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 detail. 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 halogenide ions NH4+ <----> I-, ND4+ <----> I- and HH4+ <----> Br-. Despite the fact that the average symmetry of these crystals is cubic, the transitional and orientational disorder locally breaks the cubic symmetry, the infra-red selection rules are relaxed and, hence, a third phonon contribution appears. Deviations of the temperature dependence of the eigenfrequencies, as expected for anharmonic modes, are interpreted in terms of a rotation-translation coupling where the phonon modes interact with the orientational degrees of freedom.