X-ray diffraction (XRD) is a versatile, non-destructive technique that reveals detailed information about the chemical composition and crystallographic structure of natural and manufactured materials.
Powder X-ray Diffraction (XRD):
is one of the primary techniques used by mineralogists and solid state chemists to examine the physico-chemical make-up of unknown solids. This data is represented in a collection of single-phase X-ray powder diffraction patterns for the three most intense D values in the form of tables of interplanar spacings (D), relative intensities (I/Io), and mineral name.
The XRD technique:
takes a sample of the material and places a powdered sample in a holder, then the sample is illuminated with x-rays of a fixed wave-length and the intensity of the reflected radiation is recorded using a goniometer. This data is then analyzed for the reflection angle to calculate the inter-atomic spacing (D value in Angstrom units - 10-8 cm). The intensity(I) is measured to discriminate (using I ratios) the various D spacings and the results are to identify possible matches.
X-ray diffraction techniques:
X-ray diffraction finds the geometry or shape of a molecule using X-rays.
X-ray diffraction techniques are based on the elastic scattering of X-rays from structures that have long range order.
The most comprehensive description of scattering from crystals is given by the dynamical theory of diffraction.
- Single-crystal X-ray diffraction is a technique used to solve the complete structure of crystalline materials, ranging from simple inorganic solids to complex macromolecules, such as proteins.
- Powder diffraction (XRD) is a technique used to characterise the crystallographic structure, crystallite size (grain size), and preferred orientation in polycrystalline or powdered solid samples.
- Powder diffraction is commonly used to identify unknown substances, by comparing diffraction data against a database maintained by the International Centre for Diffraction Data.Thin film diffraction and grazing incidence X-ray diffraction may be used to characterize the crystallographic structure and preferred orientation of substrate-anchored thin films.
- High-resolution X-ray diffraction is used to characterize thickness, crystallographic structure, and strain in thin epitaxial films. It employs parallel-beam optics.
- X-ray pole figure analysis enables one to analyze and determine the distribution of crystalline orientations within a crystalline thin-film sample.
- X-ray rocking curve analysis is used to quantify grain size and mosaic spread in crystalline materials.
DIFFRACTION OF LIGHT
(This figure shows that by varying the angle theta the Bragg's Law conditions are satisfied by different d-spacings in polycrystalline materials. Plotting the angular positions and intensities of the resultant diffracted peaks of radiation produces a pattern, which is characteristic of the sample. Where a mixture of different phases is present, the resultant diffractogram is formed by addition of the individual patterns)
Based on the principle of X-ray diffraction, a wealth of structural, physical and chemical information about the material investigated can be obtained. A host of application techniques for various material classes is available, each revealing its own specific details of the sample studied.
By varying the angle theta, the Bragg's Law conditions are satisfied by different d-spacings in polycrystalline materials. Plotting the angular positions and intensities of the resultant diffracted peaks of radiation produces a pattern, which is characteristic of the sample. Where a mixture of different phases is present, the resultant diffractogram is formed by addition of the individual patterns.
- X-ray diffraction methods are especially significant for the analysis of solid materials in the forensicscience.
- They are often the only methods that allow a further differentiation of materialsunder laboratory conditions.
- Smears, minute contact traces, small sample quantities, or tiny sampleareas can be successfully analyzed as well as large quantities of materials.
- Investigations with our GADDS microdiffractometer system with devices for fixed, scanning, oscillating and rotating specimens,.
- The used strategies of measurement, and the applied sample preparation techniqueare discussed at examples from the daily case work.
Something more about x-ray diffraction:
- The Physics Department of the Forensic Science Laboratory the following techniques are at our
- disposal for material analysis: a scanning electron microscope with an energy-dispersive analysis
- system, a classical wavelength-dispersive X-ray fluorescence spectrometer, an energy-dispersive
- micro X-ray fluorescence spectrometer with capillary opticsautomobiles, buildings
- and tools, building materials, minerals, ceramics, asbestos, metals, alloys, explosives, gemstones,
- soils, abrasives, and drug impurities and extenders.
- The decisive advantage of X-ray diffraction methods in forensic science is based on the unique
- character of the diffraction patterns of crystalline substances, the ability to distinguish between
- elements and their oxides, and the possibility to identify chemical compounds, polymorphic
- forms, and mixed crystals by a non-destructive examination.
- This particular quality of X-ray diffraction examinations in the forensic science is verified here in
- the comparison of the diffraction patterns from iron, from iron oxides and from iron oxide hydroxide.
- These substances are used as pigments in paints.
- In comparison with other methods, X-ray diffraction examinations in these cases provide additional
- information about the chemical and physical properties, and make it possible
Applications of X-ray:
To Metallurgical Science:
- The methods of X-ray diffraction have led to great advances in the knowledge of the constituents present in steels, and of the processes occurring during the various heat treatments to which industrial alloys are submitted.
- The information gained in this way is now being supplemented by the methods of electron microscopy and electron diffraction,and the present time is one of great activity.
- In the study of copper alloys the combination of X-ray diffraction methods with those of the older techniques led to the foundation of the first general theory of alloy structures.
- In alloys such as those of copperzinc, copper-aluminium, and copper-tin it was obvious that the copper-rich parts of the equilibrium diagrams were of the same general form although the compositions at which the various phase boundaries occurred were quite different.
