X-ray diffraction (XRD) is a common x-ray technique for scanning and measuring compounds and its main uses are in scientific industries.
The many industries that use XRD include:
Engineering
Materials science
Environmental science
Forensic science
Pharmaceuticals
Microelectronics
Geology
Biology
Glass production.
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XRD is a major analysis technique used at all stages of drug development. It is used in trace analysis in forensics. XRD characterises crystal substrates such as silicon for use in integrated circuits in microelectronics. In glass manufacturing, XRD supports process control, identifying tiny crystal that can lead to manufacturing faults.
But given these wide applications across different industries, what are the advantages and disadvantages of XRD as a measuring technique?
How XRD Works
As light-forms, x-rays include measurable wavelengths in nanometres.
If you direct monochromatic x-rays onto substances with crystalline structures, there will be levels of interference when the x-rays then scatter, which cause patterns of varying intensity.
This process is known as constructive interference, and it is the basis of a technique for studying crystal structures and atomic spacing.
The atomic planes of crystals and crystalline structures cause this interference to diffracted x-ray beams. The distances between these planes are known as d-spacings and each mineral has its own unique set.
Therefore, if you collect the diffracted x-rays, you can analyse the structure of a sample.
D-spacings have standard reference patterns, which you can refer to as comparisons to identify the structure of a sample substance.
The way the XRD method reveals the atomic structure of crystals is based on Bragg’s law.
Under Bragg’s law, for x-ray diffraction to occur, the x-rays and the sample surface must interact in a certain way:
The angle of intersection between the beam and surface is equal to the angle of scattering, and
The pathlength difference is equal to an integer number of wavelengths.
The instrument for carrying out this technique is an x-ray diffractometer.
The x-ray diffractometer emits x-rays from a cathode ray or rotating target, directing them at the crystalline sample.
The sample and x-ray detector in the x-ray diffractometer both rotate. When this movement satisfies the conditions for Bragg’s law, constructive interference occurs.
The x-ray diffractometer includes a detector, which records and processes the x-ray signals, converting them for output to a computer.
Each crystalline structure has a unique set of d-spacings, and these are like individual fingerprints. There are standard d-spacing reference patterns you can compare your readings to, enabling you to identify the sample substance’s structure.
The Advantages of XRD
You can use XRD to determine the orientation of the individual grains of a crystal and to identify crystal structures in unknown substances.
This technique will measure the internal stress, the size and shape of small crystalline areas, and measure the average spacings between the layers of rows of atoms in samples. It can determine their minerology.
XRD will produce clear, unambiguous results, and as a technique it is both powerful and rapid.
Preparation only involves minimal sample quantities. These you grind into a fine powder, with an optimum size that is less than 10μm (micrometres).
Interpreting the results of XRD is relatively straightforward. The x-ray diffractometer continuously records data during the process, and presents peak positions and x-ray counts in a table.
In the table, the Bragg equation calculates the d-spacing of each peak. Once you have all the d-spacings for the sample, you can compare these readings to d-spacings of known materials, to identify the sample.
X-ray diffractometers are widely available testing instruments, both as benchtop and portable models.
Disadvantages of XRD
XRD is a specialist technique and has limitations in what you can apply it to.
If you are using it to identify a substance that is not known to you, then the sample must be single phase and homogeneous.
The sample must also be very small, in quantities of tenths of a gram. Proper preparation of samples is crucial.
If the sample is a non-isometric crystalline structure, then indexing its patterns can be complex. Where there are high-angle reflections, peak overlay can occur and worsen.
Despite requiring small sample sizes, XRD is much more accurate measuring large crystalline structures rather than small ones. Small structures present only in trace elements will often go undetected. If the sample consists of mixed materials, the detection limit is 2% of the sample.
