X-ray diffraction (XRD) techniques can be used to analyze the atomic or molecular structure of materials. The technique is compatible with a variety of different forms of solid materials, including crystalline materials or powders. However, variations in the degree or orientation in 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.
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The level of structural information that can be recovered from XRD methods is sufficiently sensitive to small differences in lattice spacings that can be used to distinguish even elementally identical and structurally similar polymorphs.
What is Powder XRD?
Powder XRD is used on microcrystalline powder samples. Powder XRD can be relatively quick in comparison 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.
Ideally, 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 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 the preparation of bulk powders for XRD. It is possible to make measurements either on the loose, uncompacted powders or to 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 the 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 both research and industrial applications.3 This can be used to evaluate material performance but also for aging and dating specimens of historical interest.
What is Single Crystal XRD?
Single crystal XRD differs to powder diffraction not just in terms of the sample preparation but also in terms of the equipment required. Powder samples tend to give rise to diffraction ‘rings’ that are continuous. 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. These can then be transformed into a series of coordinates to recover the underlying lattice dimensions of the sample of interest. The interpretation of single crystal XRD is much less ambiguous than powder diffraction methods but the challenge in these experiments is to be able to prepare the single crystal samples, which can often be a highly laborious and time-consuming process.
Although the structural information from single crystal XRD 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 also not translate well to describing the bulk properties of interest for a given application.4
What is the Future of XRD?
Both powder and single crystal XRD methods have become workhorse techniques for materials and pharmaceutical analysis. 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 lab-based X-ray sources that are available.
One area of development for both powder and single crystal XRD is the application of these methodologies to new sample types. This includes complex materials such as nanocomposites and polymer species as well as 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 the development of 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 also 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.
- 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