Two pure phonon theories have been used for the interpretation of the infrared absorption spectra of powdered solids: the theory of absorption and scattering of light by small particles (TAS) and the theory of the average dielectric constant (TADC). The comparison between both theories shows that they are equivalent for particle sizes smaller than 2 μm and low particle concentration. However, when the particle concentration is significant, only TADC can be used to analyze the experimental spectra, thus obtaining a qualitative estimation of particle aggregation in a powder. Matrix effects were also observed in the experimental powder spectra, which can be accounted for by TADC.
J. E. Iglesias, M. Ocaña, and C. J. Serna, "Aggregation and Matrix Effects on the Infrared Spectrum of Microcrystalline Powders," Appl. Spectrosc. 44, 418-426 (1990)