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Industrial aspects of synchrotron X-ray powder diffraction
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Pergamon
Radiat. Phys. Chem. Vol. 45, No. 3, pp. 445457, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0969-806X/95 $9.50+ 0.00
0969-806X(94)00147-2
INDUSTRIAL ASPECTS OF SYNCHROTRON X-RAY POWDER DIFFRACTION R. J. CERNIKl and P. BARNES2 ISERC Daresbury Laboratory, Warrington WA44AD, U.K. and 2Department of Crystallography, Birkbeck College, Malet Street, London WC1E 7HX, U.K. Abstract--This paper describes some of the applications of powder diffraction using synchrotron radiation with possible applications to industry. The advantages of differing synchrotron diffraction geometries, Debye-Scherrer, Analyser crystal, Hart Parrish, and Energy Dispersive, are discussed. The paper is not a comprehensive review but nevertheless considers the wider role of these powder diffraction geometries in elucidating crystal structures, highlighted examples being taken from polymers, catalyst and new drug materials, in addition to specific studies on polymer electrolyte complexes, textured materials (e.g. asbestos), pyrochlores, zeolites and cements. In the latter two cases rapid time-resolvedpowder diffraction is seen to be emerging as an important development in synchrotron-based techniques.
1. I N T R O D U C T I O N
Powder diffraction has made a large impact in many industrial environments, and is one of the most well established analytical techniques for phase identification, cell parameter measurements and strain analysis. One of the most widely used analytical techniques involves the comparison of an experimentally observed powder diffraction profile with a large number of patterns in a data base. This "fingerprinting" technique dates from the late 1930s and the pioneering work of Hanawalt et al. (1938). The simplicity of these search-match methods gained popularity to the extent that today there are over 58,000 data entries in the ICDD data base (address in References), though their main limitation in normal usage derives from uncertainties in the peak intensities of patterns obtained from laboratory powder diffractometers. However despite the continuing success of conventional diffractometers, the higher resolution patterns obtained with synchrotron radiation can be used to resolve more difficult space group or symmetry problems, or can more easily identify minority phases present in the sample. The use of synchrotron radiation in this field has extended the range and complexity of the materials that can be studied. The search-match method is very powerful and enables chemists, physicists, biologists or materials scientists to determine the constituents and proportions of phases in their samples. This is of limited value if the structure of the material is unknown, and in these cases the appropriate pattern would not appear in the data base and the structure would have to be analysed b y an ab initio method as described by McCusker (1988), Cernik et al. (1991), or Estermann and Gr6rnlich (1993). RPC 45/3~J
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In these cases the ultimate demands are placed on the powder diffraction data. This is true whether the data are collected on a laboratory based, synchrotron or neutron source. The data from the unknown materialmust be indexed (Werner et al., 1985; Louer et al., 1990), decomposed (Pawley, 1981) into individual intensities, and then solved to obtain an approximate model of the atomic structure. This can be achieved by a number of techniques including modelling (Cerius), direct methods (Casarino et al., 1992), or maximum entropy and likelihood (Gilmore et aL, 1991). Significant successes have been achieved by using powder data from laboratory-based (Benard et aL, 1991), synchrotron (Roberts and Fitch, 1991) and neutron (Cheetham et aL, 1986) diffractometer systems. These successes in structure solution can be tuned directly to the advantage of industry since many of the materials studied are of direct relevance: examples are the catalyst precursor aurichalcite, the heterogeneous catalysts such as Ni exchanged zelotite Y, and the rare earth doped pyrochlore catalysts used in converting "greenhouse gases" such as CO2 and CH 4 into useful chemical feedstock. The structural properties of polymers are also of direct relevance to industry: in fact the Boeing Corporation achieved significant weight reductions on its 757 aircraft by substituting polyethyl ether Ketone resins for aluminium with no strength loss. Perhaps the most popular laboratory based method for collecting powder diffraction profiles uses Bragg-Brentano geometry, perfected by Parrish (1962). There are many thousands of these diffractometers world wide and the method is so successful that most modem manufactured instruments still use the same basic geometry. Commercially available
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powder diffractometers have also been optimized for the study of texture and strain measurements. This has been particularly important in the field of metallurgy. One of the central problems of laboratory based systems using Bragg-Brentano geometry is that the X-ray beam is divergent from the source and this
limits the available angular resolution. The beam can be collimated but at the expense of intensity, however the results from a standard X-ray generator optimized for high resolution powder diffraction can be impressive. Perfect crystal monochromators can remove the K0~2 radiation and position sensitive detectors can speed up data collections. Under
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Industrial aspects of synchrotron X-ray powder diffraction these conditions data of excellent quality have been obtained (Lightfoot et aL, 1992) and have been used to solve and refine crystal structures, Synchrotron radiation can improve upon the resolution and intensity obtainable on the best laboratory source, and because of the near parallel nature of the beam in the vertical plane offers the possibility of decoupling the specimen and detector axes for analysing samples in grazing angle. 2. E X P E R I M E N T A L G E O M E T R I E S
(a) Debye-Scherrer geometry with synchrotron radiation A full analysis of this configuration has been
published by Langford et aL (1991) [see Fig. l(a)]. The sample is mounted in a 0.3 or 0.5 mm diameter spinning capillary tube and irradiated with a beam slightly larger than the capillary size ( ~ 0 . 6 m m ) . The beam is usually between 5 and 10 mm in width. This is much wider than the standard Debye-Scherrer geometry but the distance of the detector from the sample is considerably greater and as a consequence axial divergence only becomes a serious problem at very low angles. This geometry is dominated by the capillary width at low angles and by the angular dispersion from the monochromator at higher angles, that is in the absence of any measurable sample broadening. The instrumental resolution function of
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Fig. 1. Schematic views of the various geometries used in synchrotron powder diffractometry, together with some typical high resolution powder patterns obtained in two cases (other examples given later): (a) Debye-Scherrer geometry; (b) Analyser geometry; (c) Hart-Parrish geometry; (d) glancing angle Hart-Parrish geometry; (e) energy dispersive geometry; (f) example of a powder pattern of aurichalcite, a precursor catalyst material with a very small particle size, collected on the Daresbury SRS station 9.1 using the Debye-Scherrer geometry; (g) example of a powder pattern from ammonium germanate, a mlcroporous material with possible catalytic applications, collected on the Daresbury SRS station 2.3 using the Hart-Parrish geometry (by courtesy of A. N. Fitch).
station 9.1 at Daresbury Laboratory is well matched to this type of geometry: the intrinsic divergence of the source at l0 keV is of the order of 0.028 mrads; the Darwin width from the Si(111) monochromator is of the order of 0.038 mrads; the slit widths (0.6 mm at 15 m from the tangent point) subtend an angle of 0.04 mrads; these well matched parameters coupled with a well understood instrumental resolutions function ensure that the simple slit geometry gives very reliable, reproducible and interpretable line shapes from powder samples and a very high resolution. Other advantages of this technique are that only small quantities of material are needed and samples can easily be sealed inside capillaries ensuring a well characterized adsorbate or other solution concentration. A good example of the kind of pattern that can be collected on station 9.1 in Debye-Scherrer mode is given in Fig. l(f) for aurichalcite, a precursor catalyst material. A disadvantage is that the sample is sealed so it cannot be transformed by a reactive gas flow and perhaps more importantly the diffraction pattern is not specimen displacement independent. This is a simple matter to correct but it does introduce an extra variable when determining accurate peak positions.
