- Email: [email protected]
Synthesis and structure of a perovskite related oxide, Bi23Cu3Ti4O12
Mat. Res. Bull., Vol. 25, pp. 477-483, 1990. Printed in the USA. 0025-5408/90 $3.00 + .00 C o p y r i g h t (e) 1990 Pergamon Press plc.
SYNTHESIS
AND
STRUCTURE
OF
A
PEROVSKITE
RELATED
OXIDE,
Bi2/3Cu3Ti4012 I n g r i d B r y n t s e and P e r - E r i k W e r n e r D e p a r t m e n t of I n o r g a n i c and S t r u c t u r a l Chemistry, A r r h e n i u s Laboratory, U n i v e r s i t y of Stockholm, S-I06 91 Stockholm, Sweden
(Received December 13, 1989; Communicated by A. Magn~li)
ABSTRACT The phase B i 2 / 3 C u 3 T i 4 O l 2 has been s y n t h e s i z e d and studied w i t h e l e c t r o n and p o w d e r diffraction. The c r y s t a l l o g r a p h i c study was p e r f o r m e d by f u l l - p r o f i l e analysis of p o w d e r d i f f r a c t i o n data o b t a i n e d w i t h a STADI/P d i f f r a c t o m e t e r , using the m o d e of s y m m e t r i c t r a n s m i s s i o n geometry. The s t r u c t u r e is of the p e r o v s k i t e r e l a t e d C a C u 3 M n 4 0 1 2 type. The title c o m p o u n d c r y s t a l l i z e s in the cubic space group Im3 w i t h a = 7.4175(3) A and z = 2. B i s m u t h o c c u p i e s the i c o s a h e d r a l A - s i t e s of ideal perovskite. The eight nearest o x y g e n n e i g h b o u r s of copper form two p e r p e n d i c u l a r r e c t a n g l e s of d i f f e r e n t size, and t i t a n i u m o c c u p i e s the B - s i t e s of ideal perovskite. MATERIALS
INDEX:
Bismuth,
Copper,
Titanium,
Oxides
Introduction The d i s c o v e r y of s u p e r c o n d u c t i v i t y in the B i - S r - C a - C u - O s y s t e m (i) has p r o m o t e d interest in r e l a t e d systems. In p a r t i c u lar, p e r o v s k i t e - b a s e d crystal s t r u c t u r e s have been shown to play a c r u c i a l role in o b t a i n i n g h i g h - t e m p e r a t u r e s u p e r c o n d u c t i n g phases (2). D u r i n g studies of the t e r n a r y B i 2 0 3 - C u O - T i O 2 system we have found a p e r o v s k i t e - r e l a t e d phase w i t h the c o m p o s i t i o n B i 2 / 3 C u 3 T i 4 0 1 2 . B e c a u s e of d i f f i c u l t i e s in o b t a i n i n g single c r y s t a l s of the substance, the c r y s t a l l o g r a p h i c study was performed by f u l l - p r o f i l e R i e t v e l d a n a l y s i s of X-ray powder d i f f r a c tion data. Preparation The c o m p o u n d B i 2 / 3 C u 3 T i 4 O l 2 was m e t r i c amounts of Bi203, CuO and TiO t o g e t h e r and pelletized. The pellets plates, h e a t e d to 900°C for 24 h and
477
s y n t h e s i z e d from stoichio2. The oxides were g r o u n d w e r e p l a c e d on sapphire c o o l e d to room temperature.
478
I. B R Y N T S E ,
et 8/.
Vol. 25, No. 4
The product, a d a r k b r o w n powder, w a s a n a l y s e d w i t h a J E O L 820 s c a n n i n g e l e c t r o n m i c r o s c o p e . The m a j o r p h a s e c o n s i s t e d of c r y s t a l s w i t h cubic shape, too small (<0.01 mm) for single c r y s t a l X - r a y m e a s u r e m e n t s (Fig. i). EDS a n a l y s i s i n d i c a t e d that the w e i g h t e d - i n s t o i c h i o m e t r y was p r e s e r v e d . X-ray powder diffraction photog r a p h s w e r e taken, u s i n g a focussing c a m e r a of G u i n i e r - H ~ g g type w i t h s u b t r a c t i o n g e o m e t r y and w i t h m o n o c h r o m a t e d CuKe I r a d i a t i o n (~ = 1 . 5 4 0 5 9 8 1 A at 25°C). F i n e l y powdered potassium chloride (a = 6.2930 A at 25°C (3)) w a s a d d e d to the sample as i n t e r n a l standard. The film w a s m e a s u r e d with a computer-controlled microdens i t o m e t e r (4). The d i f f r a c t i o n patt e r n r e v e a l e d a b o d y - c e n t e r e d cubic cell, a = 7.4175(3) A, w i t h c o r r e s p o n d i n g cell volume, V = 408.1 A 3.
