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APPLICATION OF SYNCHROTRON RADIATION TO HIGH PRESSURE X-RAY DIFFRACTION
P. Hatton
To cite this version:
P. Hatton. APPLICATION OF SYNCHROTRON RADIATION TO HIGH PRESSURE X-RAY DIFFRACTION. Journal de Physique Colloques, 1984, 45 (C8), pp.C8-365-C8-368.
�10.1051/jphyscol:1984864�. �jpa-00224366�
JOURNAL DE PHYSIQUE
Colloque C&, supplément au n ° l l , Tome 45, novembre 1984 page C8-365
APPLICATION OF SYNCHROTRON RADIATION TO HIGH PRESSURE X~RAY DIFFRACTION
F.D. Hatton
SBRC Daresbuvy laboratory, Daresbury, Warrington, Cheshire WA4 4AD, U.K.
Résumé - La disponibilité de sources de radiation synchrotron à haute brillan- ce est à l'origine de nombreux progrès dans les techniques de diffraction des rayons x. En plus des techniques de dispersion polychromatique d'énergie lar- gement discutées, des progrès significatifs peuvent être accomplis grâce à l'utilisation de la radiation monochromatique lors d'un balayage angulaire ou de longueur d'onde. L'application de ces techniques développées à Daresbury, est esquissée pour des études à haute pression.
Abstract - The availability of dedicated, high brightness synchrotron radia- tion sources is leading to many improvements in x-ray diffraction techniques.
In addition to the widely discussed polychromatic energy dispersive tech- niques, significant advances can be made by the use of monochromatic radiation in either angle- or wavelength-scanning. The application of these techniques, being developed at Daresbury, for high pressure investigations is outlined.
The availability of synchrotron radiation has provided considerable advances in high pressure x-ray diffraction. Synchrotron radiation sources yield a smooth con- tinuum of electromagnetic radiation from x-ray to far-infrared wavelengths. The radiation can be shifted towards shorter wavelengths by utilising insertion devices such as wigglers. The SRS wiggler has a critical wavelength X = 0.93 A and the useful radiation extends to 0.2 A. Synchrotron radiation is of very high intensity, is highly collimated and is polarized in the plane of the ring. These desirable characteristics may be exploited for x-ray diffraction work with samples under high pressures in diamond anvil cells (d.a.c's). Four different experimental procedures are currently being developed at Daresbury.
1. ENERGY-DISPERSIVE POWDER DIFFRACTION
In this technique [1] the white x-ray beam is scattered by a powder sample through a fixed angle 29, and the energy (wavelength) distribution analysed by a semiconductor detector (eg. hyperpure Germanium crystal). Thus, the powder pattern is observed as a function of energy at some fixed angle (ca. 10°). The technique is, therefore, particularly well suited for use with d.a.c's. The high intensity and col- limation allow the pattern to be observed in seconds only. Exploration of pressure- temperature space, compressibility and kinetic studies are therefore made with some ease.
There are, however, several accompanying limitations. The white beam (contain- ing all x-ray wavelengths) may cause heating effects or radiation damage; fluores- cence from the sample may obscure part of the pattern; and the detector dead time
(10 us) limits the count rate. The most important restriction, however, arises from the energy resolution of the detector (~ 150 eV at 5.9 keV). It is, thus, often im- possible to detect changes in peak shape which may occur as a result of a high pres- sure transition, let alone resolve the components. For structures of tetragonal or lower symmetry, the number of unresolved reflections rarely allows a unique indexing, which can led to misinterpretation. Clearly it is most important, for phase transi- tion studies at high pressures, to have techniques available which allow considerably higher resolution. These are outlined below.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984864
C8-366
JOURNAL DE
PHYSIQUE2. MONOCHROMATIC ANGLE-SCANNING
By i n s e r t i n g a double c r y s t a l { ~ i ( 2 2 0 ) } monochromator i n t o t h e SRS wiggler beam, h i g h l y monochromatic r a d i a t i o n ( A h / k = 3 x can be o b t a i n e d which is t u n e a b l e between 0.2 and 2.5
8 .
