Infrared anisotropy averaging in polycrystalline samples and resonant scattering: the example of YMnO3

The reflectance of ceramic YMnO3 formed by randomly oriented large crystallites (average size about 10 µm) was measured at near normal incidence in the spectral range 100–1200 cm−1. A calibration method was used in order to compensate for the effect of diffuse reflection on the surface of the sample. The spectrum was modelled by averaging the single-crystal reflectivity over all possible crystalline orientations. This anisotropy averaging method was found to adequately describe most of the spectrum, but failed in the regions where one of the principal refractive index became smaller than unity. We argue that these discrepancies are due to enhanced resonant scattering, and discuss the effect with the help of a simplified model of independent spheres based on the Mie theory of light scattering. (paper)