FIG. i. S E M p h o t o g r a p h at 3500 x m a g n i f i c a t i o n of
Bi2/3Cu3Ti4012-
The i n d e x e d p a t t e r n is l i s t e d in T a b l e i. The X - r a y p o w d e r pattern i n d i c a t e s that the c o m p o u n d is i s o s t r u c t u r a l w i t h the p e r o v skite r e l a t e d p h a s e C a C u 3 M n 4 0 1 2 (5). The sample w a s e x a m i n e d in a J E O L 200CX t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e . Two zones of the e l e c t r o n d i f f r a c t i o n p a t t e r n s from the p h a s e are shown in Fig. 2. The <001> zone axis d i s p l a y s a square a r r a y of s h a r p d i f f r a c t i o n spots, w h e r e a s the zone shows an o r t h o g o n a l pattern. No s u p e r l a t t i c e spots c o u l d be detected.
O
a)
b) Electron
diffraction
a)
zone
FIG. 2 patterns
of B i 2 / 3 C u 3 T i 4 O l 2 : b)
<110>
zone
Vol. 25, No. 4
PEROVSKITES
TABLE
479
1
O b s e r v e d a n d C a l c u l a t e d 28 V a l u e s for the G u i n i e r - H ~ g g P o w d e r D i f f r a c t i o n P a t t e r n of B i 2 / 3 C u 3 T i 4 O l 2 . = 1.5405981
A.
A(28)
= 28ob s - 28calc.
h k 1
28ob s
A(2O)
dob s
110 200 211 220 310 222 321 400 330 420 422 510 440 53O 6OO 611 620
16 893 23 978 29 473 34 139 38 336 42 163 45.724 49.081 52.263 55.328 61.149 63.935 71.959 74.541 77.094 79.630 82.115
0.002 0.003 0.000 -0.024 -0.007 -0 006 -0 007 -0 008 -0 021 -0 018 -0 013 -0 012 0 005 0.006 0.009 0.021 0.002
5.244 3.708 3.028 2.6242 2.3460 2.1415 1.9827 1.8546 1.7490 1.6591 1.5144 1.4549 1.3112 1.2720 1.2361 1.2030 1.1728
Structure
refinement
and
I/I o 83 39 31 i000 116 98 106 351 i0 14 737 16 172 19 25 20 253
result
X - r a y p o w d e r d a t a for R i e t v e l d r e f i n e m e n t w e r e c o l l e c t e d on a STOE STADI/P powder diffractometer, using a rotating sample m o u n t e d in s y m m e t r i c t r a n s m i s s i o n m o d e . A g e r m a n i u m c r y s t a l monochromator w i t h a 220 m m r a d i u s of c u r v a t u r e w a s u s e d to obtain strictly monochromated C u K e I r a d i a t i o n . The d a t a w e r e c o l lected with a small linear-position sensitive detector covering 4.6 ° in 28. In o r d e r to r e d u c e the i n f l u e n c e of e r r o r s in i n t e n s i t y c a l i b r a t i o n of the d e t e c t o r , it w a s m o v e d in s t e p s of 0.2 ° , t h u s g i v i n g an a v e r a g e i n t e n s i t y f r o m 23 (= 4 . 6 / 0 . 2 ) m e a s u r e m e n t s at e a c h t h e t a p o s i t i o n . F u l l - p r o f i l e R i e t v e l d r e f i n e m e n t of the s t r u c t u r e w a s c a r r i e d o u t w i t h the p r o g r a m D B W 3 . 2 S (6), w h i c h is a s u c c e s s o r to the p r o g r a m d e s c r i b e d in Ref. (7). T h e a n g u l a r d e p e n d e n c e of the p e a k f u l l - w i d t h (2G) at h a l f - m a x i m u m w a s d e s c r i b e d by the u s u a l q u a d r a tic e q u a t i o n in t a n e: FWHM 2 = Utan28
+ Vtan
e + W
w h e r e U, V a n d W are p a r a m e t e r s to r e f i n e . T h e m i n i m u m a n d m a x i m u m v a l u e s for the h a l f - w i d t h s w e r e f o u n d to be 0 . 0 8 3 ° a n d 0 . 1 4 6 ° (28) r e s p e c t i v e l y . S t e p i n t e n s i t i e s in the 2 e - r a n g e 14 ° to 83 ° w e r e u s e d in the r e f i n e m e n t . T h e s t e p - l e n g t h in 28 w a s 0 . 0 1 ° . In t h e r e f i n e m e n t o n e s c a l e f a c t o r for a s m a l l i m p u r i t y of T i 0 2 , r u t i l e , w a s r e f i n e d . T h e s t r o n g e s t T i O 2 r e f l e c t i o n s h o w e d an int e n s i t y of a p p r o x i m a t e l y 1% of the s t r o n g e s t r e f l e c t i o n f r o m the main phase. In s e p a r a t e r e f i n e m e n t c y c l e s the Bi o c c u p a t i o n f a c t o r w a s c h e c k e d , a n d no s i g n i f i c a n t d e v i a t i o n f r o m the w e i g h t e d - i n s t o i chiometry Bi2/3Cu3Ti4Ol2 was detected. The following parameters
480
I. B R Y N T S E ,
et al.
