Structure of microworld - the world of discovery by invisible rays
V. Valvoda, M. Chladek, D. Rafaja, J. Vacinova, J. Lhotka, L. Dobiasova, D. Simek, J. Kub1, V. Holy2, J. Zweck3, H. Hoffmann3, R. Krishnan4
1. Institute of Physics, Czech Academy of Sciences, Prague 8, Na Slovance 2
2.
Institute of Physics, Faculty of Natural Sciences, Masaryk University, Brno
3.
Institute of Advanced Physics, University of Regensburg, Germany
4.
CNRS, Meudon, France
Ag/Ni81Fe19 multilayers
The study of this system has been finished in this year. The results are summarized in [1] and they were also presented as an invited lecture on the 45th Annual Denver X-ray Conference & Powder Diffraction Satellite Meeting of the XVII Congress of the International Union of Crystallography. The results of structure analysis of the multilayer systems under study explain their distinct magnetic behavior, particularly the change from superparamagnetic to ferromagnetic state with increasing thickness of Ni81Fe19 layers. The essential point is the different amount, location and size of crystalline precipitates observed in samples with different thickness of Ni81Fe19 layers together with different size and quality of the residual multilayer blocks. The in-situ annealing structural studies were a good guide in searching for the optimal post-deposition treatment. The highest magnetoresistance of 5% at the 5 kOe magnetic field, found in the sample with the 7L Ni81Fe19 layer thickness, yet increased after an optimized fast annealing up to 7%.
HXRD patterns for the Ag/Ni81Fe19 multilayer during the annealing process shown in the inset
Fitted satellite peak widths FWHM (a,b) and heights (c,d) of the Ag/Ni81Fe19 multilayer and of Ag and NiFe crystallites for the annealing process. Solid lines are a guide for eye.
Au/CoNi multilayers
Co/Ni and Co/Pt/Ni/Pt multilayers
These materials were investigated mainly as a model samples for a study of interdiffusion processes in thin films. The results obtained up to date have been sent for publication [3]. The expected high degree of interdiffusion of atoms in the system Co-Ni was confirmed both by the measured reflectivity curves at low angles and also by the high-angle XRD. The multilayers containing platinum as a diffusion barrier show diffraction patterns characteristic for a well ordered superlattice. To describe the observed diffraction patterns, it was necessary to use a structure model which assumes different amount of interdifussion for Co and Ni atoms. As the Co and Ni atoms are the neighboring elements in the periodic table we can obtain an unambiguous answer on the diffusion process of these two elements only when enhancing the difference of their scattering powers in the vicinity of Co and Ni K absorption edges. Preliminary calculations of diffraction patterns for distinct wavelengths in the vicinity of these absorption edges have shown a strong enhancement in intensity of satellite peaks which are almost not observable for the CuKa radiation. The project assuming the application of synchrotron radiation to solve this problem by the DAFS (Diffraction Anomalous Fine Structure) technique has been proposed and presented at the 18th European Crystallographic Meeting in Prague [4]. The project has been realized at the end of the year and the results are now evaluated.
Fe/Au multilayers
The results obtained in a study of the set of Fe/Au multilayers prepared at the laboratory of Professor Krishnan in Meudon are now prepared for publication. X-ray diffraction analysis of 13 samples with various thickness of sublayers revealed that the character of the structure of these samples is different from all other samples investigated up to now in our laboratory. Standard structural models and fitting programs used in structure analysis of multilayers failed in most cases to reproduce well the observed diffraction patterns. Thus new models and new fitting procedures were prepared to enable a better analysis of these materials. Moreover, five samples were also analyzed by TEM in the laboratory of Professor Zweck in Regensburg and an attempt to utilize the more intensive synchrotron radiation source in ESRF in Grenoble to help in decoding the real structure of Fe/Au multilayers has been made. The analysis of the data is still in progress.
The project was supported by the Grant Agency of Czech Republic, grant No. 202/97/1123 and the Grant Agency of the Charles University, project No. GA UK 140/96
- V. Valvoda, M. Chladek, X-ray analysis of magnetic multilayers, Advances in X-ray Analysis, Vol.40, Edited by J.V. Gilfrich et al., International Centre for Diffraction Data , Newtown Square 1998. (CD-ROM), 6 pages.
