Ammonium ions incorporated into alkali halide lattices. Inelastic neutron scattering studies

Abstract

In this paper we report the results of a study, by inelastic neutron scattering (I.N.S.), of the librational and localized translational motion of the ammonium, NH⁺₄, ion, incorporated at low concentrations into the alkali halides. Results are reported for all the alkali halides except the lithium and sodium salts, and the fluorides. For the caesium halides the values of the librational frequency found are in good agreement with those determined from the combination band, ν₆+ν₄, by infrared spectroscopy. In the case of the (NH₄), MX systems with the sodium chloride structure the values have not previously been determined [except for (NH₄), KCl] and are sufficiently high to indicate that at least one strong hydrogen bond is formed. We have used the data to calculate barriers to rotation and hence a lower limit for the hydrogen bond strengths in the salts.