Vol. 25, No. 4
for the m a i n p h a s e w e r e r e f i n e d : 2 p o s i t i o n a l p a r a m e t e r s , 4 isot r o p i c t e m p e r a t u r e f a c t o r s , 1 z e r o - p o i n t p a r a m e t e r , 1 s c a l e factor, 3 h a l f - w i d t h p a r a m e t e r s and 1 p r e f e r r e d - o r i e n t a t i o n param e t e r for the 224 direction. The refinement was terminated w h e n all s h i f t s in the p a r a m e t e r s w e r e less t h a n 10 % of the corr e s p o n d i n g s t a n d a r d d e v i a t i o n s . The f i n a l R - v a l u e s , Rp = 0.068, R w p = 0.087, R B = 0 . 0 3 4 and R F = 0.032, w e r e o b t a i n e d for the pos i t l o n a l p a r a m e t e r s l i s t e d in T a b l e 2. B e c a u s e of s e r i a l c o r r e l a t i o n as i n d i c a t e d by a D u r b i n - W a t s o n D - v a l u e = 0.4, the s t a n d a r d d e v i a t i o n s o b t a i n e d in the l e a s t - s q u a r e s r e f i n e m e n t s are m u l t i p l i e d by 2.5 in the table. The o b s e r v e d and c a l c u l a t e d p a t t e r n s are s h o w n in Fig. 3. S e l e c t e d i n t e r a t o m i c d i s t a n c e s are l i s t e d in T a b l e 3. A s e c t i o n of the s t r u c t u r e in the a b - p l a n e is s h o w n in Fig. 4. T h e o x y g e n p o s i t i o n of i d e a l p e r o v s k i t e is in the m i d d l e of e v e r y m e t a l s q u a r e . In the r e f i n e d s t r u c t u r e the o x y g e n s are m o v e d c l o s e r to the c o p p e r a t o m s a n d t h e r e f o r e the o c t a h e d r a are tilted approximately 18 0 a w a y f r o m the x, y a n d z d i r e c t i o n s .
TABLE
2
P o s i t i o n a l P a r a m e t e r s of B i 2 / 3 [ ] I / 3 C u 3 T i 4 0 1 2 , w i t h E s t i m a t e d S t a n d a r d D e v i a t i o n s in P a r e n t h e s e s . Atom
Position
x/a
y/b
z/c
Bi
2 (a)
0
0
0
Cu
6 (b)
0
1/2
1/2
Ti
8 (a)
1/4
1/4
1/4
0.296(2)
0.178(2)
0
O
24(g)
TABLE Selected Standard
TiO 6 octahedron 6 Ti - 0 BiOl2 12 Bi
- 0
3
Interatomic Distances (A) w i t h D e v i a t i o n s in P a r e n t h e s e s ,
1.960(5) polyhedron
Estimated
CuO 8 p o l y h e d r o n 4 Cu - O
2.01(2)
4 Cu - 0
2.83(2)
2.56(2)
T h e four n e a r e s t o x y g e n n e i g h b o u r s of c o p p e r f o r m a r e c t a n g l e a n d the b o n d i n g d i s t a n c e s (cf. T a b l e 3) a g r e e w i t h C u ( I I ) - O d i s t a n c e s f o u n d in the l i t e r a t u r e . E v e r y s e c o n d m e t a l r o w in the a b - p l a n e (cf. Fig. 4) c o n s i s t s of o n l y c o p p e r b u t the o x y g e n r e c t a n g l e s a r o u n d t h e ' c o p p e r a t o m s in t h e s e r o w s are perpendicular to e a c h o t h e r , as s h o w n by the d a s h e d l i n e s in the figure. The c o o r d i n a t i o n p o l y h e d r a of the c o p p e r and t i t a n i um c a t i o n s are s h o w n in Fig. 5.
i
Ji
i
,
481
PEROVSKITES
Vo]. 25, No. 4
3000.
aOOO,
~000
,
15OO
.
