FT-IR reflection spectra of single crystals: Resolving phonons of different symmetry without using polarised radiation

Abstract

Fourier-transform infrared (FT-IR) reflection spectra, asquired at nearnormal incidence, were recorded from single crystals belonging to six crystal systems: CsCr(SO4)2?12H2O(alum, cubic),K2CuCl4?2H2O(Mitscherlichite, tetragonal), CaCO3 (calcite, hexagonal),KHSO4 (mercallite, orthorhombic), CaSO4?2H2O(gypsum, monoclinic) and CuSO4?5H2O (chalcantite, triclinic). The acquired IR reflection spectra were further transformed into absorption spectra, employing the Kramers-Kronig transformation. Except for the cubic alums, the spectra strongly depend on the crystal face from which they were recorded; this is a consequence of anisotropy. Phonons of a given symmetry (E-species, in tetragonal/hexagonal and B-species, in monoclinic crystals) may be resolved without using a polariser. The spectrum may be simplified in the case of an orthorhombic crystal, as well. The longitudinal-optical (LO) and transversal-optical (TO) mode frequencies were calculated2 in the case of optically isotropic and the simplified spectra of optically uniaxial crystals.