- V. Valvoda, M. Chladek, Multilayer structure: A comparison of results from XRD and from structure imaging techniques, Materials Science Forum, Vols. 278-281 (1998) 158-163.
- D. Rafaja, J. Vacinova, V. Valvoda, X-ray study of the Co/Ni and Co/Pt/Ni/Pt multilayers, submitted to Thin Solid Films.
- J. Vacinova, D. Rafaja, V. Valvoda, Characterization of the real structure of ultra-thin magnetic multilayers using X-ray reflectivity, X-ray diffraction and DAFS methods, Materials Structure in Chemistry, Biology, Physics and Technology, Vol.5, special issue B, ECM-18 posters - abstracts, p.216.
V. Valvoda, D. Rafaja, R. Kuzel, A Yelnikov and G. Cappuccio1
1. Giorgio Cappuccio, INFN - LNF - Laboratory DAFNE LUCE, Via E. Fermi 40, P.O. Box 13, I-00044 Frascati, Italy
Diamond coatings grown on Ti substrates have been investigated by the glancing angle incidence X-ray diffraction. The angles of incidence were in the range from 0.5 degree to 10 degrees. The aim of this work is to determine the thickness of the coatings by the measurement of the intensity of diamond reflections as a function of the angle of incidence. The method is based on the angular dependence of absorption of X-rays in thin films. There are complications arrising from preferred orientation of crystallites, from instrumental effects and from the existence of intermediate layers of TiH2 and TiC. Attempts to use an external standard to determine the instrumental effects have been made and the results are now evaluated. The measurement of the orientation distribution function of diamond crystallites has been found to be important to correct this effect. The existence of the interlayers is a drawback in an attempt to determine the thickness of the diamond coatings from the measured intensities of titanium substrate reflections.
- A. Yelnikov, D. Rafaja and V. Valvoda, Preliminary report on the diamond coatings, Prague, June 1998
- G. Cappuccio, Diamond Thin Films, 1st Report, Frascati, December 1998.
L. Dobiasova, D. Rafaja, R. Kuzel, T. Sikola* and J. Spousta*
* Faculty of Mechanical Engineering, Technical University of Brno, Technicka 2, CZ-616 69
Ultra-thin films prepared by the ion-beam assisted deposition (IBAD) were investigated using the glancing angle X-ray diffraction (GAXRD) to determine the degree of crystallinity, the phase composition and, if possible, the lattice deformation due to the residual stress. Four nitride systems and two pure metals were studied this year: Ti-N, Al-N, Mo-N, Zr-N, Co and Ni. In the Ti-N samples only the face centred cubic (fcc) d-TiN phase was detected; the titanium nitride has grown with small and destroyed crystallites. Two phases were found in the Al-N samples: the hexagonal AlN phase and the cubic nitrogen-free aluminium. The amount of the respective phase has varied both with the bias voltage and with the substrate temperature.
In the Mo-N coatings predominantly the non-stoichiometric fcc g -Mo2N phase has grown. Depending on the deposition conditions, also the stoichiometric hexagonal MoN phase was observed. Some of the Mo-N layers were deposited on a molybdenum buffer. In these samples also the cubic molybdenum was found. In the g -Mo2N phase, the stress-free lattice parameter has increased with increasing bias voltage. Moreover, substantially higher lattice parameters were observed in samples deposited at 20C than in the samples deposited on heated substrates (200-400C). The possible reason is that the coatings can grow closer to thermodynamic equilibrium if they are deposited on heated substrates than if they are grown at the room temperature. In the thermodynamic equilibrium, the stoichiometry of the molybdenum nitride depends strongly on the partial nitrogen pressure. This is a possible reason for the strong dependence of the lattice parameter on the substrate temperature and for the absence of the hexagonal MoN phase as well.
The ZrN coatings deposited at higher temperature (400 C) contain well-developed fcc d -ZrN. Crystallinity of the samples deposited at the room temperature is poor. These samples contain probably small nuclei of the d -ZrN as well.
