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Materials characterization (SEM, TEM, XRD, XPS)

In powder XRD of a metal with the FCC structure, what are the indices of the diffraction peak that appears at the lowest angle?

  1. A(100)
  2. B(110)
  3. C(111)
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Answer: C. (111)

In FCC, diffraction appears only when h, k and l are all even or all odd; when even and odd are mixed, the scattering from the four atoms in the unit cell cancels and the intensity is zero (the extinction rules). Peaks therefore appear in the order (111), (200), (220), (311)…, and (111), with the smallest h² + k² + l², has the largest interplanar spacing and the lowest angle. (100) and (110) both mix even and odd indices and so do not appear in FCC. In BCC only planes with h + k + l even appear, so the first peak is (110), and the sequence of peaks distinguishes FCC from BCC.

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