Polarized i.r. reflection, absorption and laser raman studies on a single crystal of BaSO4

This paper reports a detailed study of the K = 0 vibrational spectra of the orthorhombic crystal BaSO4. Polarized i.r. reflection spectra have been recorded with the electric vector parallel to the principal crystal directions at 300 K and 100 K. The dielectric parameters, ε′ and ε″, have been calculated from this data for these principal directions. The laser Raman spectra have been obtained for different crystal orientations and the scattering due to the several elements of the Raman polarizability tensor isolated. The i.r. and Raman spectra have been interpreted using both the factor group and site group analyses. Some corresponding measurements have been made on SrSO4, PbSO4 and CaSO4 in an attempt to elucidate the nature of the external vibrational modes. The results of some calculations on the correlation field splitting of the sulphate ion internal vibrations are discussed.