X-rays have a wavelength that is similar in magnitude to the spacing between the atoms in most inorganic materials. This means that under certain circumstances X-ray beams can be diffracted by crystal lattices. An examination of the X-ray patterns produced will yield information regarding the arrangement of atoms within the lattice. X ray diffraction is used for a variety of purposes including:
- Identification of compounds and phases
- Determination of crystal structures
- Determination of lattice parameters
- Measurements of crystallite size
- Measurement of degree of crystallinity
- Determination of residual stress
- Quantitative analysis of preparations of phases
- Orientation of single crystals
- Texture analysis of bulk material or surface layers
A wide range of X ray diffraction techniques is available within the Department of Materials for the investigation of polycrystalline materials, single crystal and thin films. Samples may be examined in either bulk or powdered form.
The facility is currently equipped with 2 PANalytical MRDs, 2 PANalytical MPDs and a Bruker D2 desk-top instrument for rapid data collection.
Since an X-ray diffraction pattern is determined by the atomic arrangement within a specimen, any material will produce a diffraction pattern that is characteristic of its constituent compounds or phases. Identification is aided by search and match software which compares experimental data with standard patterns from the ICDD reference database.
Glancing angle X-ray diffraction
This technique greatly enhances the analysis of thin films by reducing the interference from the sample substrate and increasing the absorption path of the incident beam within the layer itself. By varying the glancing angle it is possible to undertake depth profiling of surface layers.
Preferred orientation of the crystallites within a polycrystalline material can have a profound effect upon its physical and mechanical properties. By monitoring the variation of intensity of the diffracted X-ray beam from a specific set of lattice planes, whilst the specimen is orientated, it is possible to determine both the direction and the magnitude of this preferred orientation.
Residual stress in a specimen will cause very small changes to its inter-planar spacings. The magnitude of these changes will depend upon the degree of stress and the orientation of the crystallites with respect to the stress. By determining the inter-planar spacings for crystallites of different orientations it is possible to calculate the residual stress from the measured strain in the lattice. This technique offers the advantage of determining residual stress in a specimen without the need to measure the specimen in an unstressed state.
The standard X-ray diffraction configuration requires the specimen to have a flat surface for examination. By utilising the de-focussed arrangement of parallel optics it is possible to examine curved or uneven surfaces.
High temperature X-ray diffraction
XRD measurements can be performed at elevated temperatures up to 1000°C using a combination of direct and indirect heating. This allows the investigation of the thermal behaviour of lattice parameters, crystallisation studies, and the detection and characterisation of high temperature phases. The high temperature chamber is fitted with a system to allow measurements to be made in controlled atmospheres (including oxidative) so that structural changes related to sample-gas interactions can be studied.