~OOO
.
SO0
.
F -r-' ' ' I '' '-' I SO,
~S
•
' ' ' I ~ ' ~ ' 60.
S~.
)
~ "TO.
~
'
' } ' ' i , I ' ' ' Ze ~S.
80.
,f FIG. 3 Final Rietveld plot. In the upper portion the observed data are shown as a solid line. Calculated intensities are shown as dots. The lower portion is a plot of the differences Iobs - Icalc. The strongest TiO 2 peak is marked with a cross.
I I
I|I
III
I
III
I
II
482
I. B R Y N T S E ,
o1 x
x
X
X
et B/.
o
......:.............. i
Vol. 25, No. 4
,
ii
o
FIG. 4 S e c t i o n in the a b - p l a n e of B i 2 / 3 C u 3 T i 4 O l 2 . The large circles indicate Bi-sites, and the b l a c k spots s h o w Cup o s i t i o n s . C r o s s e s and small b l a c k spots s h o w o x y g e n sites in ideal p e r o v s k i t e and this s t r u c t u r e r e s p e c tively. D a s h e d lines indicate the o r i e n t a t i o n of the o x y g e n p l a n e s a r o u n d Cu in one c o p p e r row.
FIG. 5 A s t r u c t u r e m o d e l of B i 2 / 3 C u 3 T i 4 0 1 2 , s h o w i n g TiO 6 o c t a h e d r a and CuO 4 g r o u p s d r a w n as w h i t e r e c t a n g l e s . One Bi p o s i t i o n is i n d i c a t e d w i t h a circle.
The m o s t n o t a b l e d i f f e r e n c e b e t w e e n the B i 2 / 3 C u 3 T i 4 O l 2 structure and t h a t of C a C u 3 M n 4 0 1 2 p r e s e n t e d by C h e n e v a s et al. (5) is the s t a t i s t i c a l l y d i s t r i b u t e d Bi atoms in the 2(a) p o s i t i o n . C o r r e s p o n d i n g d i s t r i b u t i o n s have b e e n d e s c r i b e d for a n u m b e r of i s o s t r u c t u r a l rare e a r t h c o m p o u n d s i n v e s t i g a t e d by B o c h u et al.
(8). Conclusions This p a p e r is p a r t of an i n v e s t i g a t i o n of the t e r n a r y s y s t e m Bi203 - C u O - TiO 2. As s h o w n in the p r e s e n t w o r k we c o u l d o b t a i n a n e w p e r o v s k i t e r e l a t e d o x i d e B i 2 / 3 C u 3 T i 4 O l 2 c r y s t a l l i z i n g in a C a C u 3 M 4 O I 2 type structure. The s t r u c t u r e was r e f i n e d from a sample c o n t a i n i n g a small i m p u r i t y of Ti02 by f u l l - p r o f i l e r e f i n e m e n t of its X - r a y p o w d e r d i f f r a c t i o n pattern. The d a t a w e r e o b t a i n e d by a S T A D I / P d i f f r a c t o m e t e r in t r a n s m i s s i o n mode. The q u a l i t y of the d a t a can be j u d g e d from the final s t r u c t u r e factor R - v a l u e 0.032.
Vol.
25, No. 4
PEROVSKITES
483
Acknowledgements This w o r k has r e c e i v e d f i n a n c i a l s u p p o r t from the S w e d i s h N a t u r a l S c i e n c e R e s e a r c h Council. One of the a u t h o r s (PEW) also w i s h e s to t h a n k the K n u t & A l i c e W a l l e n b e r g F o u n d a t i o n for a g r a n t e n a b l i n g us t o a c q u i r e a p o w d e r d i f f r a c t o m e t e r .
References I. 2. 3. 4. 5. 6. 7. 8.
H. Maeda, Y. Tanaka, M. F u k o t o m i and T. Asano, Jpn. J. Appl. Phys. 27, L209 (1988). T. Nogami, S. Kosaka, Y. Shirota, H. N a k a y a m a and H. Fujita, Mat. Res. Bull. 23, 1145 (1988). P.G. H a m b l i n g , A e t a C r y s t a l l o g r . 6, 98 (1953). K.E. J o h a n s s o n , T. P a l m and P.-E. Werner, J. Phys. E: Sci. Instrum. 13, 1289 (1980). J. C h e n e v a s , J.C. Joubert, M. M a r e z i o and B. Bochu, J. S o l i d S t a t e Chem. 14, 25 (1975). D.B. Wiles, A. S a k t h i v e l and R.A. Young, P r o g r a m D B W 3 . 2 S (1987) D.B. W i l e s and R.A. Young, J. Appl. C r y s t a l l o g r . 14, 149 (1981) . B. Bochu, M.N. D e s c h i z e a u x , J.C. Joubert, A. Collomb, J. C h e n e v a s and M. M a r e z i o , J. S o l i d State Chem. 29 291 (1979).