(b) Analyser crystal geometry One of the first synchrotron powder diffractometers to operate in a routine and reliable way was the instrument [see Fig. l(b)] on line X13A at Brookhaven (Hastings et al., 1984). The advantages of the analyser geometry are that with suitably chosen crystals the very highest angular resolution can be obtained; this can be of the order of 0.02 ° in 20. This geometry gives a focusing position that can be shifted up or down in the angular range by choosing different crystal planes for the monochromator and analyser. The analyser in this geometry is acting as a very fine slit. The extraneous non elastic background is also very low and the measured peak positions are specimen displacement independent. This is a big advantage when studying materials at high temperatures as the surface of the sample tends to move. The main disadvantage of the analyser geometry at present is that it does sacrifice a good deal of intensity, though this will not be a problem on the new generation of synchrotron machines with 40 or 50 times the flux presently available. With weakly scattering samples the counting times at present have to be rather long in order to get good quality statistics. Also the high resolution configurations may not always be necessary for a
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large number of powder problems since the inherent sample broadening can be order of magnitude larger than the instrumental broadening. Again the option of a spinning sample is available and indeed is essential for specimens with particle sizes > 2 0 # m (Parrish et al., 1986). (c) Hart Parrish geometry The choice between either high resolution or high flux is an old problem but a compromise was suggested by Parrish et al. (1986). This geometry useS a set of parallel foils which define, in the Case of the Daresbury diffractometer (Cernik et aL, 1990), an angular aperture of 0.06 ° [Fig. l(c)]. A large beam size can then be employed, and because the resolution is defined by the angular acceptance of the foils there is a significantly enhanced flux (10-20 times that obtainable with capillary geometry) with no loss in resolution. Small imperfections in the way the foils are manufactured result in a slightly lower throughput, and if the angular aperture is restricted the angular resolution can be somewhat better than predicted. Typical values for peak half widths from this instrument are of the order of 0.04-0.06 ° in 20. While this is less than the very highest resolution it is well matched to the vast majority of powder samples. The peak positions are specimen displacement independent and the flux is considerably improved. This geometry coupled with accurate diffractometer control is ideal for indexing problems and unit cell refinements (Hart et al., 1990). This geometry is also ideal for the study of thin films in shallow incidence. The geometry is illustrated in Fig. l(d): the narrow incident beam is diffracted from the spinning sample at very different power densities depending on the 20-angle. The beam is at its broadest at 90 ° to the specimen surface. If the collimators are manufactured so that they accept the whole diffracted beam at all angles, the diffracted beam will be integrated over a constant angular aperture, hence this intensity and angle will be correct for all detection angles. With thin film applications, provided there is not too much texture, an excellent quality powder scan can be obtained; spinning samples may also be used. An actual example of an excellent quality pattern collected in this mode is given by Fig. l(g), that is of a microporous material, ammonium germanate. (d) Energy dispersive diffraction Energy dispersive diffraction (EDD) was first demonstrated (Giessen and Gordon, 1968) more than a quarter of a century ago and can be operated using either laboratory (Mantler and Parrish, 1976; Sparks and Gedcke, 1972) or synchrotron (Buras et al., 1978; Bordas et al., 1977) white (continuous) X-ray sources. With EDD the scattering angle, 20, remains fixed: rather it is the wavelength, 2, that is scanned, either by a pre-sample or post-sample monochromator or more rapidly using an energy-
dispersive detector. As a result the Bragg diffraction equation 2 = 2 d . sin 0 is invariably re-written, using E = hc/2, in its energyequivalent form E d . sin 0 = hc/2 = a constant (6.2 keV. ]k) where h, c are the fundamental constants, E is the energy of the associated X-ray photon, and d is the crystal inter-planar spacing. The fixed 20-angle means that the energy-dispersive mode is effectively a fixed geometry system: the specimen cell entrance/exit windows (for the X-ray beam) are fixed [Fig. l(e)], as also is the effective diffracting volume within the sample; further when using an energy-dispersive detector there are no moving parts at all during an experiment. This all greatly facilitates the design of specimen cells, particularly those involving high temperature/pressure and/or chemical reaction vessels; indeed it is this well-defined fixed geometry that makes EDD such a versatile technique. Further, when used in conjunction with a hard intense X-ray source, such as from a wiggler magnet on a second generation synchrotron, two further advantages emerge. Firstly, the beam intensity, combined with fixed geometry, leads to data collection rates that are faster than any other realistic powder diffraction method--with current ED-detectors operating at useful count rates of up to 105 cps EDD can easily deliver sub-second diffraction patterns. The only serious drawbacks with the method are the limited resolution which is of the order of 2%, and a limited count rate available with current energy-dispersive detectors. This can be problematic i f important diffraction peaks are overlapped with other peaks as a result of using either (a) complex powder mixtures or (b) using low symmetry structure materials. Significant progress is being m a d e in improving the count rates available with solid state detectors, however the energy resolution of the system will always be limited by the capacitance of the detector crystal. 3. APPLICATIONS OF H I G H RESOLUTION POWDER DIFFRACTION
In the Introduction some examples of crystal structures determined from high resolution powder diffraction were given. In the last 10 years the technique has become much more widespread although it is still very much in its infancy. The sequence of events from obtaining a data set of excellent quality to an indexed pattern has been substantially enhanced by the use of synchrotron radiation. The subsequent decomposition into peak intensities is more reliable and easier with high resolution data with reliable peak shapes. Having obtained a set of indexed integrated intensities a structure solution can be attempted either by direct or indirect methods, such
Industrial aspects of synchrotron X-ray powder diffraction as the new developments in maximum entropy methods which have yielded promising results (Gilmore et al., 1990). Having obtained a trial structure the Rietveld method can be employed to refine the structure. In practice although the crystal structures of many industrially important m a t e r i a l s have
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been determined from powder data it is more usual in an industrial environment for a good deal to be known about the materials under study, and the powder method is used to index or identify new phases, or determine the a m o u n t of each phase present. Also powder diffraction is used widely for stress 'strain analysis of materials. An example of the application of high resolution powder diffraction to industrially important systems is the ab initio structure determination of polymer electrolyte complexes (Lightfoot et al.. 1992). There has been a great deal of interest in the replacement of liquid electrolytes in batteries with a material that is both durable, easy to handle and capable of delivering a high current. Some of the most promising candidates are the high molecular weight polythene oxides with dissolved lithium salts. If suitable concentrations of materials such as L i C F 3SO 3 can be introduced into the polymer framework mobile ions can be liberated and the material conducts. However this material is very difficult to crystallise and typically only powder samples are available High resolution powder diffraction from both laboratory and synchrotron bunch sources have been used to solve and refine these structures providing important information for synthetic chemists in their attempts to manufacture better polymer electrolyte materials. Another area of great importance is the study of microporous materials as potential catalytic materials. Figure 2 shows the framework of Ni exchanged zeolite Y . This can be used in the production of benzene from acetylene. The material is at first catalytically inactive but after heating for approx. 