- In 1926, it was shown by the present writer that the body-centred cubic p-solid solutions .
- In alloys, although of variable composition, occurred at roughly a ratio of 3 valency electrons to 2 atoms (e.g. CuZn, CusAl, CusSn), and
An eye on the observation:
It was suggested that, in some of these intermediate phases, structures might be characterised by constant electron/atom ratios. This idea was taken up enthusiastically by Westgren and Bradley, and it was established that electron/atom ratios, or electron concentrations, of 3/2, 21/13, and 7/4 were characteristic of structures of the body-centred cubic, y-brass, and close-packed hexagonal types. It was, further, shown that ternary and quaternary alloys with these structures could be obtained, provided that the atoms were mixed together so that the characteristic electron concentration was preserved. Intermediate phases of this type were known as electron compounds, and it was by a combination of X-ray diffraction methods with the older techniques that the principles underlying this type of inter-metallic phase were revealed.
In 193940 the very beautiful X-ray diffraction work of A. Guinier and of G. D. Preston led to the discovery that, in alloys where the ultimate precipitate was the 8 or CuAls phase, the mechanism of precipitation was such that copper atoms first assembled together on (100) planes of the aluminium lattice, with the formation of what are now called Guinier-Preston or G. P. Zones. The next stage is the formation of a metastable 8' phase which, in turn, changes into the final precipitate of CL&~. It is now recognized that in many cases of age-hardening, the process involves the formation of unstable intermediate structures, and that it is these, rather than the stable precipitate, which produce the 198 THE GROWING FIELD greatest hardening effect. Although highly skilled microscopical work might often reveal or suggest the formation of intermediate phases, * it was only by X-ray diffraction that their nature could be established. A wide variety of methods has been used varying from the straightforward identification of precipitating phases by means of their diffraction patterns, to the low-angle scattering techniques, first advanced by Guinier in 1939, in which the size of precipitated' particles is estimated from scattering effects analogous to those by which a halo round the moon is produced from the scattering of light by small crystals of ice, or drops of water.
More about x-ray diffraction(powdered):
- X-ray powder diffraction is a non-destructive technique widely applied for the characterisationof crystalline materials.
- The method has been traditionally used for phase identification,quantitative analysis and the determination of structure imperfections.
- In recent years,applications have been extended to new areas, such as the determination of crystal structures and the extraction of three-dimensional microstructural properties
Line profile parameters and related applications
The observed diffraction line profiles in a powder diffraction pattern are distributions of intensities I(2) defined by several parameters:
- the reflection angle position 20 at the maximum intensity (related to the lattice spacing d of the diffracting hkl plane and the wavelength by Bragg.s law,= 2d sin),
- the dispersion of the distribution, full-width at half-maximum and integral breadth,
- the line shape factor, and
- the integrated intensity (proportional to the square of the structure factor amplitude).
Diffraction Line parameter Applications:
peak position: unit-cell parameter refinement pattern indexing space group determination (20/absent reflections)
anisotropic thermal expansion macrostress: sin2 methodphase identification (d/I) intensity phase abundancereaction kineticscrystal structure analysis (whole pattern)Rietveld refinement (whole pattern)search/match, phase identificationpreferred orientation, texture analysis
shape instrumental resolution function microstructure: line profile microstructure (crystallite size, size distribution, latter distortion, structure mistakes, dislocations, composition gradient), crystallite growth kinetics three-dimensional microstructure (whole pattern)
Non-ambient and dynamic diffraction: in situ diffraction under external constraints reaction kinetics
Some more applications:
- One technique which has proven to useful in a variety of single crystal inspection applications is x-ray diffraction topography.
- X-ray diffraction topography is the name given to several x-ray diffraction techniques which permit the imaging of strains and lattice isorientations associated with surface and internal defects as small as dislocations to be examined.
- The term topography literally means "to describe a place" (topos = place, graphein = to write) and the diffraction information which is obtained by x-ray topographic methods may be bulk (transmission) or surface (back-reflection) in nature.
- the topographic techniques are based on Bragg diffraction from a periodic crystal, the images are extremely sensitive to crystal imperfections, strains, and rotations since any alteration to the interplanar position and spacing of the crystal will effect a corresponding change in the Bragg condition.
- X-ray diffraction topography can easily detect and image the presence of defects within a crystal, making it a powerful nondestructive evaluation tool for characterizing industrially important single crystal specimens such as nickel based alloy turbine blades, quartz resonators and silicon carbide substrates.
- Whether looking for gross defects (crystallographic misorientations) or smaller defects such as dislocations or even atomic (point defects) scale influences, the x-ray diffraction topographic techniques are well suited to a variety of applications.
- x-ray topography was utilized to determine whether a turbine blade grown to be a single crystal specimen was in fact a single crystal, to distinguish between "good" and "bad" quartz resonators, and to evaluate the crystalline perfection of silicon carbide substrates for subsequent thin film deposition.
- Use of x-ray diffractiones did help to resolve issues related to the film deposition process, specifically, alternate substrates were selected.ism observed in the bad sample.topography in industry, would aid in the optimization of crystal growth and processing of these and other industrially important single crystal applications