Neutron diffraction studies of the atomic thermal vibrations in complex materials: Application of the Wilson method to examination of micro- and nano-crystalline SiC

The Wilson method was applied for determination of the thermal atomic motions in micro- and nanocrystalline SiC. Limitations of application of this method to examination of complex materials with atoms vibrating with more that one amplitude were discussed. It is shown that a unique interpretation of Wilson plots for crystals with more than one type of atoms and weak vibration component(s) requires measurements performed up to a very large diffraction vector Q (>25 Å^-1). Atomic vibrations in microcrystalline SiC were evaluated based on the diffractograms calculated for models built assuming different mean square atomic displacements (vibration amplitudes) of the component atoms. For nanocrystalline SiC two different temperature atomic factors which describe vibrations of the atoms in the grain interior (B _core) and at its surface (B_shell) were determined.