ETSUO AKIBA, 1 HIROSHI HAYAKAWA, 1 SHIGENOBU HAYASHI, 1 RITSURO MIYAWAKI,2t .... Rietveld refinement introducing distance constraints for Si-O.
Clays and Clay Minerals, Vol. 45, No. 6, 781-788, 1997.
S T R U C T U R E R E F I N E M E N T OF SYNTHETIC D E U T E R A T E D KAOLINITE B Y RIETVELD A N A L Y S I S U S I N G TIME-OF-FLIGHT NEUTRON POWDER DIFFRACTION DATA ETSUO AKIBA, 1 HIROSHI HAYAKAWA, 1 SHIGENOBU HAYASHI, 1 RITSURO MIYAWAKI,2t SHINJI TOMURA, 2 YASUO SHIBASAKI, 2 FUJIO IZUMI, 3 HAJIME ASANO4 AND TAKASHI KAMIYAMA4 National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki, 305 Japan 2 National Industrial Research Institute of Nagoya, 1-1 Hirate-cho, Kita-ku, Nagoya, 462 Japan 3 National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki, 305 Japan 4 Institute for Materials Science, University of Tsukuba, Tenoudai, Tsukuba, Ibaraki, 305 Japan A b s t r a c t - - T h e crystal structure of synthetic deuterated kaolinite was refined by Rietveld analysis using time-of-flight (TOF) neutron powder diffraction data. For non-hydrogen atoms, C1 symmetry was assumed. Starting models were tested in which only the direction of O-D vectors was varied. The constraints were introduced to all A1-O, Si-O and O-D bonds. The refinement adopting the former gives PI(C1), a
= 5.169(1) /k, b = 8.960(2) ,&, c = 7.410(2) ~, ct = 91.26(2) ~ [3 = 104.99(2) ~ ~/ = 89.93(1) ~ Rwp = 3.17%, R; = 5.78% and S = 1.34 with constraints of/(A1-O) = 1.93 + 0.05 /~, /(Si-O) = 1.62 + 0.03 ,~ and /(D-O) = 0.95 + 0.15 ,~. The inner O-D vector points toward the tetrahedral sheet. All innersurface O-D groups form H bonding with basal O atoms in the next kaolinite layers. The results agreed with those obtained from natural kaolinite. Key Words---Crystal Structure, Kaolinite, Neutron Powder Diffraction, Rietveld Refinement, Synthetic Kaolinite.
INTRODUCTION K a o l i n i t e is o n e o f the m o s t i m p o r t a n t r a w m a t e r i a l s for industries s u c h as c e r a m i c s a n d papers. Its plasticity is p r o b a b l y related to the H b o n d i n g s b e t w e e n the surface o f the k a o l i n i t e particle a n d w a t e r m o l e c u l e s o n it. K n o w l e d g e o f the n a t u r e o f the H b o n d i n g s in t h e crystal structure s h o u l d b e helpful for u n d e r s t a n d i n g the c h e m i s t r y o f the k a o l i n i t e particle surface. O f course, the crystal structure o f kaolinite h a s b e e n studied e x t e n s i v e l y for m a n y years. A d a m s (1983) refined the crystal structure o f n a t u r a l k a o l i n i t e (St. A u s tell kaolinite) u s i n g the n e u t r o n p o w d e r diffraction t e c h n i q u e w i t h C1 s y m m e t r y . Y o u n g a n d c o w o r k e r s ( S n i t c h a n d Y o u n g 1983; Y o u n g a n d H e w a t 1988) rep o r t e d the c r y s t a l structure o f K e o k u k k a o l i n i t e b y R i e t v e l d analysis u s i n g n e u t r o n p o w d e r diffraction data. Its structure w a s d e s c r i b e d o n the basis o f a space g r o u p P1 a n d it w a s r e p o r t e d that the 2 i n n e r 0 - t t vectors in its u n i t cell w e r e differently oriented. B i s h a n d v o n D r e e l e (1989) d e t e r m i n e d the a t o m i c coordinates for K e o k u k k a o l i n i t e b y X - r a y d i f f r a c t i o n ( X R D ) analysis, e x c e p t h y d r o g e n atoms. B i s h (1993) m a d e a f u r t h e r r e f i n e m e n t o f the crystal structure u s i n g the data o f Y o u n g a n d H e w a t (1988). T h e c r y s t a l structure refined b y B i s h (1993) h a d s y m m e t r y C1 a n d the i n n e r O - t t v e c t o r w a s a l m o s t parallel to the (001) plane. t Current address: Department of Geology, National Science Museum, 3-23-1, Hyakunin-cho, Shinjuku, Tokyo, 169 Japan. Copyright 9 1997, The Clay Minerals Society