2 h begins to catalyse the reaction to form benzene. High resolution powder diffraction was used to study this phenomenon (Dooryhee et al., 1990). A full Rietveld refinement was carried out on data sets collected at 20 rain intervals. The results are summarized in Fig. 2 which shows the migration o f a Ni 2+ ion from the catalytically inactive S1 site to the active $2 or supercage site. The spheres represent all the possible Ni sites, however the only important site from the catalytic point of view is the supercage site. The occupied sites are shown by the lighter spheres and the active Ni 2÷ moves from the inactive position 2(a'l to the active 2(b), the total path being shown in Fig. 2(c). Such structures show not only the power of the powder diffraction technique, but also how physical chemists are approaching a better understanding of the catalytic process. Finally we note the use of pole figure measurements to analyse texture, which have been a c o m m o n place method, using either photographic film or a texture attachment to a diffractometer (Cullity, 1978), for a number of years. The use of synchrotron radiation and image plates offers an interesting and very promising way forward. Figure 3 shows an image plate exposure from a mineral fibre bundle of chrysolite asbestos. The material has both rotational fibre and some plate texture. This is best illustrated by
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R.J. CERNIK and P. BARNES
Fig. 3. Chrysolite asbestos powder pattern collected using an image plate exposure from a mineral fibre bundle, showing fibre and plate texture at the nodal reflections.
examination prospect of and texture metallurgists
of some of the nodal reflections. The obtaining simultaneous crystallographic information will be very attractive to and materials scientists.
4. A P P L I C A T I O N S OF S Y N C H R O T R O N P O W D E R D I F F R A C T I O N TO D Y N A M I C SYSTEMS
An important motivation in synchrotron powder diffraction is that of harnessing its potential for realizing rapid diffraction data capture on time-dependent
systems. In many materials science applications, unlike specialized situations such as for example with explosive materials (Wong et al., 1992) or a biologically catalysed sequence (Cassetta et al., 1993), one can envisage a need to collect diffraction patterns every 1 60 s, in comparison to those more slowly changing situations where the sample is in a nearequilibrium state, such as the example given previously of Ni in zeolite Y. F o r faster reactions and transformations one has to turn increasingly to other diffraction modes, such
Industrial aspects of synchrotron X-ray powder diffraction
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Fig. 4. (a) Close-up view of part of the energy dispersive diffraction system on the Daresbury SRS station 9.7, showing the transmission high temperature furnace (labelled "TH600") in the foreground, and the pre-collimator slit system in the background. (b) Energy dispersive patterns showing an initial irreversible structural transformation of the rare earth doped pyrochlore catalyst with time (after Ashcroft et al., 1993).
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as E D D which is ideally suited to such studies provided some loss of reciprocal space resolution can be tolerated. Continuing in the area o f catalysis, one notes the E D D studies o f A s h c r o f t et al. (1993), o n raree a r t h pyrochlores as used for the selective oxidation or dry-reforming of methane. The latter reaction, in
which greenhouse gases C O 2 a n d C H 4 produce synthesis gas (CO a n d H2), has been studied in situ with the surprising result (Fig. 4) t h a t the pyrochlore-catalyst itself exhibits rapid initial irreversible structural changes during the process. These changes are sufficiently fast to require diffraction p a t t e r n collections in
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Fig. 5. Highly schematic illustrations of the energy-dispersive powder diffraction system using an environmental sample cell heating system consisting of a cylindrical thermal heat source (block heated/cooled using internal fluid circulation system) and inner cylindrical spinning sample cell. The component parts are not drawn to the same scale, in particular the heat block and sample cell are relatively enlarged with the sample cell being typically a cylinder of diameter 10-12 mm and length 400 mm. (a) A version of the environmental cell system designed for remote mixing and stirring of components to rapidly form an ettringite-based cement product (i.e. a remote "mini cement mixer"). The feed path and mini-stirrer are indicated. An actual result obtained with this system is shown in Fig. 6(a). (b) Another version of the environmental cell system, this designed for performing autoclave-hydrothermal synthesis of zeolites. An actual result obtained with this system is shown in Fig. 7.
Industrial aspects of synchrotron X-ray powder diffraction some cases on a 30 s time scale, such a time-resolution of course being well within the capabilities of EDD. Similar time resolutions are required for studying the transformations involved in the synthesis of ceramic components and a full description of such an application, the synthesis of tetragonal/monoclinic zirconia, is given elsewhere in this issue (Turrillas et al., 1995). In these particular experiments the important additional factor was the provision of a furnacing capability up to 1300°C as illustrated in Fig. 4(a). This application underlines the importance of being able to combine high speed diffraction data capture with the sample environment design, and it is the relative ease of performing this combination with a fixed geometry system that makes EDD such an attractive and, sometimes, unique mode for many applications, particularly "one-off" experiments. This aspect has been exploited in a series of studies (Barnes, 1991, 1992; Barnes et al., 1992; Munn et aL, 1992: He et al., 1992; Muhamad et al., 1993) on a range of functional zeolites and cementitious materials, and from these we include just two examples in this current review. As a first example we refer to a study on cement hydration (Muhamad et aL, 1993) where the hydration products were known to form rapidly, so rapidly in fact that the initial mixing process and safety-search of the experimental "hutch" area had to be designed into the overall experiment. Figure 5(a) shows a schematic of the arrangement used which permits automated mixing and stirring of the initial cementitious ingredients--one might describe it as a "mini-cement mixer"--while diffraction data are being collected. Such data collected on 10 s time intervals [Fig. 6(a)] show that the main hydration product, a calcium sulpho-aluminate hydrate referred to as "ettringite", starts to form within even the first 10 s of mixing. Subsequent diffraction data up to times of 100,000 s show that this hydration product can participate in a time-dependent solid solution effect due to the rapid hydration promoting substitutional groups (hydroxy/carboxy instead of sulphate groups) into the crystalline structure. This effect is demonstrated by calculating from the EDD-patterns the unit cell a-parameter for the growing ettringite phase as a function of time: Fig. 6(b) shows, graphically, a time-sequence for one such experiment in which a 0.7% increase in the unit cell a-parameter is observed in the first 100,000 s. This is consistent with a substitution of 50-70% of the sulphate groups by hydroxy/carboxy groups in the ettringite structure. This is a good example of how in situ rapid timedependent EDD-patterns can shed new light on the nature of a complex hydration process. A second example of how a specific application feature can be designed into an experiment using EDD is provided by the area of zeolite synthesis and stability. The alumino-phosphate molecular sieve, VPI-5, can be synthesized hydrothermally under autoclave conditions using a gel-template mix (He
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et al., 1992), producing VP1-5 fairly rapidly at c.
