Electron Diffraction: Polycrystalline vs Monocrystal

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Discussion Overview

The discussion revolves around the differences in electron diffraction patterns produced by polycrystalline and monocrystalline materials, specifically focusing on the implications of crystal structure on observed diffraction patterns. The conversation includes theoretical considerations and conceptual clarifications related to the arrangement of crystal planes and their effects on diffraction outcomes.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes that a polycrystalline structure, such as graphite, produces two intense rings in electron diffraction due to multiple crystal orientations.
  • Another participant suggests that a monocrystal would yield a pattern of bright spots, which could appear as rings if visualized under certain conditions.
  • A question is raised about whether multiple rings would be observed with polycrystalline materials and if a monocrystal would only produce one ring, seeking clarification on the relationship between crystal plane arrangements and diffraction patterns.
  • One participant explains that the rings result from maxima at the same angle to the incident beam across different crystal orientations, drawing an analogy to the formation of a rainbow.
  • A suggestion is made to research "Laue diffraction" for further understanding of monocrystalline diffraction patterns.
  • A participant inquires if each ring corresponds to a different family of planes, indicating a desire for clarification on how different orientations contribute to the observed diffraction pattern.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the relationship between crystal structure and diffraction patterns, with some points of agreement on the general behavior of polycrystalline versus monocrystalline materials, but no consensus is reached on specific details or implications.

Contextual Notes

The discussion includes assumptions about the behavior of crystal structures under electron diffraction that may not be fully explored, such as the specific conditions under which patterns are formed and the limitations of the analogies used.

M. next
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In this experiment, the graphite is a polycrystalline structure. That's is why we observe two intense rings. What will happen if it was a monocrystal? And why?

Thank you.
 
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a monocrystal would give a pattern of bright spots. Imagine those spots drawn on a piece of paper then rotated quickly...you would see rings... a polycrystalline material is lots of monocrystals at different angles.
 
So if they were at different angles, I would see many rings? And if it were a monocrystal I would only see one ring? If so, why? I don't get the relation between arrangement of families of planes with rings.
 
The rings are there because the maxima from each crystal are at the same angle to the incident beam, whichever way the crystals are orientated. This is the same argument that applies to the formation of a rainbow - same angle gives the appearance of a circle.
This argument is a bit over-simplified because the crystalites can be in any orientation but, as the fringes on a flat screen are equally spaced near the axis, the different orders will still coincide to give rings - but only over a limited range of angles. Off axis, the fringes will not reinforce and the pattern will degrade.
 
M. next said:
What will happen if it was a monocrystal?

Google "Laue diffraction".
 
So one ring is formed by one family of planes? And another from a different oriented set of planes and so on? Is this the case?
 

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