781
O n the o t h e r h a n d , s o m e o f the authors h a v e b e e n i n v e s t i g a t i n g the synthesis o f h i g h - p u r i t y k a o l i n i t e as t h e m a i n c o m p o n e n t o f the h i g h - p e r f o r m a n c e artificial clay for the c e r a m i c s i n d u s t r y ( S h i b a s a k i et al. 1992). T h e synthetic k a o l i n i t e is m o d e r a t e - to h i g h - d e f e c t kaolinite w i t h a relatively l o w e r H i n c k l e y I n d e x ( P l a n ~ o n et al. 1988 a n d 1989). It was n e c e s s a r y to c h a r a c t e r i z e t h e synthetic s a m p l e to control the defect b y the synthesis. I n the p r e s e n t work, a s a m p l e o f d e u t e r a t e d k a o l i n i t e was obtained, a p p l y i n g the t e c h n i q u e o f h y d r o t h e r m a l synthesis o f kaolinite, a n d the crystai structure o f deuterated s y n t h e s i z e d k a o l i n i t e w a s refined to o b t a i n inf o r m a t i o n for f u r t h e r i m p r o v e m e n t o f k a o l i n i t e synthesis. T h e r e a s o n for u s i n g d e u t e r a t e d k a o l i n i t e is that p r o t i u m (H) h a s a large, i n c o h e r e n t scattering cross section o f 7.99 • 103 m 2, a n d c o n s e q u e n t l y it m a k e s the s i g n a l - t o - n o i s e ratio in the n e u t r o n diffraction pattern high, as c a n b e seen in the structure r e f i n e m e n t s o f natural k a o l i n i t e (see F i g u r e 2 o f Y o u n g a n d H e w a t 1988). EXPERIMENTAL Materials D e u t e r a t e d k a o l i n i t e w a s p r e p a r e d b y the p r o c e d u r e d e s c r i b e d p r e v i o u s l y ( M i y a w a k i et al. 1991) u s i n g h e a v y w a t e r ( D 2 0 ) i n s t e a d o f distilled w a t e r (H20). Silica sol a n d a l u m i n a sol ( S n o w t e x - N a n d A l u m i n a sol-200; N i s s a n C h e m i c a l Industries, Ltd.) w e r e m e c h a n i c a l l y m i x e d in a Si:A1 ratio o f 1:1 a n d h e a t e d at
782
Akiba et al. Table 1. Initial crystal structure models. z Coordinatesof the innerO--Dvectors (occupancy) O(6i):z D(1):z O(611):z D(l"~):z
Model C1 up C1 d own P1 (up-down)
Random
0.3220 (1.0) 0.3220 (1.0) 0.3220 (1,0) 0.3220
0.3530 (1.0) 0.2660 (1.0) 0.3530 (1.0) 0.3530 (0.5)~ 0.2660 (0.5)t
0.3220
0.3530
0.3220
0.2660
0.3220
0.2660
0.3220 (0.5)tt
0.2660
Table 3. R factors obtained with different distance constraints. I(A1-O)
1.89 1.91 1.93 1.95 1.97
+ 0.05 -+ 0.05 -+ 0.05 -+ 0.05 -+ 0.05
Rwp
A ,A A, A, /~
3.17% 3.16% 3.12% 3.20% 3.25%
Rv
2.48% 2.48% 2.49% 2.53% 2.57%
S
R1
RE
1.34 1.33 1.32 1.35 1.37
5.92% 5.83% 5.11% 5.95% 6.15%
4.37% 4.27% 3.28% 4.58% 4,84%
l(Si-O) = 1.62 -+ 0,03 -~, in the C1 dow n model. l(Si-O)
0.3530 (0.5)t?
~,~T: The z coordinates were refined but the sum of the occupancy of each atom was kept at unity. Symmetry codes: i) x, y - 1, z; ii) x + 1/~,y _ 89 z; iii) x + 89 y+ 89
Clays and Clay Minerals
Rwo
1.60 --+ 0.03 ,~ 1.62 --+ 0.03 A 1.64 --+ 0.03 ,~,
3.12% 3.12% 3.11%
Rp
2.49% 2.49% 2.48%
S
Rt
RF
1.32 1.32 1.32
5.12% 5.11% 5.15%
3.29% 3.28% 3.32%
l(A1-O) = 1.93 -+ 0.05 ,~, in the C1 dow n model.
z.