140°C. Until recently it was believed that such conditions could only be practicably realized with in situ diffraction by using neutron radiation (Polak et al., 1990) since the considerable penetration power of neutrons makes the construction of specimen cells with thick walls possible. However with the energetic white X-ray spectrum available from a synchrotron wiggler insertion device it is now similarly feasible to exploit the penetration of 20100 keV X-ray photons through metal/polymeric enclosures and bulk (e.g. 10 mm thick) sample material. Figure 5(b) illustrates the autoclave cell system used for synthesis and stability studies on VPI-5. Figure 7(a) and (b) show typical time-resolved EDDplots covering the autoclave synthesis of VPI-5 and
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Fig. 7. (a) In situ diffraction patterns obtained during the autoclave synthesis of VPI-5. In this case the TBA-VPI-5 recipe is being used and the final temperature is 140-142°C. The diffractometer 20.angte is Set at 119° and the resulting d-spacing range reveals one very strong growing peak at 22.7 keV (16.ll &) and a Weaker growing Peak at 45.6 keV (8.2 ~) which both belong to the TBA-VPI-5 phase being synthesized. (b) In situ diffraction patterns showing the subsequent rapid transformation of t h e autoclave-synthesized VPI-5 to the related A1PO]-, after releasing the autoclave seal between 150 and 156°C. its subsequent transformation to a related zeolite, A1PO4-8, on releasing the autoclave seal. With these data it is now possible to make meaningful and quantitative interpretations (He et al., 1992) of the effects of synthesis gel-preparation and incubation and rates of transformation. This includes the temperature effect and activation energy of the zeolite product. The more ambitious possibility now arises o f contemplating an interactive synthesis regime where zeolites, which are either new or are difficult to produce synthetically, are grown by controlling the temperature conditions in response to the information gained from the dynamic EDD-data. If this could be realized it would bring a quite different dimension to the subject of in situ dynamic diffraction.
5. C O N C L U S I O N S
This paper has outlined the main advantages of powder diffraction systems which utilize the superior photon flux (for the intensity o f patterns obtained; speed of data capture), the collimation (to obtain high resolution for separating diffraction peaks), and wavelength range (choice of wavelength in angle dispersive geometries; choice of angle in the energy dispersive geometry) of X-ray beams produced by a synchrotron. Other advantages not detailed include the tunability of the radiation to an absorption edge (for anomalous absorption powder diffraction experiments) and the horizontal linear polarization of the radiation (which can be exploited to remove intensity
Industrial aspects of synchrotron X-ray powder diffraction losses normally associated with a randomly polarized laboratory X-ray source). Structure solution/refinement o f crystalline powder material, the highlighting of minor phases, and rapid time-resolved experiments are aspects of materials science which are benefiting from these enhanced powder diffraction facilities. Future developments in (third generation) s y n c h r o tron sources will further emphasize these advantages, while instrumentation developments, such as energydispersive detectors and two-dimensional image plate systems, will further extend the range of information that can be collected.
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Ashcroft A. T., Cheetham A. K., Jones R. H., Nalarajan S., Thomas J. M., Waller D. and Clark S. M. (1993) J. Phys. Chem. 97, 3355. Barnes P. (1991) J. Phys. Chem. Solids 52, 1299. Barnes P. (1992) Amer. Inst. Phys. Conf. Proc. No. 258 "'Synchrotron Radiation and Dynamic Phenomena" (Edited by Beswick A.), pp. 517-530. Amer. Inst. of Phys, New York. Amer. Inst. Phys. Conf. Proc. No. 258, "Synchrotron Radiation and Dynamic Phenomena". Grenoble, France (1991). Barnes P., Clark S. M., Hausermann D., Henderson E., Fentiman C. H., Rashid S. and Muhamad M. N. (1992) Phase Trans. 39, 117. Benard P., Lou6r M. and Loufir D. (1991) J. Solid State Chem. 94, 27. Bordas J., Glazer A. M., Howard C. J. and Bourdillon A. J. (1977) Phil. Mag. 35, 311. Buras B., Olsen J. S. and Gerward L. (1978) Nucl. Instrum. Meth. 152, 293. Casarino G., Favia L. and Giacouazzo C. (1992) J. AppL Crystallogr. 25, 310. Cassetta A., Deacon A., Emmerich C., Habash J., Helliwell J. R., McSweeney S., Snell E., Thompson A. W. and Weisgerber S. (1993) Proc. R. Soe. Lond. A 442, 177. Cerius--Molecular Stimulations Ltd, 240 The Quorum, Barnwell Road, Cambridge CB5 8KE, England. Biouynas --Technologies Ltd, Wode Road, Baunplake, Hants RG24 ONE, England. Cernik R. J;, Murray P. K., Pattison P. and Fitch A. N. (1990) J. Appl. Crystallogr. 23, 292. Cernik R. J:, Cheetham A, K., Prout C. K., Watkins D. J., Wilkinson A. P. and Willis B. T. M. (1991) J. Appl. Crystallogr. 24, 222. Cheetham A. K., David W. I. F., Eddy M., Jahemann R. J. B., Johnson M. W. and Toradi C. C. (1986) Nature (London) 320, 46. Cullity B. D. (1978) Elements of X-ray Diffraction. Addison-Wesley, Reading, Mass.
