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), K2CuCl2·2H2O (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 calculated in the case of optically isotropic and the simplified spectra of optically uniaxial crystals.

Keywords

• reflection spectra
• anisotropy
• Kramers-Kronig transformation
• phonons
• symmetry resolving
• LO phonons
• TO phonons