the disc. Further reduction to electron transparency was done by argon ion milling on a liquid nitrogen cooled stage. Optical observations were performed on a Leitz Metalloplan under polarized light. For the transmission electron microscopy studies a Philips 400, operated at 120 kV was used. Most observations were performed with the electron beam exactly parallel to the GB misorientation axis.

3

- LOW-ANGLE BOUNDARIES

According to the classic dislocation model /1/ low-angle tilt grain boundaries are composed of dislocations that lie parallel to the tilt axis and whose Burgers vector sum is perpendicular to the GB plane. We studied a few low-angle GBs in the present work to be able to make comparisons between low- and high-angle GB structure and in order to establish a connection with previous work in NiO which has been performed on tilt GBs of relatively low misorientation /2-41. Several GBs at misorientations near 8

=

15O were investigated. In one GB, two typical segments of which are shown edge-on in Fig. 1, the tilt axis deviated from <001>

by several degrees. In such a case it is possible to image individual disloca- tions in the boundary by the defocus technique of Riihle and Sass /4/ even as the sample is viewed closely parallel to the bicrystal rotation axis. The edge-on appearance of individual dislocations as Fresnel-fringe-bounded bright and dark dots in under- and overfocus, respectively, is qualitatively the same for all dislocations within the GB. However, some dots appear larger or have stronger contrast than others. This may be ascribed to differences in the size of the Burgers vector, as suggested by Vaudin, Riihle, and Sass

/ 2 / .

If these differences in the contrast of individual dislocations are considered it can be easily

Fig. 1 - Complex faceting structures in a tilt GB (8

=

13.1) with tilt axis near

<62,55>. The structural units comprising the GB change with GB orientation. The major facets are asymmetric.

*~anufactured by VCR Group, San Francisco, CA.

a x i s ( 0 = 13.1"). The GB plane

i s

symmetric.

Image

Fig. 2b

-

The observed s t r u c t u r a l period of

7.7

nm a g r e e s reasonably

-

w e l l with t h e t h e o r e t i c a l

# 9

one (8.3 nm).

recognized from Fig.

1

t h a t even w i t h i n t h e f a c e t segments d i f f e r e n t " s t r u c t u r a l u n i t s " a r e p r e s e n t . A s t h e average GB plane changes, d i f f e r e n t f a c e t configura- t i o n s appear. The f a c e t p e r i o d s can be q u i t e l a r g e and complex r e p e a t - u n i t s a r e a l s o observed. A t t h i s point some of t h e s t r u c t u r e s have been analyzed i n terms of t h e geometrical GB theory 151. S i m i l a r t o t h e f a c e t i n g as discussed by Vaudin, Riihle, and Sass, s t r u c t u r a l p e r i o d i c i t i e s i n such G B s may be understood i n terms of t h e 0 - l a t t i c e c o n s t r u c t i o n . Figure 2a shows a GB segment without f a c e t s , but with a p e r i o d i c d i s l o c a t i o n s t r u c t u r e with a r e p e a t - u n i t of

7.7

nm i n l e n g t h . Figure 2b g i v e s t h e 0 - l a t t i c e c o n s t r u c t i o n f o r t h i s boundary. The b a s i c period- i c i t y as w e l l a s the d i s t a n c e s between i n d i v i d u a l d i s l o c a t i o n s and t h e d i s l o c a t i o n types a s implied from t h e c o n d i t i o n t h a t t h e sum of t h e Burgers v e c t o r component p a r a l l e l t o t h e d i s l o c a t i o n vanishes over one period / I / a r e w e l l r e p r e s e n t e d by t h e geometrical model. It should be noted t-hat q u i t e f r e q u e n t l y minor d e v i a t i o n s from p e r f e c t p e r i o d i c i t y a r e observed.

According t o t h e 0 - l a t t i c e t h e o r y /5/, <001> t i l t G B s

w i l l

e x h i b i t f a c e t planes symmetric t o t h e (110)1 or planes. The f a c e t i n g geometry i s g e n e r a l l y derived by following a bath

through's

maximum number of 0 - l a t t i c e p o i n t s while s t a y i n g a s c l o s e a s p o s s i b l e t o t h e average GB plane. I n Fig. 3 a GB ( 8 = 15O) whose t i l t a x i s shows a small d e v i a t i o n (- 2 O ) from <001>

i s

shown. Here t h e major f a c e t s a r e indeed symmetric [ t o ( l o o ) , i n t h i s case with t h e d i s l o c a t i o n

spacing following t h e r e l a t i o n 1? = b / [ 2 s i n ( 8 / 2 ) ] with a Burgers v e c t o r

