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

Abstract

New far-infrared, microwave, and inelastic neutron scattering experiments of the superionic conductor $\alpha$-AgI are presented. As to the optical studies, far-infrared reflection and transmission spectra between 3 and 400 ${\mathrm{cm}}^{-1\mathrm{}}$ 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 ${\mathrm{cm}}^{-1\mathrm{}}$. The drastic increase in the absorption at the transition temperature and its relatively weak temperature dependence in the $\alpha$ 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 $\alpha$-AgI has been evaluated. Besides the residual ray absorption near 110 ${\mathrm{cm}}^{-1\mathrm{}}$ the conductivity is high down to 3 ${\mathrm{cm}}^{-1\mathrm{}}$ with a second very broad resonance near 18 ${\mathrm{cm}}^{-1\mathrm{}}$. For the first time, inelastic neutron scattering experiments on a large single crystal of $\alpha$-AgI have been performed. These measurements proved the lattice-dynamical origin of the absorption between 5 and 30 ${\mathrm{cm}}^{-1\mathrm{}}$. 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.