X-ray diffraction, or XRD, is a technique for analysing the atomic or molecular structure of materials.
It is non-destructive, and works most effectively with materials that are wholly, or part, crystalline.
The technique is often known as x-ray powder diffraction because the material being analysed typically is a finely ground down to a uniform state.
Diffraction is when light bends slightly as it passes around the edge of an object or encounters an obstacle or aperture.
The degree to which it occurs depends on the relative size of a wavelength compared to the dimensions of the obstacle or aperture it encounters.
X-rays are a form of electromagnetic radiation include wavelengths measurable in nanometres (a nanometre is equivalent to one billionth of a metre).
When monochromatic x-rays scatter from a substance with a structure on this scale, it causes interferences.
This results in a pattern of lower and higher intensities due to constructive and destructive interferences according to Braggs law.
With crystalline substances, the pattern creates three-dimensional shavings of diffraction in response to x-ray wavelengths, like the spacing of planes in a crystal lattice.
This process is known as constructive interference and is used as a technique for studying crystal structures and atomic spacing.
All diffraction methods start with the emission of x-rays from a cathode tube or rotating target, which is then focused at a sample.
By collecting the diffracted x-rays, you can analyse the sample’s structure.
This is possible because each mineral has its unique set of d-spacings.
D-spacings are the distances between planes of atoms, which cause diffraction peaks.
There are standard reference patterns of d-spacings, which act as a comparison when using XRD to identify the structure of a sample substance.
The way that x-rays reveal the atomic structure of crystals is based on Bragg’s law.
Diffraction will only occur if the way the x-rays and substance interact meets the conditions of Bragg’s law.
This requires that:
This allows for a condition of maximum intensity, which then enables a calculation about the details of the crystal structure concerned.
X-ray diffraction can do the following:
The basic difference between x-ray diffraction (XRD) and x-ray fluorescence is what each method can determine:
You carry out x-ray diffraction with a X-ray diffractometer.
This instrument consists of three main elements:
The cathode tube generates x-rays through applying heat to a filament.
This produces electrons, which are then directed towards a target by applying a voltage.
These bombard the target material, dislodging inner shell electrons within, which then produces x-rays.
The x-rays are then directed onto the sample.
The source and x-ray detector in the instrument both rotate.
When this geometric movement satisfies the conditions for Bragg’s law for the sample being analysed, constructive interference occurs, causing a peak in intensity.
The detector records and processes this signal, converting it into a count rate for output to a computer. Typical represented intensity vs theta.
The three things you need for preparing a sample for x-ray diffraction are:
First, take a few tenths of a gram of the sample substance.
Grind it to a fine powder.
Typically, this should be done in a fluid to minimise any extra surface energy, which might otherwise randomise the sample.
The optimum size of powder is less than 10μm (micrometres).
Next, place this ground powder onto a sample surface, or within a holder.
It is important to create a flattened upper sample surface, with a random distribution of lattice orientations.
The x-ray diffractometer continuously records data during the process when both the source and detector rotate.
Peak intensity occurs when the d-spacings in the lattice planes of the mineral sample are appropriate to the value of the diffract x-rays.
Results are presented as peak positions and x-ray counts in a table.
The Bragg equation calculates the d-spacing of each peak.
Once you know all the d-spacings in the sample, you can compare these to the d-spacings of known materials for identification purposes typically commercially available databases.
Known d-spacings for reference are available from several sources, including the International Centre for Diffraction Data (ICDD).
The main use of XRD is the identification of unknown crystalline materials.
These can be inorganic compounds or minerals, for example.
This use of XRD is essential in:
You can apply XRD for defining thin film samples, by using the following techniques:
Various industries, sectors and disciplines use XRD as a valuable tool for measurement and analysis:
In the pharmaceutical industry, XRD can characterise and clearly define the composition of materials.
It enables chemists and scientists to clearly assess the parameters associated with the crystal structures of pharmaceuticals.
Where there are formulations involving multiple components, XRD can measure the actual percentages in samples.
It is a key method for analysis at all stages of drug development.
In forensic science, the main use of XRD is in trace analysis to detect very small amounts of substances.
These substances can include: loose powdered materials; hair; glass fragments; paint flakes; stains.
The microelectronics industry uses single crystal substrates such as silicon and gallium arsenide to produce integrated circuits.
XRD characterises these substances for use, identifying any defects within a crystal.
In glass production, XRD can identify tiny crystals, which can cause faults in bulk glass manufacturing.
It also enables manufacturers to measure crystalline coatings for use in texture.
In geological applications, XRD is an integral mineral exploration tool, to the extent that it has helped to revolutionise the geological sciences.
XRD enables the rapid identification of minerals in rock or soil samples, and it can determine the proportion of these minerals in each sample.
The main advantages of x-ray diffraction are:
XRD does, however, have certain limitations:
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