It is then p o s s i b l e t o use c o n v e n t i o n a l angle-scanning t e c h - niques i n Debye-Scherrer geometry [2,31. This approach has been a p p l i e d only r e c e n t - l y t o synchrotron s o u r c e s . The i n t e n s i t y a v a i l a b l e , although g r e a t l y reduced from t h e white r a d i a t i o n s i t u a t i o n , i s s u f f i c i e n t f o r r e l a t i v e l y f a s t scans even with s m a l l samples, w h i l s t c o n f e r r i n g c o n s i d e r a b l e g a i n s i n r e s o l u t i o n . Figure 1 i s an example of a p a t t e r n c o l l e c t e d from a Lindemann c a p i l l a r y (0.5 mm d i a m e t e r ) of potassium c h l o r i d e . In c o n j u n c t i o n with a p o s i t i o n s e n s i t i v e d e t e c t o r [ 4 , 5 1 this approach should a l l o w c o l l e c t i o n of h i g h - r e s o l u t i o n powder p a t t e r n s from samples i n d.a.cls.The freedom t o s e l e c t wavelength i s a l s o h i g h l y d e s i r a b l e f o r minimising a b s o r p t i o n problems and i n anomalous s c a t t e r i n g experiments.
Fig.1
-
Debye-Scherrer a n g l e - d i s p e r s i v e powder p a t t e r n of potassium c h l o r i d e (0.5 mm c a p i l l a r y ) .To t e s t t h e r e s o l u t i o n o b t a i n a b l e , experiments were performed with a S i ( I 11 ) a n a l y s e r placed i n t h e s c a t t e r i n g r a d i a t i o n . This device a c t s a s a very f i n e a n g u l a r s l i t (- 7 a r c s e c a t 1.5
A )
and provides t h e very high r e s o l u t i o n achieved [ 3 1 . TheFig.2
-
High r e s o l u t i o n powder p a t t e r n of bismuth vanadate.e f f e c t i v e n e s s of t h i s p r o c e d u r e is shown by r e s u l t s from bismuth v a n a d a t e , BiVO,,, Fig.2. T h i s sample is t e t r a g o n a l a t h i g h t e m p e r a t u r e s and pseudo-symmetric i n its room-temperature m o n o c l i n i c phase. Thus e a c h of t h e t e t r a g o n a l p a r e n t p h a s e Bragg p e a k s is s p l i t i n t h e a m b i e n t p a t t e r n . The a b i l i t y t o r e s o l v e most of t h e s e p e a k s is c r u c i a l f o r a n a c c u r a t e u n i t - c e l l d e t e r m i n a t i o n which i s a p r e r e q u i s i t e f o r a s t r u c - t u r a l r e f i n e m e n t . The r e s o l u t i o n o b t a i n e d i n t h e s e e x p e r i m e n t s i s c o n s i d e r a b l y h i g h e r t h a n c a n be o b t a i n e d on c o n v e n t i o n a l x-ray o r n e u t r o n s o u r c e s and w i t h t h i s r e s o l u - t i o n d i r e c t methods of s o l v i n g r e l a t i v e l y c o m p l i c a t e d s t r u c t u r e s from powders a l o n e c a n be c o n s i d e r e d .
A d i s a d v a n t a g e of t h e a n g l e - s c a n n i n g method is t h e need f o r a n u n o b s t r u c t e d view o f t h e sample o v e r a l a r g e a n g l e : t h i s i s d i f f i c u l t t o r e c o n c i l e w i t h d.a.c. d e s i g n r e q u i r e m e n t s , p a r t i c u l a r l y above 20 GPa. A t e c h n i q u e combining t h e f i x e d - a n g l e a d v a n t a g e of t h e e n e r g y - d i s p e r s i v e method w i t h t h e h i g h r e s o l u t i o n a n g l e - s c a n n i n g method would t h e r e f o r e be of g r e a t v a l u e i n x-ray d i f f r a c t i o n s t u d i e s a t v e r y h i g h p r e s s u r e s . Such a p r o c e d u r e i s , i n f a c t , p o s s i b l e .