Independent of the temperature of the deposition process, the Ni layers were found to be well crystalline. At 20 C and 250 C, the pure fcc Ni has grown. At 400 C, the tetragonal h -NiSi was observed that grows probably just at the Ni/Si interface. According to the phase diagram, the h -NiSi phase is really stable at 400 C. The Ni layers deposited at the room temperature and zero bias show a compressive residual stress below -1 GPa. With increasing temperature of the deposition process, the residual stress becomes tensile, reaching 0.67 GPa at 250 C and 0.76 GPa at 400 C. We believe that the residual stress is predominantly a result of the different temperature expansions of the layer and substrate.
The Co layers deposited at the room temperature are not well crystalline. At higher temperatures, the samples contain a mixture of the low-temperature hexagonal close packed (hcp) cobalt and the high-temperature fcc cobalt. Probably, there is a high density of the stacking faults, which is typical for a mixed hcp/fcc structure. At 400 C, tensile residual stress of approximately 1 GPa was detected, which has been decreasing with increasing bias.
This work was supported by the Grant Agency of the Czech Republic as the project # GACR-101/98/0781.
- D. Rafaja, V. Valvoda, T. Sikola, J. Spousta, Ab initio study of thin metallic and ceramic films, Thin Solid Films 324 (1998) 198-208.
- T. Sikola, J. Spousta, R. Ceska, J. Zlamal, L. Dittrichova, A. Nebojsa, K. Navratil, D. Rafaja, J. Zemek and V. Perina, Deposition of Metal Nitrides by IBAD, Surf. Coat. Technol. 109 (1-3) (1998) 284-291.
* University of Chemical Technology, Prague,
! Siemens, Germany
The high-temperature X-ray diffraction (HTXRD) is a very useful tool to study the phase equilibria and phase transformations in situ. This technique is especially important for investigation of such high-temperature phases, which cannot be rapidly quenched and studied post-mortem. Another application of the in situ HTXRD is the study of the initial phases of diffusion processes. The current theories of the diffusion processes assume that all phases grow concurrently and that the whole phase-band structure arises just at the beginning of the diffusion process if the chemical activity of the in-diffusing species in the environment is sufficiently high. However, this assumption implies automatically another premise - the diffusion flow through the sample surface must be unlimited. Moreover, as the starting concentration profile has the form of a concentration sink, the concentration gradient must be infinitely large at the sample surface. The same holds also for the flow of the in-diffusing species through the surface. On the contrary, if the diffusion flow is limited, there must be a consecutive evolution of individual phases, starting with the solid solution of the non-metal in the metal and ending with the non-metal rich phase. There are some experimental indications as, for instance, the existence of the incubation time, but a direct experimental evidence is still missing.
The first in situ HTXRD experiments were done with the lead wolframate, PbWO4. The main aim of this work was the study of the phase stability of PbWO4 in the temperature range between 200 C and 900 C. Because of a good chemical stability at those temperatures, the lead wolframate, PbWO4, does not react with potential impurities in the high-temperature chamber. Thus, it was a suitable material for starting experiments. The evaluation of results is still in progress. Upon the foremost measurements, the experimental procedure was automated and optimised. Now, using a position-sensitive detector it is possible to perform HTXRD measurements having a resolution comparable with conventional powder diffractometers. The entire measuring time does not exceed several minutes. The statistical quality of such measurements is rather high, as the intensity maxima reach several thousands counts in total.
This work is supported by the Grant Agency of Charles University as project # GAUK-66/98.
V. Ucakar*, D. Rafaja and W. Lengauer*
* Institute for Chemical Technology of Inorganic Materials, Vienna University of Technology, Getreidemarkt 9/161, A-1060 Vienna, Austria
This project was carried out within the frame of the co-operation between the Department of Semiconductors Physics (Charles University Prague) and the Institute for Chemical Technology of Inorganic Materials (Vienna University of Technology). The first goal of the project was the investigation of the phase stability in the Ga-N system. The Ga-N was prepared in form of layers in a horizontal open-flow system. In the source zone, gallium molten in a Pythagorean boot has reacted with diluted HCl (carried by the N2 serving as the inert gas) to form gaseous GaCl, which was transported into the deposition zone. In the deposition zone, the gaseous GaCl was mixed with NH3, which resulted in the formation of GaN. The GaN was deposited on different substrates, which were heated up to the growth temperature. This part of the work has been done at the Vienna University of Technology.
