How to index single crystal Bragg peaks

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SUMMARY

This discussion focuses on indexing single crystal Bragg peaks, specifically for cubic crystals of NaCl using X-ray diffraction (XRD) with Cu Kα radiation (wavelength = 154 pm). The user encountered discrepancies in the expected sequence of allowed reflections, indicating a misunderstanding of the indexing process for single crystals versus powders. The conversation emphasizes the importance of orientation in single crystal XRD and suggests experimental approaches to validate results, including rotating the crystal and comparing it to powder diffraction patterns.

PREREQUISITES
  • Understanding of Bragg's Law and its application in X-ray diffraction.
  • Familiarity with the concept of Miller indices (hkl) and their significance in crystallography.
  • Knowledge of X-ray diffraction techniques, particularly for single crystals versus powders.
  • Experience with X-ray diffractometers and their operational principles.
NEXT STEPS
  • Learn about the indexing process for single crystal XRD, focusing on cubic lattices.
  • Research methods for determining Miller indices from diffraction angles.
  • Explore the differences in diffraction patterns between single crystals and powdered samples.
  • Investigate techniques for manipulating single crystals to simulate powder-like behavior in XRD analysis.
USEFUL FOR

Crystallographers, materials scientists, and researchers involved in X-ray diffraction analysis, particularly those working with single crystals and seeking to understand indexing methods and peak identification.

Hello890
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I am able to find methods on how to index Bragg peaks for powder diffraction, and was wondering if the method is the same for single cubic crystals of NaCl? I have tried and cannot get the expected sequence of allowed reflections for cubic lattices (h^2 + k^2 + l^2= 3, 4, 8, 11, 12, 16, 19, ...). Instead I am getting the sequence as 3, 13, 32.

The experiment was done simply by observing the XRD pattern from a single cubic crystal of NaCl, collected using Cu kalpha (wavelength = 154pm).

Also, how do I determine the hkl values from the angle of diffraction? ie: which planes will lead to constructive interference?
 
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Powder will give you many orientations so all possible internal reflections will appear.
Depending on it's orientation, a single crystal will provide a greatly restricted number of peaks.

XRD a single crystal, then crush it to powder and repeat the XRD. Is there a difference?
 
Last edited:
Baluncore said:
Powder will give you many orientations so all possible internal reflections will appear.
Depending on it's orientation, a single crystal will provide a greatly restricted number of peaks.

XRF a single crystal, then crush it to powder and repeat the XRF. Is there a difference?
Many thanks for your reply. Unfortunately I am not allowed to powder the sample. Also, could I use standard techniques for powder sample calcuations to calculate the relative intensities of the peaks for this single crystal?
 
I would suggest that you experiment; obtain another salt crystal from the kitchen, then obtain data from that oriented crystal before crushing it to powder and repeating the process.

I do not know the make and model of your X-Ray diffractometer, or how it is arranged. If you rotate the single cubic crystal while it is axially aligned with the rotation, then the pattern will be of a 2D, not a 3D structure. Many spacing combinations may be missing.

To make a single crystal look like a powder you would need to tumble it on two axes. Depending on the instrument, that may be possible.
 

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