Effect of Particle Size and Shape on the Infrared Absorption of Magnesium Oxide Powders

Magnesium oxide powders obtained from three sources exhibit strikingly different infrared absorption spectra. These differences are correlated herein with variations in the size and shape of the particles in the powders, and are interpreted in terms of both Fröhlich-type (surface) modes and classical electromagnetic scattering theory. (1) Reagent-grade MgO powder exhibits a strong and broad absorption centered at 550 ${\mathrm{cm}}^{-1\mathrm{}}$, the position of which is determined by the Fröhlich-type fundamental frequency for spherical particles that are small compared with the radiation wavelength. The absorption broadening results from variations in particle size and shape. The presence of double particles gives rise to a secondary absorption near 460 ${\mathrm{cm}}^{-1\mathrm{}}$. (2) MgO smoke in various matrices absorbs strongly at the bulk-mode frequency (401 ${\mathrm{cm}}^{-1\mathrm{}}$) owing to the presence of both individual large cubical particles and long chains of small cubical particles. A second strong absorption occurs at 490 ${\mathrm{cm}}^{-1\mathrm{}}$ in Nujol, or at 546 ${\mathrm{cm}}^{-1\mathrm{}}$ on polyethylene plates; these frequencies represent a surface mode of small MgO cubes. A shoulder at still higher frequencies is ascribed to surface modes of rod-shaped particles. (3) MgO powder obtained from thermally decomposed MgC${\mathrm{O}}_3$ absorbs strongly at the bulk-mode frequency (401 ${\mathrm{cm}}^{-1\mathrm{}}$), because of the presence of large flakelike particles having a very fine grain structure. With increasing grain size, the principal absorption shifts to lower frequencies, and a secondary absorption, near 490 ${\mathrm{cm}}^{-1\mathrm{}}$ in Nujol, appears as a result of absorption by small MgO particles.