Three kinds of substrates were applied: (a) sapphire single crystals, (b) Al2O3 glass and (c) SiO2 glass. The aim of the second part of the work was to investigate the phase composition of layers, the residual stresses and the preferred orientation of crystallites. The glancing angle X-ray diffraction (GAXRD) operating with the quasi-parallel beam optics was employed as a very powerful technique for structure investigation of coatings. In this diffraction geometry, the sample is irradiated by a divergent primary beam (! 1 ) at a constant angle of incidence. The diffracted beam is passing a long Soller collimator with the opening ! 0.3 . The nearly parallel part of the diffracted radiation is focused by a curved graphite monochromator into the detector. The detector moves together with the Soller collimator and the graphite monochromator to scan the reciprocal space. To obtain a better information on depth homogeneity of the layers, the GAXRD measurements were performed at three different angles of incidence, g = 2 , 5 and 15 , which provide sufficiently different penetration depths.
Crystalline gallium nitride, GaN, was found in all samples. However, as GaN has a hexagonal structure, the well-known modification of the sin2y method for a constant angle of incidence, which works with different diffraction lines, could not be applied to calculate the lattice deformation caused by the residual stress. This method is only applicable for cubic materials, as the lattice parameters obtained directly from positions of individual diffraction lines can be used as a commensurate quantity. As this is not possible for materials with a lower crystal symmetry, the sin2y method was modified again to be able to refine the lattice parameters together with the parameters characterising the lattice deformations.
In certain GaN-layers deposited either on the sapphire single crystals or on the Al2O3 glass, a very strong preferred orientation of crystallites {001} was detected. The preferred orientation has complicated the measurement of the residual stresses substantially. In these samples, the preferred orientation was studied additionally using the W -scan. Two kinds of the microstructure has been found. The crystallites were either distributed with the direction {001} perpendicularly to the sample surface or the distribution could better be characterised by a bi-modal distribution having two maxima mutually slanted about 3 . The validity of these models was confirmed by the scanning electron microscopy.
This project was supported by the Joint Czech-Austrian Grant Agency as the project AKTION # 19P11.
D. Rafaja, C. Kral*, K. Fatrdla*, B. Wollein* and W. Lengauer*
* Institute for Chemical Technology of Inorganic Materials, Vienna University of Technology, Getreidemarkt 9/161, A-1060 Vienna, Austria
The advanced model of diffusion processes created in the last years was applied to simulate the layer growth and the evolution of concentration profiles in multiphase binary carbides and nitrides, in which only the non-metal diffuses. Both, the layer growth and the changes in the concentration profiles were demonstrated in form of "movies", which illustrated the influence of finite diffusion geometry on the speed of the non-metal in-diffusion. These movies are shown at the URL address http://info.tuwien.ac.at/physmet/movies.html.
Besides, we have worked on the description of the interdiffusion process in binary systems, in which both species diffuse. The aim was to estimate the diffusion time needed for development of a sufficiently broad interdiffusion zone, which should be applied as starting material for further experiments. There are two significant differences between the diffusion of one species and the mutual interdiffusion of two species. First the boundary conditions and secondly the concentration dependencies of the diffusion coefficients are different. Upon the diffusion processes, the concentration at the sample surface is assumed to be constant. This boundary condition implies that the diffusion flow reaches its maximum at the sample surface and that the resource of the in-diffusing species is unlimited. The boundary conditions for the interdiffusion process follow the assumption that there is no macroscopic atomic exchange between the diffusion couple and the environment. Thus, the diffusion flow is equal to zero at the sample surface.