SYNTHESIS
AND
STRUCTURE
OF
A
PEROVSKITE
RELATED
OXIDE,
Bi2/3Cu3Ti4012 I n g r i d B r y n t s e and P e r - E r i k W e r n e r D e p a r t m e n t of I n o r g a n i c and S t r u c t u r a l Chemistry, A r r h e n i u s Laboratory, U n i v e r s i t y of Stockholm, S-I06 91 Stockholm, Sweden
(Received December 13, 1989; Communicated by A. Magn~li)
ABSTRACT The phase B i 2 / 3 C u 3 T i 4 O l 2 has been s y n t h e s i z e d and studied w i t h e l e c t r o n and p o w d e r diffraction. The c r y s t a l l o g r a p h i c study was p e r f o r m e d by f u l l - p r o f i l e analysis of p o w d e r d i f f r a c t i o n data o b t a i n e d w i t h a STADI/P d i f f r a c t o m e t e r , using the m o d e of s y m m e t r i c t r a n s m i s s i o n geometry. The s t r u c t u r e is of the p e r o v s k i t e r e l a t e d C a C u 3 M n 4 0 1 2 type. The title c o m p o u n d c r y s t a l l i z e s in the cubic space group Im3 w i t h a = 7.4175(3) A and z = 2. B i s m u t h o c c u p i e s the i c o s a h e d r a l A - s i t e s of ideal perovskite. The eight nearest o x y g e n n e i g h b o u r s of copper form two p e r p e n d i c u l a r r e c t a n g l e s of d i f f e r e n t size, and t i t a n i u m o c c u p i e s the B - s i t e s of ideal perovskite. MATERIALS
INDEX:
Bismuth,
Copper,
Titanium,
Oxides
Introduction The d i s c o v e r y of s u p e r c o n d u c t i v i t y in the B i - S r - C a - C u - O s y s t e m (i) has p r o m o t e d interest in r e l a t e d systems. In p a r t i c u lar, p e r o v s k i t e - b a s e d crystal s t r u c t u r e s have been shown to play a c r u c i a l role in o b t a i n i n g h i g h - t e m p e r a t u r e s u p e r c o n d u c t i n g phases (2). D u r i n g studies of the t e r n a r y B i 2 0 3 - C u O - T i O 2 system we have found a p e r o v s k i t e - r e l a t e d phase w i t h the c o m p o s i t i o n B i 2 / 3 C u 3 T i 4 0 1 2 . B e c a u s e of d i f f i c u l t i e s in o b t a i n i n g single c r y s t a l s of the substance, the c r y s t a l l o g r a p h i c study was performed by f u l l - p r o f i l e R i e t v e l d a n a l y s i s of X-ray powder d i f f r a c tion data. Preparation The c o m p o u n d B i 2 / 3 C u 3 T i 4 O l 2 was m e t r i c amounts of Bi203, CuO and TiO t o g e t h e r and pelletized. The pellets plates, h e a t e d to 900°C for 24 h and
477
s y n t h e s i z e d from stoichio2. The oxides were g r o u n d w e r e p l a c e d on sapphire c o o l e d to room temperature.
478
I. B R Y N T S E ,
et 8/.
Vol. 25, No. 4
The product, a d a r k b r o w n powder, w a s a n a l y s e d w i t h a J E O L 820 s c a n n i n g e l e c t r o n m i c r o s c o p e . The m a j o r p h a s e c o n s i s t e d of c r y s t a l s w i t h cubic shape, too small (<0.01 mm) for single c r y s t a l X - r a y m e a s u r e m e n t s (Fig. i). EDS a n a l y s i s i n d i c a t e d that the w e i g h t e d - i n s t o i c h i o m e t r y was p r e s e r v e d . X-ray powder diffraction photog r a p h s w e r e taken, u s i n g a focussing c a m e r a of G u i n i e r - H ~ g g type w i t h s u b t r a c t i o n g e o m e t r y and w i t h m o n o c h r o m a t e d CuKe I r a d i a t i o n (~ = 1 . 5 4 0 5 9 8 1 A at 25°C). F i n e l y powdered potassium chloride (a = 6.2930 A at 25°C (3)) w a s a d d e d to the sample as i n t e r n a l standard. The film w a s m e a s u r e d with a computer-controlled microdens i t o m e t e r (4). The d i f f r a c t i o n patt e r n r e v e a l e d a b o d y - c e n t e r e d cubic cell, a = 7.4175(3) A, w i t h c o r r e s p o n d i n g cell volume, V = 408.1 A 3.
