Collective Excitations in Microcrystal Aggregates and Composites

The optical properties of aggregates of microcrystals are dominated by surface excitations. The Raman and ir absorption spectra of polar microcrystals show, however, peaks at the bulk TO‐phonon frequencies, too, the occurrence of which depends on the sample preparation, i.e. on the aggregate microstructure. A recently developed theory for the dielectric function of composites of different topologies is applied to the calculation of the phonon spectra. It is shown that this theory explains the occurrence of bulk TO‐phonon peaks in appropriate microstructures.