873 K for 28.8 ks. T h e n the m i x t u r e w a s c r u s h e d a n d p a s s e d t h r o u g h a 7 0 - m e s h sieve. F o u r g r a m s o f the m i x t u r e a n d 16 c m 3 o f h e a v y w a t e r ( W a k o J u n y a k u , > 9 9 . 7 5 % ) w e r e p l a c e d i n a Teflon p r e s s u r e vessel w i t h a c a p a c i t y o f 25 c m 3 (San-ai K a g a k u Ltd., H U - 2 5 ) a n d h e a t e d at 493 K for 4 3 2 ks in a n electric furnace. X - r a y diffraction p a t t e r n s o f d e u t e r a t e d synthetic kaolinite w e r e essentially identical w i t h those p r e p a r e d u s i n g H20. H o w e v e r , the X R D pattern o f the s y n t h e t i c k a o l i n i t e s u g g e s t e d that profiles o f the diffraction peaks were broader than those of a standard sample such as A P I #9. T h e k a o l i n i t e c o n t e n t in the p r o d u c t w a s e s t i m a t e d f r o m a w e i g h t loss u s i n g t h e r m o g r a v i m etry a n d f o u n d to b e a b o u t 90%. T h i s m e a n t that a certain a m o u n t o f a m o r p h o u s m a t e r i a l s was i n c l u d e d in the sample. T h e d e g r e e o f d e u t e r a t i o n was c o n firmed b y i n f r a r e d (IR) spectroscopy. Signals due to O - H stretching ( 3 7 0 0 - 3 6 0 0 c m l) were v e r y weak, w h i l e s h a r p p e a k s d u e to O - D stretching were clearly o b s e r v e d at 2 7 5 0 - 2 6 5 0 c m -l. T h e s e results s h o w that d e u t e r a t i o n o f k a o l i n i t e w a s nearly perfect. F o r c o m p a r i s o n , w e m e a s u r e d X R D data o f the lowdefect k a o l i n i t e (API #9). T h e H i n c k l e y indices were 0.8 for synthetic k a o l i n i t e a n d 1.5 for A P I #9.
1987), at the n e u t r o n facility ( K E N S ) at the N a t i o n a l L a b o r a t o r y for H i g h E n e r g y P h y s i c s (KEK). T h e sample was p l a c e d in a c y l i n d r i c a l v a n a d i u m c o n t a i n e r 10 n u n in d i a m e t e r a n d 25 txm in thickness. T h e diffraction a n g l e was fixed at 170 ~, a n d the gate w i d t h was set at 4 - 1 6 Ixs, d e p e n d i n g o n T O E T h e diffraction data w e r e c o l l e c t e d at r o o m t e m p e r a t u r e for a b o u t 43 ks. The computer program RIETAN, which was develo p e d b y o n e o f the authors, was u s e d for X - r a y a n d n e u t r o n R i e t v e l d analysis ( I z u m i 1993). I n t e r a t o m i c distances a n d b o n d angles w e r e calculated u s i n g D A P H (Sakurai 1967) f r o m structural p a r a m e t e r s obtained b y R I E T A N . X - r a y p o w d e r diffraction data w e r e o b t a i n e d at room temperature using a Rigaku RAD-A diffractometer w i t h CuKtx radiation. P r e f e r r e d o r i e n t a t i o n w a s red u c e d u s i n g the m e t h o d r e p o r t e d b y M c M u r d i e et al. (1986) for X - r a y m e a s u r e m e n t s . T h e results o f Rietv e l d analysis w i t h a n d w i t h o u t i n t r o d u c i n g p r e f e r r e d o r i e n t a t i o n p a r a m e t e r s w e r e essentially identical for b o t h the X - r a y a n d n e u t r o n data, w h i c h suggests that preferred o r i e n t a t i o n c a n b e ignored.
Neutron and X-ray Powder Diffraction Measurements
Structural m o d e l s for R i e t v e l d analysis are as follows. T h e f r a m e w o r k c o m p o s e d o f Si, AI, O a t o m s is
N e u t r o n p o w d e r d i f f r a c t i o n data w e r e m e a s u r e d o n a T O F p o w d e r diffractometer, H R P ( W a t a n a b e et al. Table 2. Metal-oxygen bond lengths and R factors before introducing constraints. Si(1)-O Si(2)-O AI(1)-O Al(2)-O
1.55(6), 1.79(4), 1.65(6), 1.87(5), 1.66(4), 1.60(5), 1.84(7), 1.81(7), 2.47(6), 1.64(7), 1.69(6) ,~ 1.38(7), 1.73(6), 2.18(6), 2.19(6), 2.39(7)
R~p = 2 . 5 4 % Rr = 2.33%.
( S = Rwp/R e =
1.07),
1.69(5) 1.57(6),~ 1.90(6),
RESULTS AND DISCUSSION R i e t v e l d R e f i n e m e n t U s i n g the T O F N e u t r o n D a t a
Table 4. Metal-oxygen bond lengths and R factors by the Rietveld refinement introducing distance constraints for Si-O and A1-O. Si(1)-O 1.67(3), 1.59(2), 1.65(4), Si(2)-O 1.67(3), 1.65(2), 1.66(4), AI(1)-O 1.99(4), 1.87(5), 1.88(5), 1.88(5), 1.88(4) A Al(2)-O 1.98(5), 1.99(5), 1.87(5), 1.99(4), 1.98(5)/~
1.65(4)/~ 1.65(5) /~ 1.98(4), 1.96(5),
2.08(6), Rp =
1.99%,
R l = 2.83%,
Rwp = 3.03% (S = Rwp/Re = 1.28), Rp = 2.40%, R t = 4.13%, RF = 2.55%. Constraints: I(A1-O) = 1.93 _+ 0.05 ,~ and l(Si-O) = 1.62 -+ 0.03 A.