The second difference concerns the concentration dependence of diffusion coefficients. Whereas many diffusion coefficients of the non-metal in a metal matrix are nearly constant, i.e., independent of the concentration within the whole homogeneity range of the respective phase, the interdiffusion coefficients depend basically on the local concentration of both species. As for the atomic diffusion, we tried apply the forward finite difference (FFD) method to solve the diffusion equations for interdiffusion. Unfortunately, the FFD method is only applicable for a weak dependence of the diffusion coefficients on concentration, as it was confirmed when inspecting the numerical algorithm. Thus, the diffusion equations with the concentration dependent diffusion coefficients must be solved analytically.
The last topic related to diffusion was the investigation of the phase stability and the microstructure of phases in the quarternary Ti-W-C-N system. Samples with different starting compositions were hot-pressed and subsequently annealed in nitrogen atmosphere at 10 and 30 bar N2. The X-ray diffraction (XRD) was used to study the phase composition, the size of crystallites, the lattice deformation and the residual stresses in the near-to-surface region. The actual average concentration and concentration profiles of the non-metal were obtained by the gas chromatography (GC) and by the electron probe microanalysis with wavelength-dispersive spectroscopy (EPMA/WDS).
Depending on the material's composition, the following phases were found in the samples: hexagonal WC, hexagonal W2C, cubic tungsten and cubic (Ti1-xWx) (C1-yNy). In many samples, the presence of two cubic phases with slightly different lattice parameters, i.e., tungsten-rich (Ti, W) C and nitrogen-rich Ti (C, N), were detected using the profile analysis of the X-ray diffraction pattern. Asymmetrical diffraction measurements performed using the Seemann-Bohlin geometry or the parallel beam optics confirmed the anticipated development of the compressive residual stresses in the near-to-surface region. These stresses are caused by the lattice expansion due to the increase of the nitrogen concentration, which occurs during the annealing in N2.
- D. Rafaja and W. Lengauer, Simulation of reaction diffusion using the FFD method, Modelling and Simul. Mater. Sci. and Eng. 6(2) (1998) 141.
- D. Rafaja, W. Lengauer, H. Wiesenberger and M. Joguet, Combined Refinement of Diffusion Coefficients Applied on Nb-C and Nb-N Systems, Metall. Trans. A29 (1998) 439.
- D. Rafaja, W. Lengauer and H. Wiesenberger, Non-metal diffusion coefficients for the Ta-C and Ta-N systems, Acta Mater. 46(10) (1998) 141-152.
D. Rafaja, A. Yelnikov, M. Joguet* and W. Lengauer*
* Institute for Chemical Technology of Inorganic Materials, Vienna University of Technology, Getreidemarkt 9/161, A-1060 Vienna, Austria
Homogeneous samples of d -NbN1-x with [N]/[Nb] = 0.73 - 1.00 were prepared in a reactive diffusion process at 1400-1800 C and analysed by using X-ray diffraction. To get an information on the real structure of the cubic d -NbN1-x phase for different stoichiometries, diffraction patterns were evaluated using the Rietveld full pattern analysis. To avoid the anticipated correlation between parameters of the structure model, symmetrical scans measured in the symmetrical diffraction geometry were completed by texture measurements, which were performed on an Eulerian cradle. It was found that at a lower nitrogen content, niobium occupies not only the metal positions but also the non-metal positions in the NaCl structure of d -NbN1-x.
Thus, the structure model was based on the NaCl structure type, in which the randomly distributed vacancies at the nitrogen positions can also be occupied by niobium. As no preferred orientation was found in the powder samples, no texture correction was applied for the refinement. Thus, four structure parameters (the lattice parameter, the occupancies of the Wyckoff positions 4a and 4b and the overall temperature factor) and nine instrumental parameters (six of them describing the background, two describing the line broadening and one for the line shape) were refined. Occupancies of the individual atomic positions were constrained to comply with the stoichiometry obtained from the chemical analysis. It was found that the vacant non-metal positions in d -NbN1-x are occupied up to 2 at % of niobium. This could explain the very weak concentration dependence of the diffusion coefficient of nitrogen in d -NbN1-x observed in diffusion experiments as well as the non-linearity of the lattice parameter of d -NbN1-x with increasing nitrogen contents observed in the XRD measurements.
