FTIR analysis of well-defined α-Fe₂O₃ particles

The FTIR spectra of well-defined hematite (α-Fe₂O₃) particles varying in size, shape and internal structure have systematically been analyzed. It has been found that the absorption peak frequencies of the pseudocubic particles are located between the corresponding theoretical peak frequencies of spheres and cubes and shift to lower frequencies as particle size increases, corresponding approximately with the Mie theory. However, the basic spectrum pattern was determined by the morphology of the particles. The observation of ellipsoids with various aspect ratios revealed some essential discrepancies in the peak frequency between experiment and the theory on the surface phonon modes in small particles, even if the particles were completely dispersed in the medium. The internal structure mainly affects the peak width, which increases with the decreasing subcrystal size. Finally, the effects of aggregation on the IR spectrum are discussed.