Days 10 and 11 - Powder Diffraction

 It's the weekend!! Days 10 and 11 of the Industrial Liaison advent calendar will introduce the technique of powder diffraction; particularly useful in the charaterisation of materials

Powder diffraction is an important technique for characterising the crystallographic structure of materials.

The technique allows systems with single or multiple crystalline phases to be studied and can be used to identify the constituent phases within a material, follow phase transitions and perform full crystallographic structural determination of unknown materials.

Powder diffraction is a popular and powerful tool for studying a great variety of systems, from naturally occurring materials and minerals such as calcium carbonate, to novel man made pharmaceutical medicines, and determining the strain within engineering components.

When an X-ray is focused on a crystal, it diffracts in a pattern characteristic of the crystal structure. In powder X-ray diffraction (XRD), the diffraction pattern for the bulk material of a polycrystalline solid is obtained, rather than of a single crystal, which doesn't necessarily represent the overall material. A diffraction pattern plots intensity against the angle of the detector, 2θ.

X-ray diffraction is based on constructive interference of monochromatic X-rays and a crystalline sample. X-rays are generated, filtered to produce monochromatic radiation, collimated to concentrate, and directed toward the sample. When the X-rays reach the sample, the atoms of the sample act just like a diffraction grating, producing bright spots at particular angles. By measuring the angle where these bright spots occur, the spacing of the diffraction grating can be determined by Bragg's law. Because the sample itself is the diffraction grating, this spacing is the atomic spacing.

In powder diffraction, every possible crystalline orientation is represented equally in a powdered sample, and powder X-ray diffraction operates under the assumption that the sample is randomly arranged. Therefore, a statistically significant number of each of the planes of the crystal structure will be in the proper orientation to diffract the X-rays. Therefore, each plane will be represented in the signal. In practice, it is sometimes necessary to rotate the sample orientation to eliminate the effects of texturing and achieve true randomness. These diffracted X-rays are then detected, processed and counted. By scanning the sample through a range of 2θ angles, all possible diffraction directions of the sample can be gathered due to the random orientation of the powdered material. This will produce cones of diffracted rays which intersect a flat photographic plate as circles, known as Debye-Scherrer rings, named after the early powder diffraction camera invented by Debye, Scherrer and Hull.

Materials are made of atoms. Knowledge of how atoms are arranged into crystal structures and microstructures is the foundation on which we build our understanding of the synthesis, structure and properties of materials.

X-ray powder diffraction is one of the primary techniques used to characterise materials, providing structural information even when the crystallite size is too small for single crystal X-ray diffraction methods. It is most widely used for the identification of unknown crystalline materials (e.g. minerals, inorganic compounds). Determination of unknown solids is critical to studies in geology, environmental science, material science, engineering and biology.

Other applications include:

• characterisation of crystalline materials;
   
• identification of fine-grained minerals such as clays and mixed layer clays that are difficult to determine optically;
   
• determination of unit cell dimensions;
   
• measurement of sample purity.

With specialised techniques, X-ray powder diffraction can be also be used to:

• determine crystal structures using Rietveld refinement
   
• determine modal amounts of minerals (quantitative analysis)
   
• characterise thin films samples by: 
  
  - determining lattice mismatch between film and substrate and to infer stress and strain;
  - determining dislocation density and quality of the film by rocking curve measurements;
  - measuring superlattices in multilayered epitaxial structures;
  - determining the thickness, roughness and density of the film using glancing incidence X-ray reflectivity measurements. 

• make textural measurements, such as the orientation of grains, in a polycrystalline sample.

Would you like to know more about X-ray powder diffraction and how you can apply it to your research? Do you perhaps have a structural problem that you are unable to solve in your lab or a material you wish to find out more about? Then please get in touch with the Industrial Liaison Team at Diamond.

The Industrial Liaison team at Diamond is a group of professional, experienced scientists with a diverse range of expertise, dedicated to helping scientists and researchers from industry access the facilities at Diamond. We’re all specialists in different techniques and have a diverse range of backgrounds so we’re able to provide a multi-disciplinary approach to solving your research problems. We offer services ranging from full service; a bespoke experimental design, data collection, data analysis and reporting service through to providing facilities for you to conduct your own experiments.

We’re always happy to discuss any enquiries or talk about ways in which access to Diamond’s facilities may be beneficial to your business so please do give us a call on 01235 778797 or send us an e-mail. You can keep in touch with the latest development by following us on Twitter @DiamondILO orLinkedIn.