-,-

b = a<100>]. However, t h e boundary c l e a r l y does not s t a y c l o s e t o t h e average GB plane. I n s t e a d , q u i t e long (-150 nm) almost p l a n a r f a c e t s a r e joined, by s h o r t segments i n which t h e GB plane

is

r o t a t e d by -18". These secondary f a c e t s a r e asymmetric. They could, i n t h e geometrical model, be formed by a f i n e l y stepped arrangement of d i s l o c a t i o n s i n r e p e a t u n i t s of t h r e e d i s l o c a t i o n s c o n s i s t i n g of one with b = a<100> and two d i s l o c a t i o n s with b =; <110>. C l e a r l y , t h e geomet- r i c a l model cannot p r e d i c t t h e formation of t h e long f a c e t s . I n f a c t , i f one drops t h e assumption t h a t t h e r e g u l a r spacing and s m a l l e s t f a c e t l e n g t h c o n s i s t e n t with t h e 0-net

i s

t h e e n e r g e t i c a l l y favored GB, one has n e a r l y unlimited possi- b i l i t i e s f o r a r r a n g i n g f a c e t l e n g t h and s t e p h e i g h t i n any given GB. The f a c t t h a t the l a r g e s t e p s with t h e i r e f f e c t i v e asymmetric GB plane a r e observed i n Fig.

3 must mean t h a t such an arrangement i s of lower energy than a uniform d i s t r i b u - t i o n of s t e p s and f a c e t s . The long f a c e t s i n Fig. 3a a r e not p e r f e c t l y planar but show some secondary f a c e t i n g a s suggested by he s l i g h t l y d i s p l a c e d double s t r e a k s

C4-98

JOURNAL

DE PHYSIQUE

'a. .

Fig. 3

-

^{8 =} 15" tilt GB with m i s o r i e n t a t i d n a x i s near <001>. ( a ) Major f a c e t planes a r e symmetric r e l a t i v e t o (100)1,2 and a r e s e p a r a t e d by l a r g e s t e p s (-9 nm). The s t e p h e i g h t ( b )

i s

d r a s t i c a l l y reduced ( c ) a s t h e average GB plane r o t a t e s towards t h e symmetric o r i e n t a t i o n . Double s t r e a k s i n c i n d i c a t e secondary f i n e s c a l e f a c e t i n g i n ( a ) , ( b ) .

i n t h e d i f f r a c t i o n p a t t e r n . F i g u r e 3c shows a s h o r t segment of t h e same GB i n a d i f f e r e n t p a r t of t h e c r y s t a l . A r o t a t i o n of t h e average GB plane by 4' has been accomplished by a r e d u c t i o n of t h e height of t h e l a r g e s t e p s i n Figs. 3a and b.

4

-

HIGH-ANGLE BOUNDARIES

Read and Shockley suggested t h a t t h e c l a s s i c model of low-angle t i l t g r a i n boundaries can formally be extended t o high-angle g r a i n boundaries

/ I / .

The uniform d i s l o c a t i o n spacing p o s s i b l e f o r s p e c i a l m i s o r i e n t a t i o n s w i l l g e n e r a l l y be modulated f o r a r b i t r a r y m i s o r i e n t a t i o n a n g l e s and can g i v e r i s e t o s t r u c t u r a l

Fig. 4

-

Through-focus s e r i e s of high angle ( 0 = 41°) < 0 0 1 > tilt GB. AF = 0 corresponds t o t h e minimum c o n t r a s t c o n d i t i o n .

GB of ceramic m a t e r i a l s

is

of p a r t i c u l a r i n t e r e s t . Recent computer c a l c u l a t i o n s on t w i s t a s w e l l a s t i l t boundaries i n N i O have i n d i c a t e d t h a t a c o n s i d e r a b l e e f f e c t i v e volume expansion

is

p r e s e n t i n t h e immediate v i c i n i t y of t h e GB 16-8/.

E l e c t r o n d i f f r a c t i o n s t u d i e s on t w i s t GBs i n N i O I 3 1 a l s o i n d i c a t e a c o n s i d e r a b l e i n c r e a s e i n i n t e r p l a n a r s p a c i n g a t t h e GB.

