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In bcc lattice why XRD 100 peak is not observed ?

  1. Sep 19, 2016 #1
    Can someone tell me that in bcc lattice why xrd 100 peak does not appear???
     
  2. jcsd
  3. Sep 21, 2016 #2

    DrDu

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    The planes passing through the base atoms are a/2 away from the planes passing through the planes defined by the atoms on the edge of the cell. And is only a shifted copy of the latter. Hence it will reflect x rays exactly as does one of the edge planes and you get destructive interference.
     
  4. Sep 30, 2016 #3
    If you calculate the structure factor of the BCC lattice using:
    $$S(hkl)=f{1+exp[-i\pi(h+k+l)]}$$, where f= form factor. The value o f S is zero when the exponential has the value -1, which is whenever the argument is: -i*pi* (odd integer).
    So we have:
    S=0 when h+k+l= odd integer
    S=2f when h+k+l= even integer
    So therefore the diffraction pattern does not contain these planes: (100), (300), (111), (221)
    But it occurs for fx.: (200), (110), (222)

    The (100) reflection occurs when reflections from the planes that bound the cubic cell differ in phase by 2*pi. In BCC there is an intervening plane of atoms, which is equal to in scattering power to the other planes. Situated between them it gives a reflection retarded in phase by pi w.r.t. the first plane, thereby cancelling the contribution from that plane. So the cancellation occurs since the planes are identical in composition.
     
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