Vol. 45, No. 6, 1997
Structure refinement of synthetic deuterated kaolinite
Table 5. R factors obtained by the Rietveld refinement changing hydrogen positions. Start model
C1 down'f C1 u p t PI(2)J" Random'~82 2 sites#
Rwp
Rp
3.27% 2.58% 3.33% 2.64% 3.31% 2.66% . . . 2.94% 2.32%
S
Rt
Rr
1.38 1.41 1.40 . 1.25
6.92% 6.72% 7.32% . 4.18%
4.73% 5.05% 5.21% 2.83%
t With constraints of I(A1-O) = 1.93 • 0.05 J,, l(Si-O) = 1.62 -+ 0.03 ,~ and /(D-O) = 0.95 -+ 0.15 J,, using the C1 down. 82Refinement was not converged in this model. # Two positions for the D atom side by side in O(7)-D(2)0(4) H bondings with constraints of I(A1-O) = 1.93 • 0.05 and I(Si-O) = 1.62 • 0.03 A using the C1 down model.
a s s u m e d to b e C1, as several authors p o i n t e d out (Ada m s 1983; S u i t c h a n d Y o u n g 1983; Y o u n g a n d H e w a t 1988; B i s h a n d v o n D r e e l e 1989; B i s h 1993). A starting m o d e l for n o n - h y d r o g e n a t o m s is that o f B i s h a n d v o n D r e e l e (1989). L a b e l s for a t o m s in the p r e s e n t w o r k are identical to t h o s e that t h e y d e s c r i b e d e x c e p t that the x a n d y c o o r d i n a t e s o f A l ( 2 ) are x - 1/~ a n d y + 89 o f those r e p o r t e d b y t h e m a n d that 0 ( 5 + n) a n d D ( n ) a t o m s c o r r e s p o n d to O a n d D a t o m s o f O H ( n ) g r o u p s in t h e i r paper. T h e initial c o o r d i n a t e s for the hydrogen atoms were taken from Young and Hewat (1988). S i n c e R factors d e c r e a s e w i t h a n i n c r e a s e i n the n u m b e r o f refinable p a r a m e t e r s w i t h o u t a n y substantial i m p r o v e m e n t in R i e t v e l d r e f i n e m e n t , w e k e p t the total n u m b e r s o f refinable p a r a m e t e r s in the final r e f i n e m e n t s w i t h i n the r a n g e o f 6 8 - 7 5 . In the p r e s e n t work, all o c c u p a t i o n factors w e r e fixed at 1.0 a n d all isotropic t h e r m a l p a r a m e t e r s at 0.5. T h e c o o r d i n a t e s o f the i n n e r O - D groups in the present starting m o d e l s are listed i n Table 1. T h e C1 up m o d e l h a s C1 s y m m e t r y , a n d the i n n e r O - D v e c t o r p o i n t s t o w a r d the o c t a h e d r a l sheet. T h e i n n e r 0 - 1 ) vector i n the C1 d o w n m o d e l p o i n t s a w a y f r o m the octahedral sheet, in o t h e r words, t o w a r d the tetrahedral sheet. T h e i n n e r O - D v e c t o r in the P1 m o d e l orients alternatively t o w a r d a n d a w a y f r o m the o c t a h e d r a l sheet. I n the r a n d o m m o d e l , i n n e r O - D vectors rand o m i y orient. A b r o a d p e a k d u e to a n a m o r p h o u s p h a s e was obs e r v e d in a d r a n g e o f 3 . 3 - 4 . 7 ,~ i n the X R D patterns. However, o n l y the intensity data b e t w e e n d = 0.8 a n d 3.3 ,~ w e r e u s e d in the R i e t v e l d r e f i n e m e n t s o f the T O F data b e c a u s e the intensities o f diffraction p e a k s at d - v a l u e s larger t h a n 3 A w e r e weak. Therefore, this b r o a d p e a k d u e to the a m o r p h o u s p h a s e does n o t affect the analysis o f the n e u t r o n data. A m e r i t o f the T O F d i f f r a c t o m e t e r is that reliable i n f o r m a t i o n o n reflections w i t h s m a l l d - v a l u e s c a n b e o b t a i n e d b e c a u s e o f the h i g h r e s o l u t i o n in the h i g h Q or low w a v e l e n g t h region. Therefore, it fits the structure r e f i n e m e n t o f this material.
783
Table 6. Interatomic distances and R factors obtained by the Rietveld refinement introducing distance constraints for Si-O, A1-O and D-O. Si(1)-O 1.66(3), 1.66(2), 1.66(5), Si(2)-O 1.67(3), 1.65(2), 1.66(4), AI(1)-O 2.00(5), 1.88(5), 1.88(5), 1.87(5), 1.88(5) A A1(2)-O 1.98(6), 1.99(5), 1.93(6), 1.99(5), 1.88(6) A 0.93(6), 0.94(4), 0.90(5), D-O
1.66(4) 1.65(5) ,~ 1.99(5), 1.90(5), 0.83(5) ,~
Rwp = 3.17% (S = Rwp/R~ = 1.34), Rp = 2.49%,/71 = 5.78%, Re = 4.07%. Constraints: I(A1-O) = 1.93 --- 0.05 A, I(Si-O) = 1.62 +-0.03 J, and/(D-O) = 0.95 • 0.15 ,~.
