Polarized IR spectra of synthetic smoky quartz

Abstract

Polarized IR spectra of planeparallel (0001) plates of synthetic smoky quartz, with E rotating around [0001], show that the absorption figures of OH⁻ related absorption bands at 3380 (room temperature), 3365 and 3305 cm⁻¹ (liquid nitrogen temperature, -196° C) are strongly anisotropic and violate the trigonal symmetry of low quartz. This effect is correlated with a non-uniform substitution of Si by Al on the three symmetrically equivalent Si sites, as revealed by EPR measurements. Random distribution of Al over the three Si sites, obtained by dry annealing of the samples in air, yields isotropic absorption figures in the (0001) plates. It is thus experimentally evident that the absorption bands at 3380, 3365 and 3305cm⁻¹ are caused by the OH⁻ stretching vibrations coupled with Al substituting for Si. For each experimentally determined integral absorption coefficient of the three absorption bands a theoretical absorption coefficient was calculated, based on the symmetry of low quartz and the given Al distribution. This was done for various orientations of the OH⁻ dipoles with respect to the a axes of low quartz. By comparing the experimentally determined and calculated absorption coefficients, the orientation of the corresponding OH⁻ dipoles with respect to the a axes could be determined.