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Dooryhee E., Greaves G. N., Steel A. T., ToWnsend R. P., Carr S. W., Thomas J. M. and Catlow C. R. A. (1990) Faraday Disc. Chem. Soc. 89, 119. Estermann M. and Grfmlich V. (1993) J. Appl. Crystallogr 26, 396. Giessen B. C. and Gordon G. E. (1968) Science 159, 973. Gilmore C. L, Bricogne G. and Bannister C. (1990) Acta Crystallogr: 21, 571. Gilmore C. J.i Henderson KI and Bricogne G. (1991) Acta Crystallogr) A47, 830. Hanawalt J. D:, Rinn H. and Frevel L. K. (1938) Ind. Eng. Chem A n a l Ed. 10, 457. Hart M., Cernik R. J.; Parrish W. and Toraya H. (1990) J. Appl. Crystallogrl 23, 286. Hastings J. B., Thomlinson W. and Cox D. E: (1984) J. Appl. Crystallogr. 12, 856. He H., Barnes P., Munn J., Turrillas X. and Klinowski J. (1992) Chem Phys. Lett. 196, 267. ICDD Powder diffraction file, 12 Campus Boulevard, Newtown Square, PA 19073-3273, U.S.A. Langford J. I., Cernik R. J. and Lou6r D. (1991) J. Appl. Crystallogr. 24, 913. Lightfoot P., Mehta M. A. and Bruce P. G. (1992) J. Mater. Chem. 2, 361. Lou~r D., Boultif A., Gotor F. J. and Criado J. M. (1990) Powder Diff. 5, 162. Mantler M. and Parrish W. (1976) Adv. X-ray Anal. 20, 171. McCusker L. B. (1988) J. Appl. Cryst. 21, 305. Muhamad M. N., Barnes P., Fentiman C. H., Hausermann D., Pollman H. and Rashid S. (1993) Cement Concrete Res. 23, 267. Munn J., Barnes P:, Hausermann D., Axon S. A. and Klinowski J. (1992) Phase Trans. 39, i29. Parrish W. (1962) Advances in X-ray Diffractometry and X-ray Spectrography. Centrex, Eindhoven. Parrish W., Hart M. and Huang T. C. (1986) J. Appl. Crystallogr. 19, 92. Pawley G. S. (1981) J. Appl. Crystallogr. 14, 357. Also Le Bail's method, Murray and Fitch program MPROFIL, to be published. Polak E., Munn J., Barnes P, Tarling S. E. and Fitter C. (1990) J, Appl. Crystallogr. 23, 258. Roberts M. A. and Fitch A. N. (1991) J. Phys. Chem. Solids 52, 1209. Sparks C. J. and Gedcke D. A. (1972) Adv. X-ray Anal. 15, 240~ Turrillas X., Barnes P~, Gaseoigne D., Turner J. Z , Jones S. L., Norman C. J., Pygall C. F. and Dent A. J. (1995) Radiat. Phys. Chem. 45, 491. Werner P. E., Erikson L. and Westdahl M. (1985) J. Appl. Crystallogr. 18, 367. Wong J., Larson E. M., Holt J. B., Waide P. A., Rupp B., Frahm R. and Nutt G. (1992) Amer. lnst. Phys. Conf. Proc. No. 258 "'Synchrotron Radiation and Dynamic Phenomena" (Edited by Beswick A.), pp. 652-661. Amer. Inst. of Phys., New York.
Pergamon
Radiat. Phys. Chem. Vol. 45, No. 3, pp. 445457, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0969-806X/95 $9.50+ 0.00
0969-806X(94)00147-2
INDUSTRIAL ASPECTS OF SYNCHROTRON X-RAY POWDER DIFFRACTION R. J. CERNIKl and P. BARNES2 ISERC Daresbury Laboratory, Warrington WA44AD, U.K. and 2Department of Crystallography, Birkbeck College, Malet Street, London WC1E 7HX, U.K. Abstract--This paper describes some of the applications of powder diffraction using synchrotron radiation with possible applications to industry. The advantages of differing synchrotron diffraction geometries, Debye-Scherrer, Analyser crystal, Hart Parrish, and Energy Dispersive, are discussed. The paper is not a comprehensive review but nevertheless considers the wider role of these powder diffraction geometries in elucidating crystal structures, highlighted examples being taken from polymers, catalyst and new drug materials, in addition to specific studies on polymer electrolyte complexes, textured materials (e.g. asbestos), pyrochlores, zeolites and cements. In the latter two cases rapid time-resolvedpowder diffraction is seen to be emerging as an important development in synchrotron-based techniques.
1. I N T R O D U C T I O N
Powder diffraction has made a large impact in many industrial environments, and is one of the most well established analytical techniques for phase identification, cell parameter measurements and strain analysis. One of the most widely used analytical techniques involves the comparison of an experimentally observed powder diffraction profile with a large number of patterns in a data base. This "fingerprinting" technique dates from the late 1930s and the pioneering work of Hanawalt et al. (1938). The simplicity of these search-match methods gained popularity to the extent that today there are over 58,000 data entries in the ICDD data base (address in References), though their main limitation in normal usage derives from uncertainties in the peak intensities of patterns obtained from laboratory powder diffractometers. However despite the continuing success of conventional diffractometers, the higher resolution patterns obtained with synchrotron radiation can be used to resolve more difficult space group or symmetry problems, or can more easily identify minority phases present in the sample. The use of synchrotron radiation in this field has extended the range and complexity of the materials that can be studied. The search-match method is very powerful and enables chemists, physicists, biologists or materials scientists to determine the constituents and proportions of phases in their samples. This is of limited value if the structure of the material is unknown, and in these cases the appropriate pattern would not appear in the data base and the structure would have to be analysed b y an ab initio method as described by McCusker (1988), Cernik et al. (1991), or Estermann and Gr6rnlich (1993). RPC 45/3~J
445
In these cases the ultimate demands are placed on the powder diffraction data. This is true whether the data are collected on a laboratory based, synchrotron or neutron source. The data from the unknown materialmust be indexed (Werner et al., 1985; Louer et al., 1990), decomposed (Pawley, 1981) into individual intensities, and then solved to obtain an approximate model of the atomic structure. This can be achieved by a number of techniques including modelling (Cerius), direct methods (Casarino et al., 1992), or maximum entropy and likelihood (Gilmore et aL, 1991). Significant successes have been achieved by using powder data from laboratory-based (Benard et aL, 1991), synchrotron (Roberts and Fitch, 1991) and neutron (Cheetham et aL, 1986) diffractometer systems. These successes in structure solution can be tuned directly to the advantage of industry since many of the materials studied are of direct relevance: examples are the catalyst precursor aurichalcite, the heterogeneous catalysts such as Ni exchanged zelotite Y, and the rare earth doped pyrochlore catalysts used in converting "greenhouse gases" such as CO2 and CH 4 into useful chemical feedstock. The structural properties of polymers are also of direct relevance to industry: in fact the Boeing Corporation achieved significant weight reductions on its 757 aircraft by substituting polyethyl ether Ketone resins for aluminium with no strength loss. Perhaps the most popular laboratory based method for collecting powder diffraction profiles uses Bragg-Brentano geometry, perfected by Parrish (1962). There are many thousands of these diffractometers world wide and the method is so successful that most modem manufactured instruments still use the same basic geometry. Commercially available
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R, J. CERNIKand P. BARNES
powder diffractometers have also been optimized for the study of texture and strain measurements. This has been particularly important in the field of metallurgy. One of the central problems of laboratory based systems using Bragg-Brentano geometry is that the X-ray beam is divergent from the source and this
limits the available angular resolution. The beam can be collimated but at the expense of intensity, however the results from a standard X-ray generator optimized for high resolution powder diffraction can be impressive. Perfect crystal monochromators can remove the K0~2 radiation and position sensitive detectors can speed up data collections. Under
Datector (a)
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447