3. WAVELENGTH (A)-SCANNING
By s c a n n i n g t h e w a v e l e n g t h of t h e monochromator ( i . e . by c h a n g i n g t h e a n g l e o f t h e monochromator c r y s t a l w i t h r e s p e c t to t h e beam) t h e e n e r g y - d i s p e r s i v e p a t t e r n may be g r a d u a l l y assembled. High r e s o l u t i o n is m a i n t a i n e d b e c a u s e , a t any one time t h e d e t e c t o r o b s e r v e s o n l y monochromatic r a d i a t i o n , and t h e r e s o l u t i o n i s governed by t h e monochromator r a t h e r t h a n t h e d e t e c t o r . T h i s a p p r o a c h was used by G l a z e r and co-wor- k e r s (61 t o r e s o l v e t h e t e t r a g o n a l s p l i t t i n g (202 and 220) o f barium t i t a n a t e .
A v a i l a b i l i t y of b o t h angle-and wavelength-scanning methods means t h a t f o r powder d i f f r a c t i o n of s a m p l e s i n h i g h p r e s s u r e c e l l s it i s p o s s i b l e t o r e s o l v e a l l t h e r e - f l e c t i o n s n e c e s s a r y t o d e t e r m i n e t h e u n i t c e l l u n i q u e l y , even f o r low-symmetry s y s - tems. N a t u r a l l y , t h i s r e q u i r e s peak p o s i t i o n s o n l y and n o t i n t e g r a t e d i n t e n s i t i e s . T y p i c a l s a m p l e s i n a d.a.c. c o n t a i n o n l y a s m a l l number of c r y s t a l l i t e s , which t o g e t h e r w i t h t h e e x c e l l e n t c o l l i m a t i o n of s y n c h r o t r o n r a d i a t i o n c a n l e a d t o c o n s i d e r a b l e p r e f e r r e d o r i e n t a t i o n problems and u n r e l i a b l e i n t e n s i t y d a t a [ 7 1 . 4 . LAUE DIFFRACTION FROM MICRO-CRYSTALS
I n g e n e r a l , s p e c i f i c a t i o n of a t o m i c p o s i t i o n a l p a r a m e t e r s r e q u i r e s a
s i n g l e - c r y s t a l s t u d y . Such e x p e r i m e n t s a r e d i f f i c u l t , p a r t i c u l a r l y above 10 GPa.
R e c e n t work a t Daresbury h a s s u g g e s t e d t h a t Laue d i f r a c t i o n may have a u s e f u l r b l e t o p l a y i n h i g h p r e s s u r e s i n g l e - c r y s t a l s t u d i e s .
The Laue method u t i l i z e s t h e w h i t e beam and d a t a c o l l e c t i o n is t h e r e f o r e v e r y f a s t ( a few s e c o n d s ) . F u r t h e r , c r y s t a l s w i t h l i n e a r d i m e n s i o n s a s s m a l l a s a few m i c r o n s can be s t u d i e d , which h o l d s g r e a t promise f o r work t o t h e v e r y h i g h e s t p r e s - s u r e s . The c r y s t a l s t r u c t u r e of a-A1P04 ( b e r l i n i t e ) was r e c e n t l y r e f i n e d L81 from Laue p h o t o g r a p h s of 200 pn c r y s t a l , y i e l d i n g t h e e i g h t p o s i t i o n a l p a r a m e t e r s i n good a g r e e m e n t w i t h t h o s e from a c o n v e n t i o n a l s i n g l e - c r y s t a l s t u d y . The u s e of Laue d i f - f r a c t i o n h a s a l s o been reviewed i n c o n n e c t i o n w i t h p r o t e i n c r y s t a l l o g r a p h y 191. Laue methods may t h e r e f o r e be of g r e a t u s e i n t h e s t u d y of h i g h p r e s s u r e c r y s t a l l o g r a p h y f o r a s s e s s i n g t h e c r y s t a l l i n i t y of a micro-sample, t o e x p l o r e i t s symmetry and t o e s t a b l i s h a p p r o x i m a t e c e l l d i m e n s i o n s . I n f a v o u r a b l e c i r c u m s t a n c e s t h e p o s i t i o n a l p a r a m e t e r s may a l s o be r e f i n e d .
CONCLUSIONS
The p r e s e n t s i t u a t i o n f o r s y n c h r o t r o n x-ray d i f f r a c t i o n b o t h of powders and s i n g l e - c r y s t a l s h e l d u n d e r v e r y h i g h p r e s s u r e s is one of v e r y g r e a t promise. A com- b i n a t i o n o f t h e methods d e s c r i b e d h e r e i s e s p e c i a l l y p o w e r f u l .
JOURNAL DE PHYSIQUE
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