Infrared reflectance spectra of some optically biaxial crystals: on the origin of isosbestic-like points in the polarized reflectance spectra

An investigation of the IR polarized reflectance spectra of the orthorhombic K2SO4, KHSO4 and monoclinic CaSO4·2H2O, K2Ni(SO4)2·6H2O, (NH4)2Ni(SO4)2·6H2O was performed in order to explain the appearance of particular intersection points in their spectra. The analogy in the origin of the appearance of intersection points and the well-known isosbestic points in UV–vis and IR spectra was discussed. The reason for such an appearance was identified in the way that the individual reflectances (for radiation polarized along principal dielectric axes) sum up to give the reflectance under arbitrary polarization. This summation may also produce supplementary bands in the reflectance spectra, not predicted by the group theory. It was shown that relatively large LO–TO splitting is needed for the supplementary band(s) to appear, but also the overlapping region must be taken into account.