MATRIX-ISOLATION TO ORIENTATIONAL GLASS - NH4-METAL HALIDE MIXTURES( IONS IN AMMONIUM ALKALI)

The rotational excitations of NH4+ ions in dilute solutions in alkali metal halides have been investigated at temperatures below 20 K by ine lastic neutron scattering. Results as a function of ammonium concentra tion (c) show that the tunnelling spectra are very sensitive to concen tration. At low values of c, all the salts show sharp tunnelling lines , representing isolated almost free rotors. Increasing values of c lea d to an increase of the effective potential and consequently affects t he tunnelling behaviour. At c congruent-to 0.2 the motions are so hind ered that only motions similar to classical diffusion are observed. Th is is associated with the onset of a dipolar-orientational glass state . Results from a ternary mixed salt suggest that the glass-like proper ties stem from interactions of the NH4+ ions with the strain field, ra ther than NH4+-NH4+ interactions.