This work was a part of the project # GAUK-66/98.
A. Yelnikov, D. Rafaja, M. Joguet and W. Lengauer, On the ordering in the d -NbN1-x, Metal Physics and Advanced Technologies, in press.
D. Rafaja, R. Kuzel, J. Kub* and G. Cappuccio!
*Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, Praha 8
! INFN - LNF - Laboratory DAFNE LUCE, Via E. Fermi 40, P.O. Box 13, I-00044 Frascati, Italy
It is well known that the data obtained from experiments are always a convolution of the "physical" signal and the unwanted instrumental function. Thus, the knowledge of instrumental effects is needed to get a high-quality information from the measurement. This project was devoted to the inspection of the instrumental effects arising (i) in the X-ray reflectivity (XRR) measurements performed with a triple-axis diffractometer and (ii) in the glancing angle X-ray diffraction (GAXRD) measurements performed with a quasi-parallel beam optics.
The first experimental set-up consists of the source of radiation, which was an 18 kW rotating anode, the perfect flat Ge (111) monochromator, the horizontal primary slit (70-100 m m) and a sample appropriate for the XRR measurements. The reflected radiation was analysed either by the four-reflection channel-cut Si (111) monochromator or by the horizontal secondary slit with the opening of 100 m m. The investigation of instrumental phenomena has shown that the instrumental function can be described by a parallelogram in the angular representation of the reciprocal space, which length (the size in the q /2q direction) and width (the size in the W direction) depends on the size of the focus, the opening of the primary slit, the distance between the focus and the sample, the distance between the sample and the secondary slit and/or on the acceptance of the analyser placed in the reflected beam.
Moreover, the width of the instrumental function is increasing with decreasing angle of incidence, that is the angle between the monochromatised primary beam and the sample surface. This increase is rather steep, especially for the diffuse-scattered radiation. This is because both the instrumental asymmetry and the pair-correlation of structural disturbances are becoming more dominant with a larger irradiated area, that is, with decreasing angle of incidence. Further, it was found that the width of the instrumental function is different for the specularly reflected radiation and for the diffuse-scattered radiation.
Besides, special instrumental effects induced by the sample geometry have been studied. The most important one is probably the limited size of the sample, which causes large aberrations in the intensities measured at extremely small diffraction angles. For parallel incident beam, the decrease of intensity follows the function sin q , when the size of the irradiated area exceeds the length of the sample. An additional decrease of the reflected intensity was observed with curved samples. In such a case, the ridge of the specular reflectivity is broaden at very low angles of incidence, typically in the total reflection region. As the intensity integrated over all inclinations of the sample (the W -positions) remains constant, a decrease of the maximum reflected intensity is observed. Both phenomena, the small sample and the curvature of the sample surface cause an apparent decrease of the surface roughness calculated from the reflectivity curves.
The second part of this work was devoted to the study of instrumental effects arising in the quasi-parallel beam geometry used in the GAXRD. In this diffraction geometry, the samples are irradiated by a divergent primary beam with a divergence of 0.5 - 1 . From the diffracted radiation, a nearly parallel part is selected by a long Soller collimator having the angular opening of 0.4 or 0.7 . The diffracted radiation passing the Soller collimator is focused by a curved graphite monochromator into the detector. This diffraction geometry is not sensitive to the defocusation of radiation, which typically arises when the sample is inclined from the symmetrical position.
Upon the instrumental effects, primarily a decrease of intensity was observed due to the limited sample size, similarly to the XRR measurements. For the quasi-parallel beam geometry working with the divergent primary beam, the instrumental function is more complicated than for the really parallel incident beam. The effect of the limited sample size becomes important, if the measurements are performed at different angles of incidence. The second instrumental effect was observed at a constant angle of incidence, if the reciprocal space is just scanned by the detector. A detailed analysis based on the ray tracing has shown that not only the irradiated area is asymmetric, but also the acceptance of the long Soller collimator is asymmetric. Both asymmetries are inverse each other. Moreover, they are the strongest at low angles, i.e., at low angles of incidence and at low angles between the sample surface and the detector position. For certain angles of incidence, the asymmetry is so strong that the overlap of the irradiated area and the acceptance of the Soller collimator changes drastically with the diffraction angle, which was observed as a jump in measured intensities.