Industrial aspects of synchrotron X-ray powder diffraction these conditions data of excellent quality have been obtained (Lightfoot et aL, 1992) and have been used to solve and refine crystal structures, Synchrotron radiation can improve upon the resolution and intensity obtainable on the best laboratory source, and because of the near parallel nature of the beam in the vertical plane offers the possibility of decoupling the specimen and detector axes for analysing samples in grazing angle. 2. E X P E R I M E N T A L G E O M E T R I E S
(a) Debye-Scherrer geometry with synchrotron radiation A full analysis of this configuration has been
published by Langford et aL (1991) [see Fig. l(a)]. The sample is mounted in a 0.3 or 0.5 mm diameter spinning capillary tube and irradiated with a beam slightly larger than the capillary size ( ~ 0 . 6 m m ) . The beam is usually between 5 and 10 mm in width. This is much wider than the standard Debye-Scherrer geometry but the distance of the detector from the sample is considerably greater and as a consequence axial divergence only becomes a serious problem at very low angles. This geometry is dominated by the capillary width at low angles and by the angular dispersion from the monochromator at higher angles, that is in the absence of any measurable sample broadening. The instrumental resolution function of
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Industrial aspects of synchrotron X-ray powder diffraction
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Fig. 1. Schematic views of the various geometries used in synchrotron powder diffractometry, together with some typical high resolution powder patterns obtained in two cases (other examples given later): (a) Debye-Scherrer geometry; (b) Analyser geometry; (c) Hart-Parrish geometry; (d) glancing angle Hart-Parrish geometry; (e) energy dispersive geometry; (f) example of a powder pattern of aurichalcite, a precursor catalyst material with a very small particle size, collected on the Daresbury SRS station 9.1 using the Debye-Scherrer geometry; (g) example of a powder pattern from ammonium germanate, a mlcroporous material with possible catalytic applications, collected on the Daresbury SRS station 2.3 using the Hart-Parrish geometry (by courtesy of A. N. Fitch).
station 9.1 at Daresbury Laboratory is well matched to this type of geometry: the intrinsic divergence of the source at l0 keV is of the order of 0.028 mrads; the Darwin width from the Si(111) monochromator is of the order of 0.038 mrads; the slit widths (0.6 mm at 15 m from the tangent point) subtend an angle of 0.04 mrads; these well matched parameters coupled with a well understood instrumental resolutions function ensure that the simple slit geometry gives very reliable, reproducible and interpretable line shapes from powder samples and a very high resolution. Other advantages of this technique are that only small quantities of material are needed and samples can easily be sealed inside capillaries ensuring a well characterized adsorbate or other solution concentration. A good example of the kind of pattern that can be collected on station 9.1 in Debye-Scherrer mode is given in Fig. l(f) for aurichalcite, a precursor catalyst material. A disadvantage is that the sample is sealed so it cannot be transformed by a reactive gas flow and perhaps more importantly the diffraction pattern is not specimen displacement independent. This is a simple matter to correct but it does introduce an extra variable when determining accurate peak positions.
(b) Analyser crystal geometry One of the first synchrotron powder diffractometers to operate in a routine and reliable way was the instrument [see Fig. l(b)] on line X13A at Brookhaven (Hastings et al., 1984). The advantages of the analyser geometry are that with suitably chosen crystals the very highest angular resolution can be obtained; this can be of the order of 0.02 ° in 20. This geometry gives a focusing position that can be shifted up or down in the angular range by choosing different crystal planes for the monochromator and analyser. The analyser in this geometry is acting as a very fine slit. The extraneous non elastic background is also very low and the measured peak positions are specimen displacement independent. This is a big advantage when studying materials at high temperatures as the surface of the sample tends to move. The main disadvantage of the analyser geometry at present is that it does sacrifice a good deal of intensity, though this will not be a problem on the new generation of synchrotron machines with 40 or 50 times the flux presently available. With weakly scattering samples the counting times at present have to be rather long in order to get good quality statistics. Also the high resolution configurations may not always be necessary for a
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R.J. CERNIKand P. BARNES
large number of powder problems since the inherent sample broadening can be order of magnitude larger than the instrumental broadening. Again the option of a spinning sample is available and indeed is essential for specimens with particle sizes > 2 0 # m (Parrish et al., 1986). (c) Hart Parrish geometry The choice between either high resolution or high flux is an old problem but a compromise was suggested by Parrish et al. (1986). This geometry useS a set of parallel foils which define, in the Case of the Daresbury diffractometer (Cernik et aL, 1990), an angular aperture of 0.06 ° [Fig. l(c)]. A large beam size can then be employed, and because the resolution is defined by the angular acceptance of the foils there is a significantly enhanced flux (10-20 times that obtainable with capillary geometry) with no loss in resolution. Small imperfections in the way the foils are manufactured result in a slightly lower throughput, and if the angular aperture is restricted the angular resolution can be somewhat better than predicted. Typical values for peak half widths from this instrument are of the order of 0.04-0.06 ° in 20. While this is less than the very highest resolution it is well matched to the vast majority of powder samples. The peak positions are specimen displacement independent and the flux is considerably improved. This geometry coupled with accurate diffractometer control is ideal for indexing problems and unit cell refinements (Hart et al., 1990). This geometry is also ideal for the study of thin films in shallow incidence. The geometry is illustrated in Fig. l(d): the narrow incident beam is diffracted from the spinning sample at very different power densities depending on the 20-angle. The beam is at its broadest at 90 ° to the specimen surface. If the collimators are manufactured so that they accept the whole diffracted beam at all angles, the diffracted beam will be integrated over a constant angular aperture, hence this intensity and angle will be correct for all detection angles. With thin film applications, provided there is not too much texture, an excellent quality powder scan can be obtained; spinning samples may also be used. An actual example of an excellent quality pattern collected in this mode is given by Fig. l(g), that is of a microporous material, ammonium germanate. (d) Energy dispersive diffraction Energy dispersive diffraction (EDD) was first demonstrated (Giessen and Gordon, 1968) more than a quarter of a century ago and can be operated using either laboratory (Mantler and Parrish, 1976; Sparks and Gedcke, 1972) or synchrotron (Buras et al., 1978; Bordas et al., 1977) white (continuous) X-ray sources. With EDD the scattering angle, 20, remains fixed: rather it is the wavelength, 2, that is scanned, either by a pre-sample or post-sample monochromator or more rapidly using an energy-
