X-ray diffraction, or XRD analysis, is a method used to study the atomic or molecular structure of materials. It is non-destructive and operates most efficiently with materials that are fully or partially crystalline.
The technique is also referred to as x-ray powder diffraction because the substance being examined is usually finely ground to a uniform state. Diffraction happens as the light bends slightly when it moves around the edge of an object or meets an obstruction or aperture.
The degree to which it happens depends on the relative size of the wavelength compared to the size of the barrier or the opening it meets.
How does XRD work?
X-rays are a type of electromagnetic radiation, including wavelengths that can be measured in nanometres (a nanometre is equivalent to one billionth of a meter). As monochromatic x-rays are dispersed from a material with a structure on this scale, they cause interference.
This results in a trend of lower and higher severity due to positive and disruptive interference under Bragg’s law. With crystalline substances, the pattern produces three-dimensional diffraction shavings in response to x-ray wavelengths, such as plane spacing in a crystal lattice.
This method is known as constructive interference and is used as a tool for the study of crystal structures and atomic spacing.
Both diffraction methods begin with the emission of x-rays from the cathode tube or rotating target, which is then concentrated in the sample. By collecting the diffracted x-rays, you can analyze the sample’s structure. This is possible because each mineral has its own special set of d-spaces.
D-spaces are distances between atom planes that cause diffraction peaks. There are common reference patterns for d-spacing, which serve as a comparison when XRD is used to classify the structure of the sample substance. The way x-rays show the atomic structure of crystals is based on the law of Bragg.
Applications of XRD
X-ray diffraction powder is most commonly used for the detection of unknown crystalline materials (e.g. minerals, inorganic compounds). The determination of unknown solids is important to studies in geology, environmental sciences, material sciences, engineering, and biology.
Other implementations shall include:
- Characterization of crystalline materials
- Identification of fine-grained minerals such as clays and mixed-layer clays that are difficult to assess optically
- Determination of the cell dimensions of the device
- Measurement of the purity of samples
Using advanced methods, XRD can be used to:
- Determine crystal structures using Rietveld refining
- Determine the modal quantity of minerals (quantitative analysis)
- Characterize thin film samples by:
- Determining the lattice mismatch between film and substrate and inferring stress and strain
- Determining the dislocation density and consistency of the film by calculating the rocking curve
- Measurement of superlattices in multilayer epitaxial structures
- Determining the thickness, roughness, and density of the film using X-ray reflectivity gauge measurements
- Make textural measurements, such as grain orientation, in a polycrystalline sample.
Advantages and Limitations:
- The strong and rapid technique (< 20 min) for the detection of unknown minerals
- In most cases, it allows for an unequivocal determination of minerals
- Minimum sample preparation is needed
- XRD units are commonly available for use
- The analysis of data is relatively straight forward
- Homogeneous and single-phase material is best used to classify the unknown.
- Must have access to a regular inorganic compound reference file (d-spacings, hkls)
- Needs tenths of a gram of material to be ground into a powder.
- For mixed materials, the detection limit is ~ 2% of the sample.
- For unit cell determinations, indexing patterns for non-isometric crystal systems is difficult.
- Peak overlay can occur and worsen due to high angle ‘reflections’
Several industries and disciplines use XRD analysis as a valuable instrument for calculation and analysis:
1. Pharmaceuticals: In the pharmaceutical industry, XRD will clearly describe and define the structure of the products. It enables chemists and scientists to determine precisely the parameters of the crystal structures of pharmaceuticals.
Where multiple component formulations are available, XRD can calculate the actual percentages in the samples. It is a primary method of research at all levels of drug production.
2. Technology of Forensics: In forensic science, the primary application of XRD is to identify very tiny quantities of substances in trace analysis. These substances may include loose powdered materials, hair, bits of glass, flakes of paint, or stains.
3. Microelectronics: The microelectronics industry uses single-crystal substrates such as silicon and gallium arsenide to manufacture integrated circuits. XRD characterizes these compounds for use, recognizing any flaws within the crystal.
4. Glass: In the production of glass, XRD may detect tiny crystals that can cause defects in the production of bulk glass. It also allows manufacturers to calculate crystalline coatings for use in textures.
5. Geological knowledge: In geological applications, XRD is an integral method for mineral exploration to the degree that it has helped revolutionize geological sciences.