TRANSVERSE OPTIC MODES IN MONOCLINIC ALPHA-BI2O3

Measurements of infrared polarized reflectance spectra of single-cryst al alpha-Bi2O3 have been made at room temperature in the wavelength ra nge from 30 to 4000 cm(-1) for different orientations of the wave vect or k and the electric field E of the incident radiation relative to th e crystallographic axes. On the basis of Born and Huang's theory, rela tions between characteristics of TO modes of the monoclinic lattice an d the reflectivity for the orientations used are established. Dipole m omentum orientations, frequencies, strengths and linewidths of almost all of the IR-active modes predicted by the factor-group analysis for the alpha-Bi2O3 were determined. Good agreement between experimentally measured and model spectra was achieved. It was shown that in order t o obtain reliable values of parameters of B-u modes three spectra for different orientations of E in the ac-plane should be fitted simultane ously. It was found that B-u modes have significantly greater intensit ies than A(u) modes. The effect of the rotation of the principal diele ctric axes within the ac-plane was studied.