Determination of the orientation of the crystallographic axes in anisometric particles by infrared Spectroscopy

The applicability of IR spectroscopy for the determination of the orientation of the crystallographic axes in uniform anisometric particles is discussed for the case of monocrystalline hematite particles. Two samples consisting of uniform particles of similar size (similar to300 nm in length) and shape (axial ratio similar to5) obtained by different preparation methods have been used in this study. It is shown that in spite of the similar composition, crystalline structure, and morphological characteristics of the samples, their IR spectra present important variations in the position and relative intensity of the absorption bands due to the different orientations of the crystallographic axes. Such behavior can be accounted for by the effective medium theory using the optical constants of hematite. Therefore, from the analysis of the IR spectrum of a powder, it is possible to identify the orientation of the crystallographic axes of the constituent anisometric particles. These results have been corroborated by electron diffraction carried out on individual particles.