X-ray diffraction (XRD) techniques can be used to analyze the atomic or molecular structure of materials. The technique is compatible with various solid materials, including crystalline materials or powders. However, variations in the degree or orientation of different substrate types mean that the XRD methodology needs to be adapted to look at these different classes of substrates to recover the desired structural information. This article will examine the differences between powder x-ray diffraction and single crystal x-ray diffraction, as well as the future of each and their applications.
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The structural information recovered from XRD methods is sufficiently sensitive to small differences in lattice spacings that can be used to distinguish elementally identical and structurally similar polymorphs.
What is Powder XRD?
Powder x-ray diffraction is used on microcrystalline powder samples. Powder XRD can be relatively quick compared to single crystal XRD due to the significantly reduced difficulty in the sample preparation step. It can be very challenging to grow high-quality single crystals of sufficient size to perform single crystal XRD measurements for many materials but powder XRD can be performed on much smaller crystal sizes.
When preparing a sample for powder XRD, the particle size should be as homogenous as possible and less than ~ 10 μm in diameter. All the microcrystals will be randomly oriented but too small particle sizes or too much variation in the particle sizes in a sample can lead to the broadening of the peak structures and potentially complicate the structural assignments.
Techniques such as ball-milling or manual grinding with a mortar and pestle can be suitable for preparing bulk powders for XRD. It is possible to make measurements on the loose, uncompacted powders or compact the powder into pellets to try and achieve higher signal-to-noise ratios.
One issue with powder XRD is that while the sample preparation is relatively straightforward, it is demanding in terms of the amounts of sample required for a measurement.
Powder XRD is popular in pharmaceutical analysis due to the short sample preparation times and the amount of information that can be recovered on the crystalline phases of the substrate.2 Powder-based methods are also widely used in mineralogy for research and industrial applications.3 This can be used to evaluate material performance and for aging and dating specimens of historical interest.
What is Single Crystal XRD?
Single crystal x-ray diffraction differs from powder diffraction in sample preparation and equipment required. Powder samples tend to give rise to continuous diffraction ‘rings’. This can result in ambiguities in the data interpretation and the need for trialing different fittings to the data to interpret the final structures.
In single crystal XRD, single, discrete diffraction peaks are observed. Using a crystal x-ray diffractometer, peak positions can be transformed into a series of coordinates to recover the underlying crystal lattice dimensions or crystal orientations of the sample of interest. The interpretation of single crystal diffraction is much less ambiguous than powder diffraction methods but the challenge in these experiments is to prepare the single crystal samples, which can often be a highly laborious and time-consuming process.
Although the crystal structure information is more straightforward to interpret, the spatial properties of a single crystal may not necessarily be reflective of the bulk solid. This can also mean that measurements of stress and strain on the crystal may not translate well to describing the bulk properties of interest for a given application.4
What is the Future of XRD?
Powder and single crystal XRD methods have become workhorse techniques for materials and pharmaceutical data collection. While many measurements still benefit from the energy tunability and photon doses available at advanced light source infrastructures such as synchrotrons, there has been a great proliferation in the number of available lab-based X-ray sources.
One area of development for powder and single crystal XRD is the application of these methodologies to new sample types. This includes complex materials such as nanocomposites, polymer species, and thin film.5
There are also developments to try and make XRD measurements under more extreme conditions, such as the high temperatures and pressures that materials may experience when being used in each application. This includes developing sample delivery systems that mimic ‘in operando’ conditions or high temperatures and pressures to explore how the material behavior changes as a function of these additional variables.6
Multiplex X-ray measurements, where XRD measurements are performed alongside other experiments, such as X-ray fluorescence or absorption, are becoming increasingly common, where a comprehensive characterization of both elemental composition and structural arrangements can be performed in a single experiment.
References and Further Reading
- Bunaciu, A. A., Udriştioiu, E., Aboul-enein, H. Y., Bunaciu, A. A., Udriştioiu, E., Aboul-enein, H. Y., Bunaciu, A. A., & S, E. G. U. (2015). X-Ray Diffraction : Instrumentation and Applications X-Ray Diffraction : Instrumentation and Applications. Critical Reviews in Analytical Chemistry, 45, 289–299. https://doi.org/10.1080/10408347.2014.949616
- Thakral, N. K., Zanon, R. L., Kelly, R. C., & Thakral, S. (2018). Applications of Powder X-Ray Diffraction in Small Molecule Pharmaceuticals: Achievements and Aspirations. Journal of Pharmaceutical Sciences, 107(12), 2969–2982. https://doi.org/10.1016/j.xphs.2018.08.010
- Bish, D. L., & Plötze, M. (2010). X-ray powder diffraction with emphasis on qualitative and quantitative analysis in industrial mineralogy. Advances in the characterization of industrial minerals. https://doi.org/10.1180/EMU-notes.9.3
- Chatterjee, S. K. (2010). X-ray diffraction: Its theory and applications. PHI Learning Pvt. Ltd.
- Nagaraj, S. K., Shivanna, S., & Subramani, N. K. (2016). Revisiting Powder X-ray Diffraction Technique: A Powerful Tool to Characterize Polymers and their Composite Films. Research & Reviews: Journal of Material Science, 4(4), 1–5. https://doi.org/10.4172/2321-6212.1000158
- Hirao, N., Kawaguchi, S. I., Hirose, K., Shimizu, K., Ohtani, E., & Ohishi, Y. (2020). New developments in high-pressure X-ray diffraction beamline for diamond anvil cell at SPring-8 New developments in high-pressure X-ray diffraction beamline for diamond anvil cell at SPring-8. Matter and Radiation at Extreme Conditions, 5, 018403. https://doi.org/10.1063/1.5126038
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