FIG. i. S E M p h o t o g r a p h at 3500 x m a g n i f i c a t i o n of
Bi2/3Cu3Ti4012-
The i n d e x e d p a t t e r n is l i s t e d in T a b l e i. The X - r a y p o w d e r pattern i n d i c a t e s that the c o m p o u n d is i s o s t r u c t u r a l w i t h the p e r o v skite r e l a t e d p h a s e C a C u 3 M n 4 0 1 2 (5). The sample w a s e x a m i n e d in a J E O L 200CX t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e . Two zones of the e l e c t r o n d i f f r a c t i o n p a t t e r n s from the p h a s e are shown in Fig. 2. The <001> zone axis d i s p l a y s a square a r r a y of s h a r p d i f f r a c t i o n spots, w h e r e a s the
O
a)
b) Electron
diffraction
a)
zone
FIG. 2 patterns
of B i 2 / 3 C u 3 T i 4 O l 2 : b)
<110>
zone
Vol. 25, No. 4
PEROVSKITES
TABLE
479
1
O b s e r v e d a n d C a l c u l a t e d 28 V a l u e s for the G u i n i e r - H ~ g g P o w d e r D i f f r a c t i o n P a t t e r n of B i 2 / 3 C u 3 T i 4 O l 2 . = 1.5405981
A.
A(28)
= 28ob s - 28calc.
h k 1
28ob s
A(2O)
dob s
110 200 211 220 310 222 321 400 330 420 422 510 440 53O 6OO 611 620
16 893 23 978 29 473 34 139 38 336 42 163 45.724 49.081 52.263 55.328 61.149 63.935 71.959 74.541 77.094 79.630 82.115
0.002 0.003 0.000 -0.024 -0.007 -0 006 -0 007 -0 008 -0 021 -0 018 -0 013 -0 012 0 005 0.006 0.009 0.021 0.002
5.244 3.708 3.028 2.6242 2.3460 2.1415 1.9827 1.8546 1.7490 1.6591 1.5144 1.4549 1.3112 1.2720 1.2361 1.2030 1.1728
Structure
refinement
and
I/I o 83 39 31 i000 116 98 106 351 i0 14 737 16 172 19 25 20 253
result
X - r a y p o w d e r d a t a for R i e t v e l d r e f i n e m e n t w e r e c o l l e c t e d on a STOE STADI/P powder diffractometer, using a rotating sample m o u n t e d in s y m m e t r i c t r a n s m i s s i o n m o d e . A g e r m a n i u m c r y s t a l monochromator w i t h a 220 m m r a d i u s of c u r v a t u r e w a s u s e d to obtain strictly monochromated C u K e I r a d i a t i o n . The d a t a w e r e c o l lected with a small linear-position sensitive detector covering 4.6 ° in 28. In o r d e r to r e d u c e the i n f l u e n c e of e r r o r s in i n t e n s i t y c a l i b r a t i o n of the d e t e c t o r , it w a s m o v e d in s t e p s of 0.2 ° , t h u s g i v i n g an a v e r a g e i n t e n s i t y f r o m 23 (= 4 . 6 / 0 . 2 ) m e a s u r e m e n t s at e a c h t h e t a p o s i t i o n . F u l l - p r o f i l e R i e t v e l d r e f i n e m e n t of the s t r u c t u r e w a s c a r r i e d o u t w i t h the p r o g r a m D B W 3 . 2 S (6), w h i c h is a s u c c e s s o r to the p r o g r a m d e s c r i b e d in Ref. (7). T h e a n g u l a r d e p e n d e n c e of the p e a k f u l l - w i d t h (2G) at h a l f - m a x i m u m w a s d e s c r i b e d by the u s u a l q u a d r a tic e q u a t i o n in t a n e: FWHM 2 = Utan28
+ Vtan
e + W
w h e r e U, V a n d W are p a r a m e t e r s to r e f i n e . T h e m i n i m u m a n d m a x i m u m v a l u e s for the h a l f - w i d t h s w e r e f o u n d to be 0 . 0 8 3 ° a n d 0 . 1 4 6 ° (28) r e s p e c t i v e l y . S t e p i n t e n s i t i e s in the 2 e - r a n g e 14 ° to 83 ° w e r e u s e d in the r e f i n e m e n t . T h e s t e p - l e n g t h in 28 w a s 0 . 0 1 ° . In t h e r e f i n e m e n t o n e s c a l e f a c t o r for a s m a l l i m p u r i t y of T i 0 2 , r u t i l e , w a s r e f i n e d . T h e s t r o n g e s t T i O 2 r e f l e c t i o n s h o w e d an int e n s i t y of a p p r o x i m a t e l y 1% of the s t r o n g e s t r e f l e c t i o n f r o m the main phase. In s e p a r a t e r e f i n e m e n t c y c l e s the Bi o c c u p a t i o n f a c t o r w a s c h e c k e d , a n d no s i g n i f i c a n t d e v i a t i o n f r o m the w e i g h t e d - i n s t o i chiometry Bi2/3Cu3Ti4Ol2 was detected. The following parameters
480
I. B R Y N T S E ,
et al.