Table 2 s h o w s the r e s u l t i n g R factors a n d s o m e int e r a t o m i c distances. A s B i s h a n d v o n D r e e l e (1989) reported, the o b t a i n e d distances h a v e a w i d e r a n g e c o m p a r e d w i t h t h o s e o f dickite ( A d a m s a n d H e w a t 1981; J o s w i g a n d Drits 1986) a n d nacrite ( B l o u n t et al. 1969). In order to o b t a i n r e a s o n a b l e distances, n o n l i n e a r c o n s t r a i n t s were i m p o s e d o n A1-O a n d S i - O dist a n c e s (Table 3). O n e x a m i n a t i o n o f Table 3, w e adopte d the c o n s t r a i n t s o f I(A1-O) = 1.93 + 0.05 ,~ a n d l(Si-O) = 1.62 + 0.03 /~. Table 4 s h o w s the results o f R i e t v e l d analysis after i n t r o d u c i n g the d i s t a n c e constraints. I n t e r a t o m i c dist a n c e s b e t w e e n D(2) a n d O a t o m s are 1.63(4) ,~ for O ( 7 ) - D ( 2 ) a n d 1.63(4) ,~ for O(4)-D(2). T h a t is, the D ( 2 ) a t o m is located at the m i d d l e o f the 2 0 a t o m s in the interlayer space. T h i s H p o s i t i o n is unrealistic. Therefore, 3 m o d e l s w e r e i n t r o d u c e d for f u r t h e r ref i n e m e n t o f H positions. O n e is a m o d e l w i t h a relatively " s o f t " O - D d i s t a n c e constraint. T h e s e c o n d o n e
Figure 1. Hydrogen positions refined using the 2 hydrogen sites' model. Interatomic distances are O(4)-D(21): 1.51 A, O(7)-D(21): 1.50 ,~; O(4)-D(22): 2.06 ~ and O(7)-D(22): 2.42 A. Bond angles are O(4)-D(21)-O(7): 151 ~ and 0(4)D(22)-O(7): 81 ~
784
Clays and Clay Minerals
Akiba et al.
2400 t-
1800
O
C) 1200
120
80
160
200
240
280
d~ pm Figure 2. Results of Rietveld analysis for synthetic deuterated kaolinite. Observed and calculated diffraction profiles are shown in dots and the line, respectively. Short bars indicate Bragg angles, and Ay shows the difference between observed and calculated data.
is a m o d e l w i t h 2 H sites side b y side b e t w e e n 0 ( 7 ) a n d 0 ( 4 ) . T h e results o f the r e f i n e m e n t s a d o p t i n g the 2 m o d e l s are s h o w n in Tables 5 a n d 6. F i g u r e 1 s h o w s H p o s i t i o n s refined u s i n g the 2 H-site models. O n e H a t o m , D ( 2 1 ) , still locates at t h e m i d d l e o f the t w o O atoms, b u t a n o t h e r h y d r o g e n , D(22), m o v e s far f r o m the original position. T h e i n t e r a t o m i c distances bet w e e n the D ( 2 2 ) a t o m a n d t h e n e i g h b o r i n g O a t o m s Table 7. Crystal structure parameters of deuterated kaolinite. Space group~ PI(C1); V = 331.4,~3; a = 5.1691(11).~; b = 8.9595(19) A; c = 7.4101(18) ,~; a = 91.262(16)~ ~ = 104.98(2)~ ~ = 89.928(14)~ Kaolinite 2(Si2A12Os(OD)4 ) Atom
AI(1) Al(2) Si(1) Si(2) O(1) 0(2) O(3) 0(4) 0(5) O(6) 0(7) 0(8) 0(9) D(1) D(2) D(3) D(4)
la la la la la la la la la la la la la la 1a la la
gt
x
y
z
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
0.291(6) 0.326(8) 0.984(8) 0.522(7) 0.063(6) 0.128(7) 0 0.206(5) 0.236(4) 0.082(6) 0.012(7) 0.049(6) 0.052(7) 0.152(8) 0.118(6) 0.007(7) 0.073(7)
0.521(5) 0.840(5) 0.341(2) 0.164(2) 0.366(4) 0.675(4) 89 0.220(4) 0.727(3) 0.996(5) 0.184(4) 0.506(4) 0.885(5) 0.080(3) 0.225(4) 0.526(4) 0.839(4)
0.501(4) 0.507(6) 0.107(3) 0.113(3) 0.338(4) 0.345(4) 0 0.065(5) 0.001(4) 0.396(4) 0.652(5) 0.649(5) 0.624(5) 0.350(5) 0.794(5) 0.757(4) 0.723(5)
Br
0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