The o u t s t a n d i n g f e a t u r e observed by us s o f a r on a l l high-angle GB i n N i O

(2

20') i s i l l u s t r a t e d i n Fig. 4. The through-focus s e r i e s of a 8 = 41' <001> t i l t GB, t a k e n under k i n e m a t i c a l c o n d i t i o n s , shows a b r i g h t band i n under-focus and a d a r k band i n over-focus. S i m i l a r t o t h e o b s e r v a t i o n s of Riihle and Sass on i n d i v i d u a l d i s l o c a t i o n s / 4 / , t h i s behavior i n d i c a t e s a reduced mean i n n e r p o t e n t i a l w i t h i n a band 0.5 t o 0.8 nm wide a t t h e GB. This

i s

commensurate w i t h t h e r e g i o n of e f f e c - t i v e reduced d e n s i t y found i n l a t t i c e s t a t i c s c a l c u l a t i o n s 16-8/. It should b e noted t h a t t h i s through-focus behavior

i s

t y p i c a l n o t o n l y f o r high-angle tilt GB but a l s o f o r high-angle

t w i s t

GBs. S i m i l a r defocus e f f e c t s , a l t h o u g h g e n e r a l l y o v e r a c o n s i d e r a b l y g r e a t e r width a r e o f t e n observed i n ceramics when an amorphous GB phase

i s

p r e s e n t / 9 / . I n t h e p r e s e n t c a s e a n amorphous GB phase can be exclud- ed on account of t h e d a r k - f i e l d imaging behavior of t h e GB, r e c e n t l a t t i c e f r i n g e o b s e r v a t i o n s , and t h e observed presence of s t r u c t u r a l p e r i o d i c i t i e s i n t h e GB t h a t

w i l l

be d i s c u s s e d below. N e v e r t h e l e s s , t h e i n f l u e n c e of p o s s i b l e i m p u r i t y and charge e f f e c t s a t t h e GB w i l l have t o be taken i n t o account i n a q u a n t i t a t i v e e v a l u a t i o n of t h e defocus c o n t r a s t behavior. The above o b s e r v a t i o n s i n N i O a r e i n c o n t r a s t t o s t u d i e s of GBs i n m e t a l s f o r which a comparable e f f e c t i n t h e defocus behavior has not been r e p o r t e d /10,11/. A q u a l i t a t i v e d i f f e r e n c e between high- a n g l e GB s t r u c t u r e i n m e t a l s and i o n i c s o l i d s seems t o be t h e g r e a t e r openness of t h e s t r u c t u r e i n ceramic o x i d e s which a p p a r e n t l y can be maintained by v i r t u e of t h e s t r o n g i o n i c i n t e r a c t i o n s 16-8/.

While i n low-angle GBs i n d i v i d u a l d i s l o c a t i o n s a r e e a s i l y r e s o l v e d by c o n v e n t i o n a l TEM, i t becomes q u i t e d i f f i c u l t t o d i s c e r n t h e primary d i s l o c a t i o n p e r i o d when t h e t i l t angle exceeds -20". P e r i o d i c i t i e s corresponding t o t h e expected d i s l o c a t i o n spacing based on an e x t e n s i o n of t h e low a n g l e model have, however, been observed down t o a p e r i o d of 0.5

nm

and a t m i s o r i e n t a t i o n s up t o 35". While a d e s c r i p t i o n of t h e GB i n terms of d i s 1 o c a t i o n s " m a y f o r m a l l y be p o s s i b l e a t h i g h m i s o r i e n t a - t i o n , t h e c l o s e p r o x i m i t y o r even o v e r l a p p i n g a t d i s l o c a t i o n c o r e s should make such a p i c t u r e n o t t o o u s e f u l , a l t h o u g h some of t h e rudimentary g e o m e t r i c a l a s p e c t s may i n f a c t be r e t a i n e d a s evidenced by t h e observed p e r i o d i c i t i e s . It should be noted t h a t d e s p i t e t h e f a c t t h a t f i n e - s c a l e p e r i o d i c i t i e s c o r r e s p o n d i n g t o t h e expected primary d i s l o c a t i o n s p a c i n g s have been observed a t s e v e r a l h i g h a n g l e GBs, t h e s e p e r i o d i c i t i e s a r e g e n e r a l l y n o t s e e n a t 8

>

25O.

I n a g e n e r a l edge-on o r i e n t a t i o n and under k i n e m a t i c a l c o n d i t i o n s , most high-angle GBs appear r e l a t i v e l y s t r u c t u r e l e s s i n t h e d i r e c t i o n of t h e GB p l a n e ( s e e Fig.

4 ) . However, i n o r i e n t a t i o n s c l o s e t o t h e tilt a x i s and f o r f a v o r a b l e imaging c o n d i t i o n s most high-angle <001> tilt GBs show s t r u c t u r a l p e r i o d i c i t i e s . The l a t t e r change w i t h GB p l a n e o r i e n t a t i o n and t a k e t h e form of f a c e t s and p e r i o d i c - i t i e s w i t h i n t h e f a c e t s . The s t e p h e i g h t i s g e n e r a l l y q u i t e s m a l l compared t o low-angle boundaries. Most high-angle GB f a c e t s a r e asymmetric, i . e . , d i f f e r e n t c r y s t a l l o g r a p h i c p l a n e s a r e joined a t t h e GB. Such a boundary

is

shown i n F i g . 5. The f a c e t s a r e q u i t e long (-22 nm) and t h e s t e p h e i g h t

is

-0.8 nm. The r e c i p r o c a l d i s t a n c e between t h e c l o s e l y spaced double s t r e a k s i n t h e d i f f r a c t i o n p a t t e r n a g r e e w e l l w i t h t h e observed l e n g t h of t h e f a c e t s . The b a r e l y v i s i b l e s t r e a k s n e a r (2001, s u g g e s t t h e presence of a d d i t i o n a l s t r u c t u r a l modulation i n t h e GB w i t h p e r i o d s of

1.1

and 1.3

nm.

