The 390 cm−1 feature of polycrystalline hematite—An optical crystallite size effect

The 390 cm−1 spectral feature of polycrystalline hematite is shown to be caused by a crystallite size effect which is caused by the anisotropic nature of single crystalline hematite. This effect occurs if the probing light is able to resolve the heterogeneous nature of a polycrystalline material and if the corresponding single crystalline material is optically anisotropic. Therefore it does not depend on whether the material of interest is a powder or consolidated. As a consequence, the resulting macroscopic reflectance and transmittance of the polycrystalline material is an average of the microscopic reflectance and transmittance of the individual crystallites. Therefore, randomly oriented polycrystalline materials with large crystallites show characteristic changes in their spectral profile compared to the spectral profiles of polycrystalline materials consisting of crystallites that are small in comparison with the wavelength. The extent of the spectral changes depends on the degree of optical anisotropy of the corresponding single crystalline material. The spectral changes also comprise non-zero cross-polarization terms despite of random orientation. Therefore a characterization of a polycrystalline material with a scalar dielectric function is possible in general only if the material consists of randomly oriented crystallites small compared to the wavelength.