Interactive Tutorial about Diffraction
Interactive example: Phase angles II

Phase problem
Shift in real space
Centrosymmetric vs. Acentric

Interactive examples
Single atom
Pair of atoms
One atom in unit cell
Two atoms in unit cell


This time you can create a pair of atoms on the X-axis. You are free to choose identical or different atoms at any position on the X-axis. The positions of the atoms/electrons are given in A. You can also select whether to calculate X-ray or neutron intensities. The calculated range for the scattering intensity h is fixed to -2.0Å-1 to 2.0Å-1 parallel to the x-axis.

Start exploring and try to answer the questions below:

  • What is the effect of the positions /type on the intensity, the real part, the imaginary part and the phase angle of the diffraction pattern ?
  • What is the difference between a centrosymmetric structure and a non-centrosymmetric ?
  • Simulate two identical atoms at identical distance left and right from the origin. Now shift both atoms by the same amount. This pair of atoms is still centrosymmetric, with the center of symmetry in the middle of the atoms. Why is the phase angle no longer that of a centrosymmetric structure ?
  • Try to work out the relationship between the magnitude of the interatomic distance and the phase angle ?
  • Why does the phase angle change periodically, although the other features of the diffraction pattern do not ?
Once you are done, click here to verify your answers.
© Th. Proffen and R.B. Neder, 2003