J. Vacinova, D. Rafaja, V. Valvoda, L. Drozdova1, J.L. Hodeau2, H. Renevier2,3, D.T. Bowron,3
1
J. Heyrovsky Institute, Czech Academy of Sciences, Dolejskova 3, 182 23 Prague 8, CZ
2
Laboratoire de Cristallographie CNRS, BP 166X, 38042 Grenoble Cedex, France
3
European Synchrotron Radiation Facilities, BP 220,38042 Grenoble Cedex, France
In relation with the membership of the Czech republic in European Synchrotron Radiation Facilities in France, the X-ray diffraction group introduces new methods for structural investigations using the SR:
Anomalous Diffraction: This method is based on the resonant scattering phenomenon in a vicinity of an absorption edge, where the energy-dependent anomalous terms f' and f" of the atomic scattering factor f(E) reach their maximum magnitudes. The term f' is the dispersive term, f" is the absorption term and is proportional to the atomic cross section s (E). Near the edge both terms are very sensitive to the local symmetry and chemical state of the atom-absorber. The anomalous diffraction method is used for:
- localisation and determination of substituted atoms,
- phase determination of structure factors,
- selective study of two distinct frames in a multilayer
Extended X-ray Absorption Fine Structure (EXAFS): this method is probably one of the most extended synchrotron radiation techniques which can be applied to a large spectrum of compounds. Using properties of synchrotron radiation sources, which are the energy tunability, high intensity flux and low divergence, one can measure a variation in energy of transmitted intensity over an absorption edge and obtain a selective information of the atom-absorber (distances of nearest neighbours, coordination number). The main advantage of this technique is its applicability on very disordered or diluted samples.
Diffraction Anomalous Fine Structure (DAFS): this method is relatively new and combines all of the capabilities of diffraction (information on ordered, long range atomic structure) and x-ray absorption fine structure (local structure and chemical information) into a single technique. It allows :
- determination of the local environment of the atom-absorber in mixed occupied sites,
- valence state determination of the cations in different crystallographic sites,
- higher precision in localisation of atoms with close atomic numbers Z
The above techniques will be applied for structural characterisation of magnetic multilayers, intercalated compounds and platinum clusters inserted in zeolite matrices Na-Y and Na-X.
The interface quality strongly affects physical properties of magnetic multilayers. These artificial structures were already intensively studied using the "classical" characterisation methods. Using the anomalous diffraction and DAFS techniques we want to distinguish two phenomena: interdiffusion and interface roughness.
To obtain highly efficient Pt catalysts, Pt is required in a highly dispersed form on supports. The synthesis of small clusters has attained new dimensions when zeolites were employed as matrices because they can accommodate metallic clusters of limited size in identical environment throughout the whole material. The usage of EXAFS seems to be essential for the solution of the problem concerning the determination of the cluster size and morphology. Also some information on the close environment of the cluster accommodated in the cavity and probable interaction with the framework is expected.
Preliminary measurements of platinum clusters intercalated in zeolite matrices were carried out at the ESRF via the Rapid access facility on the BM29. These measurement were done by reason of optimisation of the experimental conditions for a forthcoming project. The experiment will be proposed via the Experimental proposal for ESRF in January 1999.
V. Valvoda, R. Kuzel, D. Rafaja
V. Valvoda: Texture effects in powder diffraction and their correction by simple empirical functions, in: R. Snyder, J. Fiala, H.-J. Bunge, eds, Real Structure Analysis by XRD, Oxford University Press, manuscript submitted.