dispersive detector. As a result the Bragg diffraction equation 2 = 2 d . sin 0 is invariably re-written, using E = hc/2, in its energyequivalent form E d . sin 0 = hc/2 = a constant (6.2 keV. ]k) where h, c are the fundamental constants, E is the energy of the associated X-ray photon, and d is the crystal inter-planar spacing. The fixed 20-angle means that the energy-dispersive mode is effectively a fixed geometry system: the specimen cell entrance/exit windows (for the X-ray beam) are fixed [Fig. l(e)], as also is the effective diffracting volume within the sample; further when using an energy-dispersive detector there are no moving parts at all during an experiment. This all greatly facilitates the design of specimen cells, particularly those involving high temperature/pressure and/or chemical reaction vessels; indeed it is this well-defined fixed geometry that makes EDD such a versatile technique. Further, when used in conjunction with a hard intense X-ray source, such as from a wiggler magnet on a second generation synchrotron, two further advantages emerge. Firstly, the beam intensity, combined with fixed geometry, leads to data collection rates that are faster than any other realistic powder diffraction method--with current ED-detectors operating at useful count rates of up to 105 cps EDD can easily deliver sub-second diffraction patterns. The only serious drawbacks with the method are the limited resolution which is of the order of 2%, and a limited count rate available with current energy-dispersive detectors. This can be problematic i f important diffraction peaks are overlapped with other peaks as a result of using either (a) complex powder mixtures or (b) using low symmetry structure materials. Significant progress is being m a d e in improving the count rates available with solid state detectors, however the energy resolution of the system will always be limited by the capacitance of the detector crystal. 3. APPLICATIONS OF H I G H RESOLUTION POWDER DIFFRACTION
In the Introduction some examples of crystal structures determined from high resolution powder diffraction were given. In the last 10 years the technique has become much more widespread although it is still very much in its infancy. The sequence of events from obtaining a data set of excellent quality to an indexed pattern has been substantially enhanced by the use of synchrotron radiation. The subsequent decomposition into peak intensities is more reliable and easier with high resolution data with reliable peak shapes. Having obtained a set of indexed integrated intensities a structure solution can be attempted either by direct or indirect methods, such
Industrial aspects of synchrotron X-ray powder diffraction as the new developments in maximum entropy methods which have yielded promising results (Gilmore et al., 1990). Having obtained a trial structure the Rietveld method can be employed to refine the structure. In practice although the crystal structures of many industrially important m a t e r i a l s have
(a) =
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.
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Fig. 2. Ni-exchanged zeolite Y, showing the migration of the nickel ion from the S1 site to the catalytically active supercage site (after Dooryhee et al., 1990).
451
been determined from powder data it is more usual in an industrial environment for a good deal to be known about the materials under study, and the powder method is used to index or identify new phases, or determine the a m o u n t of each phase present. Also powder diffraction is used widely for stress 'strain analysis of materials. An example of the application of high resolution powder diffraction to industrially important systems is the ab initio structure determination of polymer electrolyte complexes (Lightfoot et al.. 1992). There has been a great deal of interest in the replacement of liquid electrolytes in batteries with a material that is both durable, easy to handle and capable of delivering a high current. Some of the most promising candidates are the high molecular weight polythene oxides with dissolved lithium salts. If suitable concentrations of materials such as L i C F 3SO 3 can be introduced into the polymer framework mobile ions can be liberated and the material conducts. However this material is very difficult to crystallise and typically only powder samples are available High resolution powder diffraction from both laboratory and synchrotron bunch sources have been used to solve and refine these structures providing important information for synthetic chemists in their attempts to manufacture better polymer electrolyte materials. Another area of great importance is the study of microporous materials as potential catalytic materials. Figure 2 shows the framework of Ni exchanged zeolite Y . This can be used in the production of benzene from acetylene. The material is at first catalytically inactive but after heating for approx. 2 h begins to catalyse the reaction to form benzene. High resolution powder diffraction was used to study this phenomenon (Dooryhee et al., 1990). A full Rietveld refinement was carried out on data sets collected at 20 rain intervals. The results are summarized in Fig. 2 which shows the migration o f a Ni 2+ ion from the catalytically inactive S1 site to the active $2 or supercage site. The spheres represent all the possible Ni sites, however the only important site from the catalytic point of view is the supercage site. The occupied sites are shown by the lighter spheres and the active Ni 2÷ moves from the inactive position 2(a'l to the active 2(b), the total path being shown in Fig. 2(c). Such structures show not only the power of the powder diffraction technique, but also how physical chemists are approaching a better understanding of the catalytic process. Finally we note the use of pole figure measurements to analyse texture, which have been a c o m m o n place method, using either photographic film or a texture attachment to a diffractometer (Cullity, 1978), for a number of years. The use of synchrotron radiation and image plates offers an interesting and very promising way forward. Figure 3 shows an image plate exposure from a mineral fibre bundle of chrysolite asbestos. The material has both rotational fibre and some plate texture. This is best illustrated by
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R.J. CERNIK and P. BARNES
Fig. 3. Chrysolite asbestos powder pattern collected using an image plate exposure from a mineral fibre bundle, showing fibre and plate texture at the nodal reflections.
examination prospect of and texture metallurgists
of some of the nodal reflections. The obtaining simultaneous crystallographic information will be very attractive to and materials scientists.
4. A P P L I C A T I O N S OF S Y N C H R O T R O N P O W D E R D I F F R A C T I O N TO D Y N A M I C SYSTEMS
An important motivation in synchrotron powder diffraction is that of harnessing its potential for realizing rapid diffraction data capture on time-dependent
systems. In many materials science applications, unlike specialized situations such as for example with explosive materials (Wong et al., 1992) or a biologically catalysed sequence (Cassetta et al., 1993), one can envisage a need to collect diffraction patterns every 1 60 s, in comparison to those more slowly changing situations where the sample is in a nearequilibrium state, such as the example given previously of Ni in zeolite Y. F o r faster reactions and transformations one has to turn increasingly to other diffraction modes, such
Industrial aspects of synchrotron X-ray powder diffraction
(a)
Fig. 4. (a) Close-up view of part of the energy dispersive diffraction system on the Daresbury SRS station 9.7, showing the transmission high temperature furnace (labelled "TH600") in the foreground, and the pre-collimator slit system in the background. (b) Energy dispersive patterns showing an initial irreversible structural transformation of the rare earth doped pyrochlore catalyst with time (after Ashcroft et al., 1993).