Vol. 25, No. 4
for the m a i n p h a s e w e r e r e f i n e d : 2 p o s i t i o n a l p a r a m e t e r s , 4 isot r o p i c t e m p e r a t u r e f a c t o r s , 1 z e r o - p o i n t p a r a m e t e r , 1 s c a l e factor, 3 h a l f - w i d t h p a r a m e t e r s and 1 p r e f e r r e d - o r i e n t a t i o n param e t e r for the 224 direction. The refinement was terminated w h e n all s h i f t s in the p a r a m e t e r s w e r e less t h a n 10 % of the corr e s p o n d i n g s t a n d a r d d e v i a t i o n s . The f i n a l R - v a l u e s , Rp = 0.068, R w p = 0.087, R B = 0 . 0 3 4 and R F = 0.032, w e r e o b t a i n e d for the pos i t l o n a l p a r a m e t e r s l i s t e d in T a b l e 2. B e c a u s e of s e r i a l c o r r e l a t i o n as i n d i c a t e d by a D u r b i n - W a t s o n D - v a l u e = 0.4, the s t a n d a r d d e v i a t i o n s o b t a i n e d in the l e a s t - s q u a r e s r e f i n e m e n t s are m u l t i p l i e d by 2.5 in the table. The o b s e r v e d and c a l c u l a t e d p a t t e r n s are s h o w n in Fig. 3. S e l e c t e d i n t e r a t o m i c d i s t a n c e s are l i s t e d in T a b l e 3. A s e c t i o n of the s t r u c t u r e in the a b - p l a n e is s h o w n in Fig. 4. T h e o x y g e n p o s i t i o n of i d e a l p e r o v s k i t e is in the m i d d l e of e v e r y m e t a l s q u a r e . In the r e f i n e d s t r u c t u r e the o x y g e n s are m o v e d c l o s e r to the c o p p e r a t o m s a n d t h e r e f o r e the o c t a h e d r a are tilted approximately 18 0 a w a y f r o m the x, y a n d z d i r e c t i o n s .
TABLE
2
P o s i t i o n a l P a r a m e t e r s of B i 2 / 3 [ ] I / 3 C u 3 T i 4 0 1 2 , w i t h E s t i m a t e d S t a n d a r d D e v i a t i o n s in P a r e n t h e s e s . Atom
Position
x/a
y/b
z/c
Bi
2 (a)
0
0
0
Cu
6 (b)
0
1/2
1/2
Ti
8 (a)
1/4
1/4
1/4
0.296(2)
0.178(2)
0
O
24(g)
TABLE Selected Standard
TiO 6 octahedron 6 Ti - 0 BiOl2 12 Bi
- 0
3
Interatomic Distances (A) w i t h D e v i a t i o n s in P a r e n t h e s e s ,
1.960(5) polyhedron
Estimated
CuO 8 p o l y h e d r o n 4 Cu - O
2.01(2)
4 Cu - 0
2.83(2)
2.56(2)
T h e four n e a r e s t o x y g e n n e i g h b o u r s of c o p p e r f o r m a r e c t a n g l e a n d the b o n d i n g d i s t a n c e s (cf. T a b l e 3) a g r e e w i t h C u ( I I ) - O d i s t a n c e s f o u n d in the l i t e r a t u r e . E v e r y s e c o n d m e t a l r o w in the a b - p l a n e (cf. Fig. 4) c o n s i s t s of o n l y c o p p e r b u t the o x y g e n r e c t a n g l e s a r o u n d t h e ' c o p p e r a t o m s in t h e s e r o w s are perpendicular to e a c h o t h e r , as s h o w n by the d a s h e d l i n e s in the figure. The c o o r d i n a t i o n p o l y h e d r a of the c o p p e r and t i t a n i um c a t i o n s are s h o w n in Fig. 5.
i
Ji
i
,
481
PEROVSKITES
Vo]. 25, No. 4
3000.
aOOO,
~000
,
15OO
.
~OOO
.
SO0
.
F -r-' ' ' I '' '-' I SO,
~S
•
' ' ' I ~ ' ~ ' 60.
S~.
)
~ "TO.
