Finite Size Effects on Lattice Vibrations of Ionic Crystals and Measurements of Infrared Absorption Spectra

Calculations of absorption and total cross sections of spherical particles based on Englmaii and Ruppin's [1, 2] theory were carried out for different particle size distributions, and compared with the experimental results on NaCl and ZnS. The transmittance curves of powder samples containing spherical particles exhibit, in the case of one optically active mode of lattice vibrations, two absorption maxima whose positions as well as relative height and half-widths depend on the particle size, size distribution, and optical dielectric constant of the surrounding medium. In the limiting case R -> 0, the maximum between omega(TO) and omega(LO) prevails in accordance with Frohlich's formula [3].