Study of vibrational spectra of lanthanum sulphate nonahydrate at different temperatures in the region from 4000–50 cm−1

Abstract

Infrared spectra of lanthanum sulphate nonahydrate are studied over the 4000–50 cm⁻¹ range. The bands pertaining to different internal and external modes of sulphate ion and water molecules are observed and assigned. It is shown that coordinated water molecules exist at two site symmetries C₁ and C_i, which provides confirmation of crystallographic data. Lattice water is not found in these crystals. The effect of low temperature on librational modes of coordinated water and their lattice vibrations are studied in the 550–50 cm⁻¹ region. The potential force field constants are calculated for rocking, wagging and twisting librational modes of water at 290 K, 80 K and 60 K. The higher value of the potential constant for the rocking mode from the wagging librational mode shows the in-plane bend of the hydrogen bond in lanthanum sulphate crystals.