Glass transition in K1−x(NH4)xI mixed crystals

Abstract X-ray diffraction studies were performed on K 1− x (NH 4 ) x I mixed crystals for the range of ammonium ion concentration 0.23⩽ x ⩽ 0.68. Systematically we observed the anomalous expansion of lattice constants and the appearance of diffuse scattering around the Brillouin-zone boundary of an FCC lattice (X point) below the glass transition temperature T g . Neutron incoherent inelastic scattering experiments were also performed to elucidate the freezing of NH 4 + ion motion. Experimental results suggest that a low-frequency lattice vibration of iodine atoms which couples with reorientational freezing of NH 4 + ion sis relevant for the glass transition.