Low-Frequency Motions of H2O Molecules in Crystals

Abstract

A number of selected samples containing water of crystallization or adsorbed water have been systematically examined by neutron inelastic scattering using the Brookhaven slow chopper to study translational and librational motions of the H2O molecules in a region lying below 800 cm—1. To gain an understanding of these motions in various configurations, spectra were measured for polycrystalline samples of natrolite (isolated H2O molecules in a ``cavity''); chabazite (waterlike H2O molecules); analcime (two H2O molecules per ``cavity''); apophyllite (distorted H2O molecules); copper sulfate pentahydrate (H2O molecules forming hydrogen bonds of various lengths with the lattice); and beryl (adsorbed monomeric H2O molecules). Despite the large variety of lattice configurations and bonding forces, all these compounds, with the exception of beryl, show peaks corresponding to torsional motions of the H2O molecules in a narrow frequency region (480 to 600 cm—1). Beryl, however, shows only a broad spectrum centered approximately at kT, as expected for weakly bound H2O molecules. Besides indicating the frequencies of torsional motions and the degree of rotational freedom of the H2O molecules, these measurements provide information concerning: distribution of water molecules among various lattice sites, influence of the distortion of the H2O molecules on the observed frequencies, and the effects of its environment on the translational modes of the H2O molecules.