X-ray diffraction (XRD) is a method used for studying the structure, composition, and physical properties of materials by analyzing their crystal structures. With XRD analysis, it is also possible to identify crystalline materials. X-ray diffraction is commonly applied in materials science, but it can also be used in product development and production process optimization in multiple industries.
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What is X-ray diffraction used for?
X-ray diffraction (XRD) is used to determine the properties of the crystallographic structure (i.e. crystal structure) of crystalline materials. Crystal structure means the order of the particles (atoms, ions, or molecules) in a crystalline matter, where they are arranged into regular arrays of repeating unit cells.
Sodium chloride (table salt) and diamond are examples of crystalline matter, but all solid materials have some kind of crystal structure. The components of this structure are usually crystal planes, i.e. atoms that have settled as planes on top of each other at certain distances. These distances can be measured with XRD.
Diffraction as a phenomenon
XRD is based on a phenomenon called diffraction. In diffraction, a regular array of scatterers produces a regular array of spherical waves of radiation when the radiation hits them and is reflected from them. This is called elastic scattering. In almost all directions, the reflected waves cancel each other out, which is called destructive interference. However, in a few specific directions, the waves add constructively and therefore amplify each other, which is called constructive interference. These specific directions appear as bright spots called reflections on the formed diffraction pattern.
This phenomenon can be represented with Bragg’s law: 2dsinθ = nλ, where d is the distance between diffracting (wave reflecting) planes or objects, θ is the angle of the reflected radiation, n is an integer representing the number of scatterers, and λ is the wavelength of the radiation used. In conclusion, the diffraction patterns result from waves of electromagnetic radiation reflecting from a regular array of scatterers.
Diffraction in XRD
Diffraction can be applied to the atomic level of different materials when X-rays are used as electromagnetic radiation to produce the diffraction pattern. X-rays are a good tool in determining crystal structures because the wavelength (λ) of X-rays is often the same order of magnitude as the distance of the spaces (d) between the crystal planes in the material.
In XRD, the X-rays scatter from the atoms in the crystal structure primarily because they interact with the electrons in the atoms. The diffraction pattern produced by the diffracted X-rays is different for every substance because of the unique order of the atoms or molecules in them.
The intensities and scattering angles of the X-rays diffracted from the material are measured with an X-ray analyzer. The final result of the measurement is a diffractogram, which is a plot having the X-ray intensity on the y-axis and the angle between the incident and the diffracted X-ray beam on the x-axis. When the angles where the constructive interference happens and the reflections occur are measured and the wavelength of the used X-rays (λ) is known, the distances between the crystal planes or atoms in the material (d) can be calculated using the mathematical formula of Bragg’s law.
Information obtained through X-ray diffraction
Various types of information can be obtained using XRD analysis. Because every crystalline matter produces its own kind of diffraction pattern and thus diffractogram, different materials can be identified by comparing the obtained diffractogram with commonly used databases of diffractograms of different materials.
It is also possible to determine the individual components and their relative amounts in a material that consists of several different phases or substances. The lattice parameters of particles in a crystalline structure can be determined with XRD, as it is possible to measure the dimensions, shapes, and geometries of the particles in the material.
Additionally, the crystallite size and strain can be measured with XRD, because the width of the bright spots in the diffraction pattern depends on the crystallite size of the matter and microstrain in the sample. Thus, information about the size and strain of the crystallites can be obtained from the broadening of the peaks in the diffractogram. Single crystals can also be investigated to determine the three-dimensional structure of their molecules with the help of single-crystal diffraction (SCD). This analysis requires a single crystal from the sample which can be produced by varying conditions like solvent or evaporation rate.
X-ray diffraction measurements can be done under ambient or non-ambient conditions. Usually, ambient XRD is used for determining the basic structure of the material. In non-ambient XRD (NA-XRD), the sample is analyzed under different conditions which can be created by adjusting environmental parameters such as temperature, pressure, relative humidity, gas environment, and mechanical load as well as electrical and magnetic fields.
The altering of the parameters in NA-XRD results in changes in the material and its structures and these changes can be examined at the same time when they happen. For example, structural changes during operation, heat treatment, calcination, and sintering, as well as hydration and dehydration processes can be studied with this method. NA-XRD can give very important information about the behavior of the material in different situations.
For diffraction to occur, X-rays have to scatter from a regular array of particles that have a long-range order in the material. This is why the samples used have to be solid crystalline materials with a regular crystal structure. Amorphous materials can also be examined with XRD but only a limited amount of information is obtained from them. It is possible to pretreat the samples before analysis by grinding them into powder. This improves the quality of the measurement by increasing the diffraction intensity of the X-rays.
Suitable sample matrices
- Solid samples
- Crystalline materials
- Active pharmaceutical ingredients (APIs
Ideal uses of XRD analysis
- Material examination, such as analyzing crystal structures or phases and structural changes in variable conditions
- Industrial research and product development in industries like mineralogy, metallurgy, chemical industry, and food industry
- Material identification
- Quality control
- Failure and defect analyses, such as internal stress measurements
- Optimization of production processes
Frequently asked questions
The most commonly used application of XRD is the identification of materials based on the diffraction pattern and diffractogram of the reflected X-rays. XRD is also used for determining the structural properties of the material, including grain size, lattice parameters, strain, preferred orientation of the particles, and phase composition.
Non-ambient XRD (NA-XRD) is widely used in research and product development in various fields of industry, such as mineralogy, metallurgy, ceramics, chemical industry, pharmacological industry, and food industry.
Additionally, thin films and coatings can be analyzed with grazing incidence X-ray diffraction (GIXRD), which is a version of XRD.
Grazing incidence X-ray diffraction (GIXRD) is a modification of XRD having the same operating principle as XRD. With GIXRD, the crystal structure and particle level properties of thin films and coatings can be analyzed by adjusting the incident angle of the X-ray beam hitting the sample relative to the critical angle of the reflected X-rays. The phenomenon works in the same way as in X-ray reflectivity (XRR), with which the structural properties of thin films can be determined.
When small enough incident angles of the X-ray beam are used, a stronger signal from the surface layer of the material is created, in which case the particle-level structure of thin films and coatings can be discovered and the substances in them can be identified. Therefore, GIXRD can be a helpful tool in the product development and quality control of different coatings, for example.
When an unknown material needs to be defined properly, the sample should be homogenous or its composition should be consistent. Identification of unknown sample materials also requires a reference data library.
XRD is best suited for samples consisting of crystalline material. The measurement also works out for amorphous matter, but the amount of information obtained from it remains small. If the sample consists of many different crystalline components, the diffractogram may be too complex to analyze the repeating unit cells or particles of the material. In addition, some samples need to be ground in order to obtain good enough data.
The crystal structure of thin films cannot be determined with the ordinary XRD method. Instead, grazing incidence X-ray diffraction (GIXRD) is needed for this purpose.
In order for the diffraction to occur, the X-rays have to scatter from a regular array of particles that have a long-range order in the material. This is why the sample material has to be solid and preferably crystalline.
With non-ambient XRD (NA-XRD), sample matrices like alloys, building materials, ceramics, refractory materials, polymers, minerals, zeolites, catalysts, drug APIs, pharmaceuticals, and foods can be examined.
Measurlabs offers a variety of laboratory analyses for product developers and quality managers. We perform some of the analyses in our own lab, but mostly we outsource them to carefully selected partner laboratories. This way we can send each sample to the lab that is best suited for the purpose, and offer high-quality analyses with more than a thousand different methods to our clients.
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