Question

A metallic crystal has an fcc lattice with a lattice constant equation. Explain whether the following planes are allowed or forbidden for X-ray diffraction:

(100), (111), (210), (220)

Calculate the X-ray diffraction angles for the allowed planes. Assume that diffraction occurs in the first order and the X-ray wavelength is equation. 

19 Jan 2026
Answer :
Word Count : 241
For an fcc lattice, the structure factor determines whether a plane (hkl) is allowed for X-ray diffraction. The condition for allowed reflections is that h, k, l are either all even or all odd. If this is not satisfied, the reflection is forbidden. Given planes: 1. (100) → h=1, k=0, l=0 → mixed (odd, even, even) → forbidden 2. (111) → h=1, k=1, l=1 → all odd
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