How to index single crystal Bragg peaks

  • #1
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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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Answers and Replies

  • #2
Baluncore
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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?
 
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  • #3
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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.

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?
 
  • #4
Baluncore
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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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