Low-Frequency Motions of H2O Molecules in Crystals. III

Abstract

The scattering of low‐energy neutrons (0.005 eV) has been used to study translational and librational motions of H₂O molecules in various configurations in crystals in a frequency region lying below 1000 cm⁻¹. An attempt is made in this paper to determine the effects of various cations and anions on the frequencies of intermolecular motions of H₂O molecules in crystalline salts. The neutron spectra were obtained for a series of hydrates in which the H₂O is hydrogen bonded to the same anion, Cl⁻, but on the sphere of coordination of different cations (Al³⁺, Cr³⁺, Sr²⁺, Ca²⁺, Fe²⁺, Mg²⁺, Co²⁺, Ni²⁺) and for Al salts with different anions (NO₃⁻, SO₄²⁻, Cl⁻). This investigation tends to suggest that the vibrational frequencies of the H₂O molecules are strongly dependent upon the degree and strength of coordination of the molecule as well as upon the degree and strength of the hydrogen bonds formed with neighboring anions or H₂O molecules.