Vibrational properies of nanostructured materials
The structures of nanocrystalline materials are unique because a significant fraction of the total volume of the material comprises of grain boundaries. The question raises how this structural particularity changes the vibrational behaviour with respect to a crystalline solid. Nuclear methods like Mößbauer spectroscopy show anomalous decreasing Debye-Waller factors, which correspond to an increasing mean square displacement of the Mößbauer isotope. Furthermore nanocrystalline materials are known to exhibit an increased specific heat.
Inelastic incoherent neutron scattering determines the overall vibrational spectrum and enables us to compute the one-phonon density of states (DOS). By means of the DOS, thermodynamical properties like specific heat can be calculated and compared to the respective measurements.
Generally, the vibrational spectrum of small particles is expected to differ from the spectrum of massive bulk material. The change of lattice forces at the surface should cause a frequency softening. Additionally the finite size of the crystals should prohibit the propagation of phonons which have a wave length longer than twice the crystal diameter.
