AN INNOVATED LABORATORY XAFS APPARATUS

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1 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume AN INNOVATED LABORATORY XAFS APPARATUS TAGUCHI Takeyoshi XRD Division, Rigaku Corporation HARADA Jimpei X-ray Research Laboratory, Rigaku Corporation TOHJI Kazuyuki and SHINODA Kohzo Deapartment of Geoscience and Technology, Tohoku University ABSTRACT X-ray Absorption Fine Structure (XAFS) spectroscopy is one of the most powerful tools to determine valence states of local atoms in various materials. XAFS spectroscopy has an advantage in providing the structural information of amorphous, liquid or catalyst, which cannot be obtained by X-ray powder diffraction technique. Results from XAFS spectroscopy are commonly reported using synchrotron radiation, but results obtained using a laboratory XAFS apparatus has rarely been reported. The shortage of data obtained from a laboratory XAFS apparatus has been mainly due to its availability. A laboratory XAFS apparatus is not as popular as a powder diffractometer or a fluorescence spectrometer since its complicated and expensive mechanism. We have recently developed an innovated laboratory XAFS apparatus, which is compact in size and easy to use. This could boost the popularity of XAFS spectroscopy. INTRODUCTION Although XAFS spectroscopy is a very powerful technique to characterize material, it is not as popular as XRD or XRF. The main reason is a lack of access to this method. In general, it is believed that Synchrotron Radiation (SR) is necessary to measure X-ray Absorption Spectra. Only a limited researchers can use those facilities regularly, but most other people can get there once a year or less. Before SR became available, many researchers built their own apparatus and measured XAFS spectra. From mid 70 s to late 80 s, many reports on laboratory XAFS facilities are found in a literature.[1][2][3] In 1980, a workshop on laboratory XAFS facilities was held at the University of Washington. In that workshop, they concluded that laboratory XAFS facilities can provide good quality DATA for moderate specimen [4], but the report out of laboratory XAFS facilities seems decreasing since then. The reason is that XAFS researcher had shifted to SR. XAFS spectra taking hours can be done in minutes at SR. A few XAFS systems (based on a rotating anode X-ray source) have been sold in Japan in last 2 decade. To compare with XRD or XRF apparatus, they are much less. The main reason is its cost, and the total system cost $50,000 or more.

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3 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume INSTRUMENTATION Optics Principle RIGAKU has manufactured rotating anode based systems for more than 20years. The system configuration had been changed gradually, but the principle of the spectrometer has not changed. The principle is so called linear spectrometer [5] which makes use of a Rowland geometry. The X-ray source S, the crystal center M and the receiving slit F are placed along Rowland circle with a radius R. They are each mounted at one end of a bar of R in length. The end of bars are linked. 2 stepping motors are installed to make translational motion keeping the X-ray source to crystal and crystal to the receiving slit distances the same so that the incident angle and the exit angle are kept the same. During an energy scan, the X-ray source (S) and the crystal center (M) are moved, and distances SM and MF are kept the same. There had been many systems suggested, designed and tested [6], but we found that this Fig.1 Principle of linear spectrometer system is the best for laboratory XAFS apparatus. Design Consideration Today users prefer a small apparatus for analytical equipment. The main apparatus of an original RIGAKU XAFS system has a dimension of 1800 x 1500mm 2. The whole system added space for a high voltage transformer, controller and computer system, requires more than double the space occupied by an ordinary power diffraction system. The size is large because of a large scanning range (30 to 120º in 2θ ). We have reconsidered the scanning range. The range is determined so that a maximum number of elements can be measured. Table 1 shows the numbers of elements, which can be measured using different monochromator crystals in 30 to 60º, 60 to 90º and 90 to 120º 2θ ranges. Only about 10 percent of elements are measured in the scanning range of 90 to 120º. From our experience, this scanning range is hardly used. Also when 2θ angle gets above 90º, X-ray intensity decreases significantly Ge(111) Ge(220) Ge(400) Si(400) Table1 Number of elements measured with various monochromator crystals