453
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R.J. CERNIK and P. BARNES
as E D D which is ideally suited to such studies provided some loss of reciprocal space resolution can be tolerated. Continuing in the area o f catalysis, one notes the E D D studies o f A s h c r o f t et al. (1993), o n raree a r t h pyrochlores as used for the selective oxidation or dry-reforming of methane. The latter reaction, in
which greenhouse gases C O 2 a n d C H 4 produce synthesis gas (CO a n d H2), has been studied in situ with the surprising result (Fig. 4) t h a t the pyrochlore-catalyst itself exhibits rapid initial irreversible structural changes during the process. These changes are sufficiently fast to require diffraction p a t t e r n collections in
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Industrial aspects of synchrotron X-ray powder diffraction some cases on a 30 s time scale, such a time-resolution of course being well within the capabilities of EDD. Similar time resolutions are required for studying the transformations involved in the synthesis of ceramic components and a full description of such an application, the synthesis of tetragonal/monoclinic zirconia, is given elsewhere in this issue (Turrillas et al., 1995). In these particular experiments the important additional factor was the provision of a furnacing capability up to 1300°C as illustrated in Fig. 4(a). This application underlines the importance of being able to combine high speed diffraction data capture with the sample environment design, and it is the relative ease of performing this combination with a fixed geometry system that makes EDD such an attractive and, sometimes, unique mode for many applications, particularly "one-off" experiments. This aspect has been exploited in a series of studies (Barnes, 1991, 1992; Barnes et al., 1992; Munn et aL, 1992: He et al., 1992; Muhamad et al., 1993) on a range of functional zeolites and cementitious materials, and from these we include just two examples in this current review. As a first example we refer to a study on cement hydration (Muhamad et aL, 1993) where the hydration products were known to form rapidly, so rapidly in fact that the initial mixing process and safety-search of the experimental "hutch" area had to be designed into the overall experiment. Figure 5(a) shows a schematic of the arrangement used which permits automated mixing and stirring of the initial cementitious ingredients--one might describe it as a "mini-cement mixer"--while diffraction data are being collected. Such data collected on 10 s time intervals [Fig. 6(a)] show that the main hydration product, a calcium sulpho-aluminate hydrate referred to as "ettringite", starts to form within even the first 10 s of mixing. Subsequent diffraction data up to times of 100,000 s show that this hydration product can participate in a time-dependent solid solution effect due to the rapid hydration promoting substitutional groups (hydroxy/carboxy instead of sulphate groups) into the crystalline structure. This effect is demonstrated by calculating from the EDD-patterns the unit cell a-parameter for the growing ettringite phase as a function of time: Fig. 6(b) shows, graphically, a time-sequence for one such experiment in which a 0.7% increase in the unit cell a-parameter is observed in the first 100,000 s. This is consistent with a substitution of 50-70% of the sulphate groups by hydroxy/carboxy groups in the ettringite structure. This is a good example of how in situ rapid timedependent EDD-patterns can shed new light on the nature of a complex hydration process. A second example of how a specific application feature can be designed into an experiment using EDD is provided by the area of zeolite synthesis and stability. The alumino-phosphate molecular sieve, VPI-5, can be synthesized hydrothermally under autoclave conditions using a gel-template mix (He
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Fig. 6. Some unique time-resolved powder diffraction sequences obtained using the speciallydesigned energy dispersive diffraction systems illustrated in Fig. 5. (a) In situ diffraction patterns obtained during the very rapid early formation of the calcium sulpho-aluminate hydrate, ettringite, in 10 s intervals after mixing. The main ettringite (110) peak is marked. On these time scales, the hydration sequence appears to commence immediately after mixing. (b) Variation of the ettringite unit cell a-parameter with time during its growth (time on logarithmic scale). This indicates a time-dependent solid solution growth effect.
et al., 1992), producing VP1-5 fairly rapidly at c.
140°C. Until recently it was believed that such conditions could only be practicably realized with in situ diffraction by using neutron radiation (Polak et al., 1990) since the considerable penetration power of neutrons makes the construction of specimen cells with thick walls possible. However with the energetic white X-ray spectrum available from a synchrotron wiggler insertion device it is now similarly feasible to exploit the penetration of 20100 keV X-ray photons through metal/polymeric enclosures and bulk (e.g. 10 mm thick) sample material. Figure 5(b) illustrates the autoclave cell system used for synthesis and stability studies on VPI-5. Figure 7(a) and (b) show typical time-resolved EDDplots covering the autoclave synthesis of VPI-5 and
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•
°
Fig. 7. (a) In situ diffraction patterns obtained during the autoclave synthesis of VPI-5. In this case the TBA-VPI-5 recipe is being used and the final temperature is 140-142°C. The diffractometer 20.angte is Set at 119° and the resulting d-spacing range reveals one very strong growing peak at 22.7 keV (16.ll &) and a Weaker growing Peak at 45.6 keV (8.2 ~) which both belong to the TBA-VPI-5 phase being synthesized. (b) In situ diffraction patterns showing the subsequent rapid transformation of t h e autoclave-synthesized VPI-5 to the related A1PO]-, after releasing the autoclave seal between 150 and 156°C. its subsequent transformation to a related zeolite, A1PO4-8, on releasing the autoclave seal. With these data it is now possible to make meaningful and quantitative interpretations (He et al., 1992) of the effects of synthesis gel-preparation and incubation and rates of transformation. This includes the temperature effect and activation energy of the zeolite product. The more ambitious possibility now arises o f contemplating an interactive synthesis regime where zeolites, which are either new or are difficult to produce synthetically, are grown by controlling the temperature conditions in response to the information gained from the dynamic EDD-data. If this could be realized it would bring a quite different dimension to the subject of in situ dynamic diffraction.
5. C O N C L U S I O N S
This paper has outlined the main advantages of powder diffraction systems which utilize the superior photon flux (for the intensity o f patterns obtained; speed of data capture), the collimation (to obtain high resolution for separating diffraction peaks), and wavelength range (choice of wavelength in angle dispersive geometries; choice of angle in the energy dispersive geometry) of X-ray beams produced by a synchrotron. Other advantages not detailed include the tunability of the radiation to an absorption edge (for anomalous absorption powder diffraction experiments) and the horizontal linear polarization of the radiation (which can be exploited to remove intensity
Industrial aspects of synchrotron X-ray powder diffraction losses normally associated with a randomly polarized laboratory X-ray source). Structure solution/refinement o f crystalline powder material, the highlighting of minor phases, and rapid time-resolved experiments are aspects of materials science which are benefiting from these enhanced powder diffraction facilities. Future developments in (third generation) s y n c h r o tron sources will further emphasize these advantages, while instrumentation developments, such as energydispersive detectors and two-dimensional image plate systems, will further extend the range of information that can be collected.
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