ROTATIONAL EXCITATIONS IN MIXED AMMONIUM POTASSIUM HALIDES - A NEUTRON-SCATTERING STUDY

The rotational dynamics of NH4+ ions in dilute solutions in potassium halides (I, Br, Cl) have been investigated at temperatures below 20 K by inelastic neutron scattering. Tunnel splitting in the (NH4)xK1-xCl mixed crystal is reported for the first time. An almost free uniaxial quantum rotation of the NH4 is observed in all the halides. An increase in NH4 concentration in all mixed salts produces a collapse of the free-rotor line into a broad intense feature to lower energies. The onset of this collapse occurs at different NH4 concentrations x in the different halides, with x(Cl) < x(Br) < x(I). The tunnel splittings of the different halides exhibit a systematic relationship to the nearest-neighbour distance of the respective matrices. The remarkable similarity to the methane-rare-gas systems is discussed.