These s t r u c t u r a l p e r i o d i c i t i e s a l o n g t h e f a c e t s c o u l d , i n t h i s c a s e , a l s o been imaged by s t r a i n - c o n t r a s t ( s e e Fig. 5b).

E l e c t r o n d i f f r a c t i o n may be t h e p r e f e r r e d method f o r e s t a b l i s h i n g t h e presence of such s t r u c t u r a l p e r i o d i c i t i e s . HREM, however, w i l l be n e c e s s a r y t o o b t a i n i n s i g h t i n t o t h e atomic arrangement.

C4-100

JOURNAL DE PHYSIQUE

Fig. 5

-

High a n g l e <001> t i l t GB ( 0 = 41°). Asymmetric f a c e t p l a n e s a r e i n d i c a t e d . B a r e l y v i s i b l e s t r u c t u r a l d e t a i l a l o n g f a c e t s i n ( a )

i s

c l e a r l y r e s o l v e d i n (b). The s t r u c t u r a l p e r i o d i c i t i e s concerning f a c e t s a s w e l l a s p e r i o d i c i t i e s w i t h i n t h e f a c e t s can be o b t a i n e d from e l e c t r o n d i f f r a c t i o n .

Obviously t h e appearance of f a c e t s i n high-angle <001> t i l t GBs i n d i c a t e s t h a t some GB p l a n e s a r e p r e f e r r e d . The tendency f o r high-angle <001> t i l t GBs t o form asymmetric f a c e t s s u g g e s t s t h a t symmetric GB p l a n e o r i e n t a t i o n s do n o t p o s s e s s p a r t i c u l a r l y low e n e r g i e s . This i s i n c o n t r a s t t o what

is

implied by t h e geomet- r i c a l model. T h e r e f o r e , t h e 0 - l a t t i c e c o n s t r u c t i o n may n o t be a p p l i c a b l e t o high- a n g l e <001> t i l t G B s i n N i O . On t h e o t h e r hand, t h e observed p e r i o d i c i t i e s a l o n g t h e f a c e t s s t r o n g l y s u g g e s t t h a t t h e atomic matching between opposing p l a n e s must p l a y an important r o l e . A s t r u c t u r a l u n i t model may p r o v i d e i n s i g h t i n t o t h e formation of complex s t r u c t u r a l p e r i o d i c i t i e s a l o n g t h e boundary. F u r t h e r prog- r e s s i n t h e u n d e r s t a n d i n g of such GBs

is

expected t o come from HREM i n v e s t i g a t i o n s i n c o n j u n c t i o n w i t h l a t t i c e s t a t i c s c a l c u l a t i o n s .

REFERENCES

1. W. T. Read and W. Shockley, Phys. Rev.

78

(1950) 275.

2. M. D. Vaudin, M. Riihle, and S.

L.

S a s s , Acta Met.

31

(1983) 1109.

3.

J. Eastman, F. Schmiickle, M. D. Vaudin, and S. L. S a s s , Adv. Ceram.

12

(1984) ( i n p r e s s ) .

4.

M.

Riihle and S. L. S a s s , P h i l . Mag. A s (1984) 759.

5. W. Bollmann, C r y s t a l D e f e c t s and C r y s t a l l i n e I n t e r f a c e s , ( S p r i n g e r , B e r l i n , 1980).

6. D.

Wolf and

B.

Benedek, Adv. C e r a m . L ( l 9 8 1 ) 107.

7. D.

Wolf, Adv. Ceram.

12

(1984) ( i n p r e s s ) .

8 . D. M. Duffy and P.

W.

Tasker, P h i l . Mag. A x (1983) 817.

9. D.

R. C l a r k e , U l t r a m i c r o s c o p y ~ ( 1 9 7 9 ) 33.

10. R. C. Pond, i n Grain Boundary S t r u c t u r e and K i n e t i c s , ASM (Metals Park, Ohio, 1980), p. 13.

11. F. Cosandey and.C.

L.

Bauer, i n T r e a t i s e on M a t e r i a l s Science and Technology, Vol. 24, K. N. Tu and R. Rosenberg e d s . (Academic P r e s s , New York, 1982), p. 113.