Single-Crystal Raman Studies on the Solid Hydrates MX 2 ·H 2 O ( M = Ba,Sr;X = OH,Br,I)

Abstract

Single-crystal Raman spectra of the isomorphic orthorhombic halides MX₂·H₂O (M = Sr, Ba; X = Br, I) and hydroxides M(OH)₂·H₂O (Sr(OH)₂·H₂O and β-Ba(OH)₂·H₂O) are presented. Unambiguous assignment of the wagging, twisting, and rocking librations of the water molecules and the librational modes of the OH⁻ ions is established. On the basis of both the H/D isotopic shifts and the intensities of the Raman bands, separation of the translational modes with respect to pure H₂O (and OH⁻) vibrations and other lattice modes is likewise possible. Unit cell group (and TO/LO) splittings of internal and external modes as well as the sequences of the water bands in terms of both energy and intensity are discussed with regard to the crystal structures, especially the intramolecular and intermolecular symmetry of the water molecules. An improved procedure of suppressing refraction and stray-light effects and recording Raman spectra of crystals as small as 0.01 mm in diameter is given.