Vibrational Entropy and Local Structures of Solids

There is now widespread evidence that vibrational entropy must be included in an understanding of solid state phase transitions. While the basic principles are known, the individual phenomena that contribute to differences in vibrational entropy of solid phases require much more investigation. We do not expect all of the vibrational modes in a solid to have the same dependence on local atomic structure. In ordered and disordered transition metal aluminides, for example, there is a change in the high energy optical modes upon ordering that can account for most of the difference in vibrational entropy. From other studies performed so far, it seems that vibrational entropy is sensitive to the packing of atoms of different size. Although much of the vibrational entropy depends on harmonic effects that can be calculated readily with the phonon DOS, anharmonic effects also make important contributions to the entropies of solid phases. Phonon DOS measurements at different temperatures will provide information about these anharmonic contributions, which have been shown to be important for the hP24 phase of Co₃V. The hope is that there will emerge, at least for specific classes of materials, systematic trends showing how differences in vibrational entropy depend on the local atomic structure in a material