Crystal-optic reflectance data from the literature, reflectance measurements at near-normal incidence of pressed powders and from faces of crystals between 1500 and 200 cm-1 are compared. Reststrahlen bands of more than twenty inorganic substances, ranging from sodium nitrate to quartz and magnesium oxide, were scrutinized. Differences among the reflectances of various pellets pressed from ground crystals, microcrystals (size below 20 µm), and smokes (smaller than 1 µm) of the same substance were mostly negligible. Factor of 3 deviations from nominal pressure (11 kbar) also were not significant. Measured powder reflectances agree with averaged crystal-optic reflectances if the low density of powders of harder substances is taken into account (as was partially proved by increasing the pressure to 110 kbar). Mixed powders often showed serious weakening of some nonoverlapping component bands; overlapping bands may become enhanced or weakened. This severely limits the usefulness of the reflection method to obtain information on the composition of mixtures. Band shifts, when they occur, are small. Analogies to two-mode behavior of mixed crystals in lattice-disorder studies may exist. The results cannot be explained by any of the effective medium mixing rules or absorption index averaging of classical oscillator theory.
Frederic E. Volz, "Infrared specular reflectance of pressed crystal powders and mixtures," Appl. Opt. 22, 1842-1855 (1983)