R ~ = 3.17% (S = R~/R~ = 1.34), Rp = 2.49%, R~ = 5.78%, 4.07%. r Occupation factor. ~'~"Isotropic thermal parameter. The refinement was carried out by adopting the C1 down model. Constraints were as follows: I(A1-O) = 1.93 + 0.05 A, l(SiO) = 1.62 _+ 0.03 ~ and/(D-O) = 0.95 --- 0.15 .~. Labels of non-hydrogen atoms are identical to those reported by Bish and von Dreele (1989) (see text). RF =
( 0 ( 7 ) a n d 0 ( 4 ) ) are 2.06 ,~ a n d 2.42 ,~, respectively. In addition, w e p u t the D ( 2 ) a t o m into 2 sites l i n e a r l y located b e t w e e n the o x y g e n 0 ( 7 ) a n d 0 ( 4 ) a t o m s as the t h i r d m o d e l . T h i s c a n b e called the " d o u b l e m i n i m u m h y d r o g e n p o s i t i o n m o d e l " . H o w e v e r , the position o f 1 site was a l m o s t identical to that i n t h e O - D c o n s t r a i n t m o d e l , b u t a n o t h e r site m o v e d far a w a y f r o m the O atoms. S i n c e s u c h results are unrealistic, w e d i d n o t take 2 H p o s i t i o n s for the D(2) site. O n the o t h e r h a n d , t h e results o f the r e f i n e m e n t w i t h the O - D distance constraint afforded reasonable interatomic d i s t a n c e s c o m p a r a b l e to t h o s e r e p o r t e d for dickite a n d n a c r i t e ( B l o u n t et al. 1969; A d a m s a n d H e w a t 1981; J o s w i g a n d Drits 1986). A m o n g the m o d e l s w i t h the d i s t a n c e c o n s t r a i n t s for A1-O, Si-O a n d D - O b o n d s , the C1 down m o d e l gives t h e l o w e s t R factors. W h e n the P1 and C1 up m o d e l s are adopted, i n n e r O-D v e c t o r t e n d s to direct to the tetrahedral sheet like the C1 down m o d e l w i t h increasing cycle o f the refinement. In the P1 and C1 up m o d els, the i n n e r 0-19 v e c t o r is finally d i r e c t e d to t h a t i n the C1 down structure, a n d the a t o m i c c o o r d i n a t e s are a l m o s t identical to t h o s e o b t a i n e d w i t h the C1 down model. T h e r e f i n e m e n t w i t h the r a n d o m m o d e l c o u l d n o t b e c o n v e r g e d u n d e r the s a m e c o n d i t i o n s as the o t h e r models. Therefore, w e c o n c l u d e that the structure o f synthetic d e u t e r a t e d k a o l i n i t e h a s the s y m m e t r y C1 a n d that the i n n e r O-D v e c t o r is o r i e n t e d to the tetrahedral sheet. F i g u r e 2 s h o w s the r e s u l t o f R i e t v e l d analysis a d o p t i n g t h e C1 down m o d e l w i t h the c o n straints for the A1-O, Si-O a n d D - O distances. Table 7 s h o w s the crystal structure p a r a m e t e r s o b t a i n e d for synthetic d e u t e r a t e d k a o l i n i t e u s i n g the C1 down m o d el. Table 8 lists the i n t e r a t o m i c d i s t a n c e s a n d b o n d angles o b t a i n e d b y t h e R i e t v e l d m e t h o d . T h e I n n e r O-D Vectors T h e d i r e c t i o n o f the i n n e r O-D vector(s) is c o n s i d ered to d e t e r m i n e the space g r o u p o f kaolinite. T h e r e -
Vol. 45, No. 6, 1997
Structure refinement of synthetic deuterated kaolinite
785
Table 8. Interatomic distances (A) and bond angles (o) of synthetic deuterated kaolinite refined with bond-distance contraints for A1-O, Si-O and D-O. Note: Estimated standard deviations in parentheses refer to the last digits. Constraints are as follows l(A1-O) = 1.93 _+ 0.05 A, l(Si-O) = 1.62 _+ 0.03/k and/(D-O) = 0.95 -+ 0.15 ,~. Tetrahedral sheet Si(1)-O(1 i) Si(1)-O(3 i) Si(1)-O(4 i) Si(1)-O(5 il)
1.66(3) 1.66(2) 1.66(5) 1.66(4)
Si(2)-O(2 il) Si(2)-O(3 ii) Si(2)-O(4) 5i(2)-O(5 ii)
1.67(3) 1.65(2) 1.66(4) 1.65(5)
Octahedral sheet AI(1)-O(1) Al(1)-O(2) Al(1)-O(6 ~) Al(1)-O(70 Al(1)-O(8) AI( 1) -O(9 ii)
2.00(5) 1.88(5) 1.88(5) 1.99(5) 1.87(5) 1.88 (5)
Al(2)-O(1 v) A1(2)-O(2) A1(2)-O(6) A1(2)-O(70 A1(2)-O(8 v) A1(2)-O(9)
