Some previous work on the effect of aggregation on the infrared absorption spectrum of small-dielectric-sphere powders is extended in order to take into account various cluster sizes and shapes. A model recently introduced is critically discussed. Trends are given for the spectral bounds, the largest absorption strengths, and the possible structure in an infrared powder spectrum, as a function of the number of crystallites. Application of our results to NiO powder spectrum is briefly made. According to our calculation, the upper half of the spectrum can be completely covered by modes belonging to the considered clusters. The assumption that crystallites are spheres is discussed. One conclusion is that it is necessary to go beyond the approximation of identical ionic crystal spheres if a powder spectrum has to be interpreted.

• Received 14 March 1977

DOI:https://doi.org/10.1103/PhysRevB.18.7176