~
'
' } ' ' i , I ' ' ' Ze ~S.
80.
,f FIG. 3 Final Rietveld plot. In the upper portion the observed data are shown as a solid line. Calculated intensities are shown as dots. The lower portion is a plot of the differences Iobs - Icalc. The strongest TiO 2 peak is marked with a cross.
I I
I|I
III
I
III
I
II
482
I. B R Y N T S E ,
o1 x
x
X
X
et B/.
o
......:.............. i
Vol. 25, No. 4
,
ii
o
FIG. 4 S e c t i o n in the a b - p l a n e of B i 2 / 3 C u 3 T i 4 O l 2 . The large circles indicate Bi-sites, and the b l a c k spots s h o w Cup o s i t i o n s . C r o s s e s and small b l a c k spots s h o w o x y g e n sites in ideal p e r o v s k i t e and this s t r u c t u r e r e s p e c tively. D a s h e d lines indicate the o r i e n t a t i o n of the o x y g e n p l a n e s a r o u n d Cu in one c o p p e r row.
FIG. 5 A s t r u c t u r e m o d e l of B i 2 / 3 C u 3 T i 4 0 1 2 , s h o w i n g TiO 6 o c t a h e d r a and CuO 4 g r o u p s d r a w n as w h i t e r e c t a n g l e s . One Bi p o s i t i o n is i n d i c a t e d w i t h a circle.
The m o s t n o t a b l e d i f f e r e n c e b e t w e e n the B i 2 / 3 C u 3 T i 4 O l 2 structure and t h a t of C a C u 3 M n 4 0 1 2 p r e s e n t e d by C h e n e v a s et al. (5) is the s t a t i s t i c a l l y d i s t r i b u t e d Bi atoms in the 2(a) p o s i t i o n . C o r r e s p o n d i n g d i s t r i b u t i o n s have b e e n d e s c r i b e d for a n u m b e r of i s o s t r u c t u r a l rare e a r t h c o m p o u n d s i n v e s t i g a t e d by B o c h u et al.
(8). Conclusions This p a p e r is p a r t of an i n v e s t i g a t i o n of the t e r n a r y s y s t e m Bi203 - C u O - TiO 2. As s h o w n in the p r e s e n t w o r k we c o u l d o b t a i n a n e w p e r o v s k i t e r e l a t e d o x i d e B i 2 / 3 C u 3 T i 4 O l 2 c r y s t a l l i z i n g in a C a C u 3 M 4 O I 2 type structure. The s t r u c t u r e was r e f i n e d from a sample c o n t a i n i n g a small i m p u r i t y of Ti02 by f u l l - p r o f i l e r e f i n e m e n t of its X - r a y p o w d e r d i f f r a c t i o n pattern. The d a t a w e r e o b t a i n e d by a S T A D I / P d i f f r a c t o m e t e r in t r a n s m i s s i o n mode. The q u a l i t y of the d a t a can be j u d g e d from the final s t r u c t u r e factor R - v a l u e 0.032.
Vol.
25, No. 4
PEROVSKITES
483
Acknowledgements This w o r k has r e c e i v e d f i n a n c i a l s u p p o r t from the S w e d i s h N a t u r a l S c i e n c e R e s e a r c h Council. One of the a u t h o r s (PEW) also w i s h e s to t h a n k the K n u t & A l i c e W a l l e n b e r g F o u n d a t i o n for a g r a n t e n a b l i n g us t o a c q u i r e a p o w d e r d i f f r a c t o m e t e r .
References I. 2. 3. 4. 5. 6. 7. 8.
H. Maeda, Y. Tanaka, M. F u k o t o m i and T. Asano, Jpn. J. Appl. Phys. 27, L209 (1988). T. Nogami, S. Kosaka, Y. Shirota, H. N a k a y a m a and H. Fujita, Mat. Res. Bull. 23, 1145 (1988). P.G. H a m b l i n g , A e t a C r y s t a l l o g r . 6, 98 (1953). K.E. J o h a n s s o n , T. P a l m and P.-E. Werner, J. Phys. E: Sci. Instrum. 13, 1289 (1980). J. C h e n e v a s , J.C. Joubert, M. M a r e z i o and B. Bochu, J. S o l i d S t a t e Chem. 14, 25 (1975). D.B. Wiles, A. S a k t h i v e l and R.A. Young, P r o g r a m D B W 3 . 2 S (1987) D.B. W i l e s and R.A. Young, J. Appl. C r y s t a l l o g r . 14, 149 (1981) . B. Bochu, M.N. D e s c h i z e a u x , J.C. Joubert, A. Collomb, J. C h e n e v a s and M. M a r e z i o , J. S o l i d State Chem. 29 291 (1979).