J. Vacinova, D. Simek, D. Rafaja, V. Valvoda, R. Renevier1,2 and J.L. Hodeau1
1 Laboratoire de Cristallographie CNRS, BP 166X, 38042, Grenoble Cedex, France
2
European Synchrotron Radiation Facilities, BP 220,38042 Grenoble Cedex, France
Our contibution was devoted to the study of the multilayer systems consisting of the alterning Co/Ni bilayers, CoxNi1-x/Au bilayers and the repeated Co/Pt/Ni/Pt quadrilayers. As the Co and Ni exhibit complete solid solubility in the cubic (a -Co,Ni) phase, a very high degree of interdiffusion was assumed in the Co/Ni system. This assuption was partially verified by X-ray reflectivity and X-ray diffraction measurements using the CuKa radiation [1]. As the Co and Ni are the neighbouring atoms elements in the periodic table the low diffraction contrast between them can be enhanced using the X-ray radiation with the energy in the vicinity of the Co or Ni K absorption edges. The anomalous diffraction experiments done in the European Synchrotron Radiation Facility have definitely confirmed the strong interdiffusion in the Co/Ni system because nearly no sattelite peaks characterising the multilayer were observed.
On the contrary to the Co/Ni system the amount of the interdiffusion in the Co/Pt/Ni/Pt system is believed to be lower as the phase diagrams of the Co-Pt and Ni-Pt contain ordered phases which are separated from the solid solution by narrow two-phase regions. To obtain a good fit of diffraction data measured using the CuK a radiation we had to suppose a certain quantity of the interdiffusion different for the Co and Ni atoms in the structural model.
In the CoxNi1-x/Au system we supposed a different interdiffusion process than in the Co/Pt/Ni/Pt system because the Co and Au atoms exhibit complete solid solubility without any ordered phase whereas in the Ni-Au system several ordered phases exist which are separeted with a misciptybility gap. Using the anomalous diffraction and DAFS experiments done at the ESRF in December 1998 we propose to clarify the interdiffusion of Co and Ni in interfaces in ultra-thin materials with respect to different thickness of magnetic layers and to the choice of distinct non-magnetic compounds. The data treatment is in process.
[1] D. Rafaja, J.Vacinova, V. Valvoda: X-ray study of the Co/Ni and Co/Pt/Ni/Pt multilayers, submitted to Thin Solid Films
1 Department of Metal Physics, Faculty of Mathematics and Physics
The aim of the project is to build a common laboratory for the liquid helium temperature X-ray diffraction and magnetic measurements. The principal task of the common laboratory will be the undestanding of the relation between the magnetic properties and the crystallographic structure of industrially important compounds. The great attention is also paied to the education of gradual and postgradual students not only in the above fields but also in the cryo- and vacuum physics.
The first preliminary measurements of binary intermetallics RCo2 (R being a rare earth element) were done to determine the dependence between the magnetic susceptibility and the thermal expansion. The well known compound ErCo2 was first used because of optimization and calibration of the new low temperature experimental set-up.
R. Kuzel, F. Chmelik1, R.K. Islamgaliev2
1) Department of Metal Physics, our faculty
2) Ufa State Aviation Technical University
The evolution of the mean crystallite size and strain with annealing temperature (within the range 20-1000C) is studied for ultrafine - grained copper, germanium, silicon and nickel produced by plastic deformation. In as-prepared samples of germanium and silicon a minor high-pressure tetragonal phases were found which disappeared after annealing above 300C.
Short-annealed Cu samples are investigated at present time. Positron annihilation studies shown remarkable correspondence with X-ray line broadening results.
- R. Kuzel, R.K. Islamgaliev and F. Chmelik, Structural Studies of Submicron-grained Copper, Germanium and Silicon. Mat. Sci. Forum, 278-281(1998) 496-501
- R.K. Islamgaliev, R. Kuzel, E.D. Obraztsova, J. Burianek, F. Chmelik and R.Z. Valiev, TEM, XRD and Raman Scattering of Germanium Processed by Severe Deformation. Mat. Sci. Eng. A249, (1998) 152-157.
The exhibition took place on the occasion of the 18th European Crystallographic Meeting in Prague. After the short historical introduction, the beginning of the X-ray diffraction analysis in Czechoslovakia was presented. The practical examples of the use of X-ray diffraction analysis in medicine, mineralogy and biochemistry were showed.
The exhibition was supported by the Grant PG 98136 of the Ministry of Education.
