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I've been struggling with what is probably a basic question about Bragg peaks, diffraction, and I guess geometric structure factors.

Basically, as I understand it, Bragg peaks depend on the symmetry of the system. We get them from constructive interference due to different planes of unit cells in the crystal. What I don't understand is why planes of atoms contained in those unit cells, which may have different (greater?) symmetry than the unit cells, cannot also give rise to Bragg peaks. In other words, why can't a sublattice be responsible for its own Bragg peaks?

I hope that makes sense, thanks.

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  • $\begingroup$ I have no idea what the question is asking to begin with, so i envy you just beacuse you even know what this stuff is lol $\endgroup$ Apr 23, 2017 at 23:51

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You can get additional peaks, or even the absence of peaks depending on the unit cell choice. There are two things at part here: the arbitrary choice of non-primitive unit cells, and the structure factors of Bragg peaks.

Take a simple cubic lattice with one atom per unit cell. Nothing is stopping you from choosing a body-centered cubic unit cell, even though the primitive unit cell is a single atom. However, when you actually look at the diffraction patterns, you will see additional reflections that wouldn't appear for a normal body centered unit cell. These peaks arise from non zero atomic periodicities within your unit cell.

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  • $\begingroup$ Thank you for your answer, but I'm still confused. I'm suggesting a case where the primitive unit cell itself features atoms on a sublattice with symmetry different than the symmetry of the primitive unit cell. Perhaps my question doesn't make sense. But I would have thought that you should get diffraction patterns from both, and then the concept of Bragg peaks being zone centers confuses me. $\endgroup$
    – RonManatee
    Apr 24, 2017 at 2:29

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