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Topic: X-ray crystallography  (Read 3890 times)

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memma

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X-ray crystallography
« on: October 03, 2006, 05:01:22 AM »
Hello,

I am just wondering whether if someone can check the folloriwing paragraph that I have written on X-ray crystallography and  Debye-waller factor; in fact, I am not sure whether if the content is right. If not, I would be grateful if someone could correct it. Thank you!

The intensity of scattering is reduced due to thermal vibrations of the atoms and crystal disorder. In fact, the Debye-Waller factor represents the decrease of intensity in diffraction due to static disorder of the crystal and disorder caused by thermal vibrations. According to this factor, the observed intensity I is proportional to exp[-2B(sin?/?)2] and thus:


                                                I = I0  exp[-2B(sin?/?)2] 


where I0 is the intensity observed for an ideally ordered crystal.
This loss of intensity increases as the angle increases and as the resolution 1/d increases (d decreases). In fact, from Bragg’s law, sin?/? is 1/d and therefore the previous equation can be rewritten as:

                                                I = I0 exp[-2B(1/d)2] 

The quantity B is called the temperature factor and it represents the breadth of smearing: in fact, if the atom has a root-mean-square displacement u, then B = 8?2u2.

Offline AWK

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Re: X-ray crystallography
« Reply #1 on: October 03, 2006, 06:48:16 AM »
Bragg law is
2d sin ? = n?

henc sin?/? = 1/d only for n=2
AWK

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