O-D bonding O(6vi)-D(1)-O(4) O(7)-D(2)-O(4 vii) O(8)-D(3)-O(3 vii) O(9)-D(4)-O(5 vii) D(lvii)-O(4vii)-D(2) angle A1-A1 AI(1)-AI(2) AI( 1)-Al(2ii) AI(10-AI(2) Si-Si Si(1)-Si(2) Si(lv~i)-Si(2) Si(1)-Si(20
1.98(6) 1.99(5) 1.93(6) 1.90(5) 1.99(5) 1.88(6)
O-D distance 0.93(6) 0.94(4) 0.90(5) 0.83(5)
O(1)-O(3) O(1)-O(4) O(1)-O(5 iii) 0(3)-0(4) O(4)-O(5 iii) O(5~)-O(3) 0(2)-0(3) O(2)-O(4i0 0(2)-0(5) 0(5)-0(3) 0(3)-0(4 ~v) O(4iv)-O(5)
2.74(3) 2.64(5) 2.88(4) 2.73(3) 2.35(3) 2.80(3) 2.90(3) 2.63(4) 2.79(5) 2.37(3) 2.60(3) 2.90(4)
O(1)-O(2) O(2)-O(6 ii) O(6ii)-O(1) 0(8)-0(70 O(7v)-O(9ii) O(9~)-O(8) O(1)-O(9 ~) 0(2)-0(8) O(7v)-O(6~)
2.79(5) 2.79(5) 2.85(5) 2.87(5) 2.69(5) 2.86(5) 2.85(4) 2.86(5) 2.61(5)
O(1)-O(8) 0(2)-0(70 0(6)-0(9)
2.62(5) 2.60(4) 2.02(5)
O(lv)-O(2) 0(2)-0(6) O(6)-O(1 v) 0(8)-0(7) 0(70-0(9) 0(9)-0(80 O(1)-O(7) 0(6)-0(80 0(2)-0(9)
2.83(5) 2.91(6) 2.87(5) 2.90(5) 2.95(5) 2.75(5) 2.94(5) 2.65(4) 2.86(5)
D-O distance 2.55(5) 2.16(5) 1.83(3) 2.27(5)
O-D-O angle 147(3) 144(4) 157(4) 167(4)
O..O distance 3.39(5) 2.97(5) 2.68(4) 3.09(5)
A1-Si AI( 1)-Si( 1viii) Al(2)-Si(liv)
3.32(4) 3.27(5)
Ai( 1)-Si(2~0 AI(2)-Si(2iv)
3.15 (4) 3.30(5)
147(2) 2.87(6) 2.91 (6) 3.19(6) 2.89(5) 3.19(5) 2.89(3)
Symmetry codes:i) x + 1, y, z;ii) x + 89 y - 89 z;iii) x y - 1, z;vii) x , y , z + 1;viii) x - 1, y,z.
1A, y - 89 z;iv) x -
89 y + 89 z ; v ) x + 89 y + 89 z;vi) x,
fore, the structural studies o f kaolinite have placed emphasis on the direction of the inner O-D vector(s). For example, A d a m s (1983) reported that the angle between the O-D vector and the xy plane is 34 ~ and the
0-1) vector directs toward the tetrahedral sheet. He found that the s y m m e t r y is C1. On the contrary, Young and H e w a t (1988) reported that the inner O-D vectors direct toward the octahedral sheet and the tet-
786
Aldba et al.
Figure 3.
Clays and Clay Minerals
Structure of synthetic deuterated kaolinite viewed along the (100) direction. rahedral sheet alternately, and that, therefore, the space group is P1. In this work, we find that the inner 0-1) vector points toward the tetrahedral sheet and that the angle between the inner O-D vector and the xy plane is 28 ~ The direction of the inner O-D vector determined by us is different from the results of Bish (1993), in which the inner 0-1) vector is almost parallel to the (001) plane. Figure 3 shows the crystal structure refined in the present work. The inner D atom, D(1), has several neighboring O atoms such as 0(7) (surface O), 0(8) (surface O), O(1) (apical O), 0(2) (apical O), 0(3) (basal O) and 0(4) (basal O). Distances from the inner D(1) atom to these O atoms are 2.67(6) ,~ for 0(7), 2.70(5) ,~ for 0(8), 2.59(5) ,~ for O(1), 2.61(6) ,~ for 0(2), 3.58(5) ,~ for 0(3) and 2.55(5) ~, for 0(4). The D(1)-O(4) distance is the shortest among them. The projection of the structure for kaolinite on the xy plane in Figure 4 clearly indicates that the inner O-D vector directs toward the basal 0(4) oxygen. This suggests formation of a H bond between the inner O-D vector and basal 0(4) atom. The Inner-Surface O-D Vectors
Figure 4. The direction of the inner O-D vector (projection to the xy plane). Basal oxygen, silicon and the inner O-D vector are shown.
The observed interatomic distances of inner-surface O-D bonds are 0.94(4) for O(7)-D(2), 0.90(5) for O(8)-D(3) and 0.83(5) ,~ for O(9)-D(4). The average distances are slightly shorter than those reported by Bish (1993), but the tendency in the order of distances,
Vol. 45, No. 6, 1997
Structure refinement of synthetic deuterated kaolinite
Figure 5.
