Far-infrared, microwave, and inelastic neutron scattering experiments of the superionic conductorα-AgI

New far-infrared, microwave, and inelastic neutron scattering experiments of the superionic conductor α-AgI are presented. As to the optical studies, far-infrared reflection and transmission spectra between 3 and 400 cm^-1 in the temperature range between 20 and 440 ^∘{}C have been measured. The far-infrared transmission data have been checked independently by microwave techniques in the frequency range between 2.5 and 8 cm^-1. The drastic increase in the absorption at the transition temperature and its relatively weak temperature dependence in the α phase is due to the almost complete disordering of the silver ions at the phase transition. Based on these experimental data the frequency-dependent conductivity of α-AgI has been evaluated. Besides the residual ray absorption near 110 cm^-1 the conductivity is high down to 3 cm^-1 with a second very broad resonance near 18 cm^-1. For the first time, inelastic neutron scattering experiments on a large single crystal of α-AgI have been performed. These measurements proved the lattice-dynamical origin of the absorption between 5 and 30 cm^-1. An extremely large linewidth for the transverse-acoustic phonon modes is observed. It is concluded that it is mainly the disorder of the silver ions which is responsible for this broadening.