Lattice vibrations in the mixed crystals (NH4I)(0.3)(KI)(0.7), (ND4I)(0.3)(KI)(0.7) and (NH4Br)(0.3)(KBr)(0.7)

Transverse optical phonons have been studied in (NH4I)(0.3)(KI)(0.7), (ND4I )(0.3)(KI)(0.7) and (NH4Br)(0.3)(KBr)(0.7) using Fourier-transform infrared techniques. In all materials three phonon modes have been observed in the far-infrared regime and were followed as a function of temperature in detai l. While the first low-frequency mode characterizes a sublattice motion of the alkali ions against the halogenides K+ <----> I- and K+ <----> Br-, the second mode involves vibrations of the ammonium molecules against the halo genide ions NH4+ <----> I-, ND4+ <----> I- and HH4+ <----> Br-. Despite the fact that the average symmetry of these crystals is cubic, the transitiona l and orientational disorder locally breaks the cubic symmetry, the infra-r ed selection rules are relaxed and, hence, a third phonon contribution appe ars. Deviations of the temperature dependence of the eigenfrequencies, as e xpected for anharmonic modes, are interpreted in terms of a rotation-transl ation coupling where the phonon modes interact with the orientational degre es of freedom.