SYNCHROTRON RADIATION FOR MAterials Science AND Powder Diffraction

Milan Dopita^1,2, Radomír Kužel¹

¹Department of Condensed Matter Physics, Faculty of Mathematics and Physics,

Charles University, Ke Karlovu 5, 121 16 Praha 2, Czech Republic

²Institute of Materials Science,Technical University Bergakademie Freiberg, Gustav Zeuner Str. 5, D-09599 Freiberg, Germany

There are many different applications of synchrotron radiation in materials science. Actually, they use nearly all well-known remarkable properties of the radiation: hih intensity, high brilliance, tunability, low divergence etc. Different kind of samples can be studied - powders, thin films, bulk samples, small particles, small areas of bulk samples. In particular, non-ambient conditions and time resolved experiments are of interest.

Main applications

High-resolution powder diffraction

One o f the most frequently used experiments of powder diffraction with synchrotron radiation is high-resolution powder diffraction with a resolution of about an order better than in conventional diffraction and at very reasonable intensity and necessary time. High-resolution powder diffraction beamlines can be found at important synchrotrons all over the world. The use of high-resolution experiments is of particular interest in the several problems. Phase analysis of complex mixtures when unique determination of neither quantitative nor qualitative determination of phase composition is difficult. The information content of the diffration pattern in terms of interplanar spacings (peak positions) and intensities is closely related to the resolution. If this is low, the phase analysis may be impossible in the principle, in particular cases. Structure refinement or structure determination especially for low-symmetry phases or structures with large unit cell which is for example the case of biological material or zeolites. High-resolution setup reduces significantly overlapping of peaks the effect which complicates any minimalization procedures used for structure determination. Since peaks are narrower, peak positions can be estimated more accurately, peak overlapping diminishes and a more complete intensity data set can be extracted. Combination of several diffraction patterns collected at different conditions can be of great use. In textured samples, different patterns at different sample orientations can be collected and the pole function for a few well separated reflections calculated. With this information the orientation distribution function for each reflection can be found, so that a set of linear equations can be established, in which the integrated intensities of the texture-free sample are the unknowns. For both phase analysis and structure determination anomalous scattering can be very useful. This can be easily generated for selected elements by choice of the wavelength from the broad spectrum. Line profile analysis can offer information on mean crystallite size, crystallite size distribution, microstrain and/or dislocation densities or even types. Practical limits of conventional analysis for crystallite size and dislocation densities are up to 200-300 nm and down to about 1014m-2, respectively. High-resolution setup can shift significantly the limits by reducing the instrumental broadening that can often be even neglected and moreover high intensity reduces the noice and makes possible investigation of fine microstructural details (distribution of crystallite size, arrangement and correlation of structural defects).

Pair distribution function

Study of pair distribution function (PDF) has been revitalized in last few years. Originally, this was usually of interest for amorphous materials but it has been shown that it is the only method for structural characterization of nanomaterials, study of local order in materials showing average disorder, and even very helpful for structure refinement. In order to determine the function it is necessary to use high energy radiation to be able to perform measurement to large Q vectors, and position sensitive detectors (like 2D detectors) for data with good statistics at high Q values.

Microdiffration

Diffraction on extremely small diffracting volumes, small set of crystals or even single microcrystals are of interest in several different scientific branches. Nominally, in pharmaceutical research, in organic chemistry, in research on the field of forensic science and art, in investigation of the microporous materials etc. The spatial resolved x-ray diffraction experiments are moreover important in investigation of the materials with function gradients and generally for the diffraction investigation of inhomogeneous samples. The X-ray microdiffraction (X-ray rocking curve imaging) with spatial resolution on µm scale or even below can be used for analysis and quantification of spatial distributions of crystal lattice misorientations, for determination of defects densities and for detection of local lattice quality in crystalline specimens. 3D XRD grain mapping, called also 3D X-ray microscopy can be nowadays performed at ESRF (ID11) or at two beamlines in Argonne national Laboratory. Shapes, orientations and even growth of individual grains with temperature can be studied.

Analysis of orientation dependence of microstructure (stresses, textures)

Studies of orientation dependence of microstructures are of great importance in materials science, for example for mechanically treated (deformed materials), thin films etc. Measurement requires complete mapping of diffraction peak positions (determination of the residual stresses), intensities (description of the preferred orientation of crystallites - texture) or even line width (microstrain determination and estimation and study of the microstructural defects types and distribution). This can be performed much more efficiently with synchrotron radiation and area position sensitive detector. Another approach to the problem is to replace global analysis by the local one which means 3D grain-by-grain (spatially resolved) mapping of the material. Such a procedure can yield unique information namely in an inhomogeneous materials, functionally graded samples, or in materials which are somehow in-homogeneously mechanically, chemically or thermally treated. Shapes, orientation and even growth of individual grains can be studied. At present, the experiments can be performed at ESRF (ID11) or in Argonne National Laboratory (two beamlines). However, demands on the beatime are very high now.

Analysis of thin films

Investigation of thin layers or low dimensional structures incorporates whole variety of problems which are often not possible to overcome using classical X-ray equipment. In addition to the classical problems as is for example the low diffracted intensity, the investigation of the thin films requires other specific features. First of all, this is necessity on reduction or setting of the analyzed depth. This is usually done by setting of the appropriate low angle of incidence of the primary beam in the glancing angle X-ray diffraction (GAXRD). High intensity of synchrotron radiation in combination with excellent energy resolution makes that technique convenient for investigation of the nanocrystalline coatings and observation of the partial coherence effects in thin layers and nanocrystalline materials. Another experimental techniques suitable and often used for the coatings, epitaxial layers, multilayers and semiconductor thin films investigation, are for example X-ray reflectometry (XRR) giving the information on electron density and surface roughness and also information on the number of layers, periods, interface roughness in the case of multilayers, or the grazing incidence diffraction (GID) which can provide information on sample structure in the lateral direction. Using of highly coherent synchrotron radiation in combination with its high intensity and possibility of the wavelength tunability greatly exceeds the potential of conventional laboratory X-ray diffraction equipment and makes the use of the synchrotron radiation necessary for the detailed investigation of thin layers/multilayers.

Measurements under non-ambient conditions

Probably one of the highest importances of synchrotron radiation, thanks to its high intensity and tunable wavelength, is the efficient possibility to perform all experiments under non-ambient conditions – low and high temperatures, high pressures and applying of external (electromagnetic) fields. In particular, the pressure is a powerful thermodynamic variable that allows the direct control of the interatomic distances and in combination with thermal treatment it can be used for novel supra-hard material synthesis or the structural characteristics obtained from temperature / pressure measurements can be used for the equation of state determination. Low temperature measurements can yield unique information on magnetic properties and generally on the electron structure of solids. Fast phase transitions, chemical reactions in solid, liquid or solid-gas state can be fully structurally characterized only when using synchrotron radiation. Moreover, the scattering of synchrotron radiation can be easily combined with in-situ spectroscopic studies like Raman or infrared. In-situ sample preparation like thin film deposition or hard material synthesis would also be of interest.