4 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume We decided to limit the maximum scanning angle to 90º. By reducing the maximum scanning angle from 120º to 90º, the translation distance (length of SM(=MF) in Fig.1) is shortened by 100 mm. When the translation distance is long, it becomes difficult to maintain accuracy and the cost of parts get more expensive. A reduction of the maximum angle improves the reliability of the equipment and reduces the cost down at the same time. An X-ray source is fixed in a rotating anode based XAFS system because the source is too heavy to move. Therefore the diffraction plane was limited to horizontal. Fig.2 shows an example of rotating-anode based XAFS system. In this system, operation panel is at the bottom of figure. When an operator changes sample or monochromator crystal, the operator has to hang over the goniometer or walk around to the other side and open a cover. From our experience of developing a vertical type goniometer [7], we found that a vertical Fig.2 Schematic diagram of a rotating anode based XAFS system goniometer gives good accessibility to all components. Therefore we decide to use a vertical goniometer. X-ray source When XAFS spectrum is measured with a laboratory system, the tube voltage is set double to absorption edge energy of the element to be examined to avoid higher harmonics. The full power of a 18kW rotating anode generator is not needed for most cases. A normal sealed tube is not suit for XAFS measurement because it uses tungsten for its filament material. Tungsten L emission lines have been a problem for using a conventional X-ray source for XAFS measurement [4]. We have developed a small fixed-anode X-ray tube that can select LaB 6 as filament material. This tube is used in the newly designed goniometer. In the new goniometer, the X-ray source and the monochromator crystal are both in motion. Hence the position and the direction of which monochromatic X-ray is emitted are not changed during an energy scan. This gives great freedom to a sample chamber, which is Fig.3 Schematic diagram of new XAFS apparatus

5 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume not possible with an original goniometer. PERFORMANCE Absorbance (arb.u.) â-fesi2 Fe Fe2O 3 Fe3O E nergy (ev ) Fig.4 XANES spectra of various Fe compound taken with new system The new system is about half size of our original rotating-anode based system, but its performance is maintained. Fig.4 shows XANES spectra of various Fe compounds obtained with the new XAFS system. Ge(400) crystal was used and X-ray generator was operated at 14 kv ma. Energy step is 1 ev and it took about 3 hours to scan an 100 ev energy range. The pre-edge feature at 7123 ev is clearly resolved. This indicates that the energy resolution of the system is better than 2 ev at this energy. Fig.5 shows the MoS 2 EXAFS spectrum taken with the new system. A Ge(840) crystal was used. The tube voltage and tube current were set 40 kv and 75 ma respectively. The total DATA acquisition time took about 12 hours to complete a 400-point scan. More than 16Å -1 of spectral region in k range can be used for a FOURIER analysis. Although the X- ray power was low and the measurement time was slightly longer than a rotating-anode based system, sample handling is far improved. rbance (arb.u.) A bso Crystal: Ge(840) Filament: W Target: W XG: 40kV-50mA Time: 12 hours (400 points) Energy (ev ) Fig.5 EXAFS spectrum of MoS 2

6 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume CONCLUSION XAFS is very useful technique to obtain structural information of amorphous, nano-scale particles and liquid, but there has not been a handy spectrometer to be used in a laboratory. We have developed a compact laboratory XAFS system. This system is easy to handle and can be used for routine XAFS experiments. REFERENCES [1] Knapp, G.S., Chen, H., and Klippert, T.E., Rev.Sci.Instrum, 1978, 49, 1658 [2] Cohen, G.C., Fischer, D.A., Colbert, J. and Schevchik, N.J., Rev.Sci.Instrum., 1980, 51, 273 [3] Tohji, K., Udagawa, Y., Kawasaki, T. and Masuda, K., Rev.Sci.Instrum., 1982, 54, 1482 [4] Stern, E.A. ed., Laboratory EXAFS Facilities-1980, 1980, 160 [5] Iwasawa, Y. ed., X-ray Absorption Fine Structure for Catalysts and Surfaces, World Scientific, 1996, 134 [6] Taguchi, T., Qi-Fan Xiao, and Harada, J., J.Synchrotron Rad., 1999, 6, 170 [7] The Rigaku Journal, 1993, 10, 2, 45