How to index single crystal Bragg peaks

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Discussion Overview

The discussion revolves around the methods for indexing Bragg peaks specifically for single crystal diffraction, with a focus on cubic crystals such as NaCl. Participants explore the differences between single crystal and powder diffraction, the challenges in obtaining expected reflection sequences, and the determination of hkl values from diffraction angles.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes difficulty in obtaining the expected sequence of allowed reflections for cubic lattices when indexing single crystal Bragg peaks, contrasting with known results from powder diffraction.
  • Another participant explains that powder diffraction captures many orientations, leading to a broader range of reflections, while single crystal diffraction is limited by the crystal's orientation.
  • There is a suggestion to crush the single crystal to powder for comparison, but one participant indicates that they are not allowed to powder the sample.
  • Participants discuss the potential use of standard powder sample calculations to determine relative peak intensities for the single crystal data.
  • A suggestion is made to experiment with different salt crystals and to consider the orientation of the crystal during XRD to avoid missing spacing combinations.

Areas of Agreement / Disagreement

Participants generally agree on the differences between single crystal and powder diffraction but do not reach a consensus on the best approach to index the Bragg peaks for single crystals or the implications of their findings.

Contextual Notes

Limitations include the dependence on the orientation of the single crystal and the unresolved details regarding the specific X-ray diffractometer used. There are also uncertainties about the applicability of powder techniques to single crystal analysis.

Who May Find This Useful

Researchers and students interested in crystallography, materials science, and X-ray diffraction techniques may find this discussion relevant.

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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