7 Random thermal motion

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Here "laue" represents the thermal motion of the atoms by displacing the atoms from their lattice sites by random amounts with rms amplitude sigma thermal. Preset 7 uses a sigma thermal of 0.3 to make the displacements evident in the electron density display. Switch sigma thermal to 0.03 and calculate to get a more interesting diffraction pattern. The thermal motion of the atoms in a crystal may be represented as the superposition of periodic distortions (the phonons) of very small amplitude. The implication for the diffraction is a collection of many very weak phonon satellites on each of the Bragg peaks. Zoom in to blow up the structure near the delta-k axis. The superposition of "phonon sidebands" leads to an incoherent mish-mash called thermal diffuse scattering.

A second important feature of the pattern is seen by returning the diffraction graph to full scale and expanding just the delta k axis near one Bragg peak. Despite the thermal disorder there is NO broadening of the Bragg peaks!

There is, however, loss of intensity (the Debye-Waller effect) from the peaks. Vary the magnitude of sigma thermal and note that it is the high order peaks that get lost the fastest.