787
Hydrogen-bond chain in the kaolinite structure.
l(O(7)-D(2)) > l(O(8)-D(3)) > l(O(9)-D(4)), is identical. The formation of H bonding has been suggested for the inner surface O-D vectors (Adams 1983; Young and Hewat 1988; Collins and Catlow 1991). The observed interatomic distances in the interlayer are 2.16(5) ,~ for D(2)-O(4), 1.83(3) A for D(3)-O(3) and 2.27(5) ,~ for D(4)-O(5). These H bondings connect the kaolinite sheets and prevent a slip between these sheets. However, the O(7)-D(2)-O(4) H bondings differ from the other 2 H bondings in bond angles. The O(7)-D(2)-O(4) bond angle is 144(4) ~ while the 0(8)D(3)-O(3) angle is 158(4) ~ and the O(9)-D(4)-O(5) angle is 167(4) ~ Such variations in the angles of the inner surface O-D vectors are also described in the reports by Young and Hewat (1988) and by Bish (1993). Bish (1993) explained the difference in the
inner-surface O-D vectors by the assignment of the OH band in infrared spectra.
CONCLUSIONS Deuterated kaolinite was synthesized using heavy water instead of H20. The structure of the deuterated kaolinite has been refined by Rietveld analysis using TOF neutron diffraction data. All the inner O-D vectors direct toward the tetrahedral sheet and, therefore, the structure of deuterated kaolinite has the symmetry C1. The inner surface O-D groups are connected to the basal O of the neighboring kaolinite sheet with H bonding. The results obtained in the present work agree with those obtained from low-defect natural kaolinite.
788
Akiba et al.
ACKNOWLEDGMENTS The authors thank K. Kuroda of Waseda University for supplying the API #9 kaolinite sample and D. L. Bish of Los Alamos National Laboratory for sending the article before publishing. A part of the present study was supported by the Special Research Program for Important Regional Technology, Agency of Industrial Science and Technology, Ministry of International Trade and Industry, Japan. Two of the authors (E. A. and S. H.) thank the financial support for the present study from Special Coordination Funds of the Science and Technology Agency, Japan. REFERENCES Adams JM. 1983. Hydrogen atom positions in kaolinite by neutron profile refinement. Clays Clay Miner 31:352-356. Adams JM, Hewat AW. 1981. Hydrogen atom positions in dickite. Clays Clay Miner 29:316--319. Bish DL. 1993. Rietveld refinement of the kaolinite structure at 1.5K. Clays Clay Miner 41:738-744. Bish DL, von Dreele RB. 1989. Rietveld refinement of nonhydrogen atomic positions in kaolinite. Clays Clay Miner 37:289-296. Blount AM, Threadgold IM, Bailey SW. 1969. Refinement of the crystal sU'ucture of nacrite. Clays Clay Miner 17:185194. Collins DR, Catlow CRA. 1991. Energy-minimized hydrogen-atom positions of kaolinite. Acta Crystallogr B47:678682. Izumi E 1993. Rietveld analysis program RIETAN and PREMOS and special application. In: Young RA, editor. The Rietveld method. Oxford: Oxford University Pr. p 232-253.
Clays and Clay Minerals
Joswig W, Drits VA. 1986. The orientation of the hydroxyl groups in dickite by X-ray diffraction. N Jb Miner Mh 1986:19-22. McMurdie HF, Morris MC, Evans EH, Paretzkin B, WongNg W, Hybbard CR. 1986. Methods of producing standard X-ray diffraction powder patterns. Powder Diffract 1:4043. Miyawaki R, Tomura S, Samejima S, Okazaki M, Mizuta H, Maruyama S, Shibasaki Y. 1991. Effects of solution chemistry on the hydrothermal synthesis of kaolinite. Clays Clay Miner 39:498-508. Plan~on A, Giese RF, Snyder R. 1988. The Hinckley index for kaolinite. Clay Miner 23:249-260. Planqon A, Giese RE, Snyder R, Drits VA, Bookin AS. 1989. Stacking faults in the kaolin-group minerals: Defect structure of kaolinite. Clays Clay Miner 37:203-210. Sakurai T. 1967. In: Universal crystallographic computation program system (I). Crystallogr Soc Jpn. p 76 [in Japanese]. Shibasaki Y, Tomura S, Miyawaki R, Maeda M, Inukai K, Oda K. 1992. Synthesis technology of kaoliniteic clay. In: Nagasawa K, editor. Proc Workshop WB-1, Clay minerals: Their natural resources and uses; 29th Int Geol Cong; 1992; Nagoya, Japan. p 137-143. Snitch PR, Young RA. 1983. Atom positions in highly ordered kaolinite. Clays Clay Miner 31:357-366. Watanabe N, Asano H, Iwasa H, Satoh S, Murata H, Karahashi K, Tomiyoshi S, Izumi F, Inoue K. 1987. High resolution neutron powder diffractometer with a solid methane moderator at pulsed spallation neutron source. Jpn J Appl Phys 26:1164-1169. Young RA, Hewat AW. 1988. Verification of the triclinic crystal structure of kaolinite. Clays Clay Miner 36:225232.
(Received 21 February 1994; accepted 15 November 1996; Ms. 2471)
