Powder X-ray diffraction is an analytical technique that can be used to acquire qualitative data from a solid sample, including the identification of the species contained and characterization of the structure. In addition, the technique can be used to obtain quantitative data, such as crystalline phase concentrations or even data for complete crystal structure solution. The method is related to single crystal X-ray diffraction and grazing incidence X-ray diffraction.

The principles of powder XRD

When radiation of a wavelength similar to the distance between atoms reaches a solid structure, it scatters, producing a multitude of rays that continue to travel through the structure. These rays can interfere with one another either in a destructive or constructive manner – a phenomenon known as diffraction. Diffraction occurs most visibly in structures that are highly ordered, which explains why the method in question highly suitable for analysis of crystalline structures. For constructive interference to occur, the scattered rays must be in the same phase, a condition that can be expressed by Bragg’s law: 2d sin θ = nλ. This law shows the relation between the incidence angle θ (the angle at which the radiation is directed towards the surface), the distance between the lattice planes d and the wavelength of the radiation λ.

In powder XRD, the studied sample is radiated with X-rays produced using an X-ray tube. A specific wavelength is selected using a monochromator, and guided to the sample using collimating lenses. After the diffracted rays have travelled through the sample, they are reflected back through the surface and guided to the detector via another collimating lens-monochromator combination. At the detector, the intensity of the reflected rays is measured. Since the wavelength is easily selectable, this makes it possible to measure the intensity for a specific value of d by simply adjusting the incidence angle θ. By presenting the intensity as a function of θ, a graph known as a diffractogram can be obtained. To make sure that all possible lattice planes are represented, the initial sample is ground intensively into a very fine state, after which it is placed into a mold to produce a sample of suitable size for the measurement.

In addition to intensity data, each lattice plane is labeled using so-called Miller indices that describe its geometry. As every species has a unique diffractogram, it can be used for identification purposes simply by comparing the one obtained for the sample to those found in databases (printed or digital). To make the comparison possible, the acquired data is scaled using the highest obtained intensity count so that the final data shows the relative intensity as a function of θ. An example of an instrument used for powder XRD analysis can be seen in Figure 1.[1][2]

Figure 1. PANalytical X'Pert Pro series instrument at Aalto University (Figure: Dimitri Ojanperä).

Example: Identification of an unknown sample using powder XRD

As an example of a powder XRD measurement, a procedure that was conducted as part of a laboratory work course at Aalto University will be described here. An unknown sample powder was obtained for which the point of interest was the identification of component species and their ratio. The only background information given was that the sample consisted of two species. The initial unknown sample was finely ground, after which it was carefully moved into the mold used for analysis. The measurement was carried out using Cu Kα1 radiation at a λ value of 0.15406 nm for an incident angle range from 5 to 100 degrees. The diffractogram and the results of the database analysis are presented in Figure 2, with the XY-data of the measurement included in the reference section.[3]

Fig. 2 Diffractogram obtained from the measurements. The sample in question was identified to be a mixture of NaCl and Na2SO4 (at a 60/40 ratio) by using the X’Pert HighScore+ analysis program (Figure: Dimitri Ojanperä).

Powder XRD databases

One of the most well-known databases for XRD measurements is the PDF, powder diffraction file, provided by ICDD, the International Centre for Diffraction Data. The files housed in this database include information on the analyzed compound, measurement parameters and the conditions for the measurement (such as the radiation used, the temperature and pressure), among others. Other databases that house XRD data for different compounds are, for example, CSD, ICSD, RRUFF and COD.

ICDD homepage
Inorganic Crystal Structure Database
Cambridge Structural Database 
RRUFF Database
Crystallography Open Database

Do note that some of these require an account registration and/or a fee to use. Most of the databases have a very thorough search function built in, making it possible to be very specific in terms of the structure searched.


1. 1

Skoog, D. A., Holler, J. F., Crounch, S. R., Principles of Instrumental Analysis, 6th Ed., 2007

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3. 1

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