Electron Diffraction: Polycrystalline vs Monocrystal

[M. next]

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.

 

[technician]

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.

 

[M. next]

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.

 

[sophiecentaur]

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.

 

[jtbell]

What will happen if it was a monocrystal?

Google "Laue diffraction".

 

[M. next]

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?