Structural and dynamical properties of silver chalcogenides are reviewed. Since Rahlfs and Strock proposed that the four Ag ions in a unit cell of αAg2S are distributed with equal probability over the 42 crystallographic sites of 6(b), 12(d) and 24(h), many measurements have been performed. Recent precise measurements of neutron scattering, EXAFS, and X-ray diffraction have suggested that Ag ions in αAgI type superionic conductors are distributed over 12(d) sites with large asymmetric anharmonic thermal vibrations. The transport and electrochemical properties of the electronic and ionic conductivities and the self-diffusion coefficient and mobility of mobile ions have been investigated also extensively. These measurements have shown a remarkable deviation from the Einstein relation in Ag-chalcogenides. To interprete this behavior, Yokota presented a new diffusion theory, caterpillar mechanism. To understand the liquid-solid duality of an unusual state of materials in which some atoms have nearly liquid like properties while other atoms retain their regular crystalline arrangement, a continuum model has been presented. It is supposed that the lattice composed of anions is immersed in the cation liquid. The structure factors with which electrons are connected are expressed symmetrically in terms of the structure factors of ions in the long wavelength limit using the fluctuation-dissipation theorem and the Kramers-Kronig relation. The calculated conductivity satisfies the f{hook}-sum rule and its ionic part has a broad peak at the optical-phonon frequency, which is the similar frequency dependence to that of αAgI. The frequency- and wave vector-dependent conductivity and the dispersion relations of collective modes are calculated. In particular the plasmon-LO phonon coupling is investigated in detail. A molecular dynamics method is applied for the further study of Ag-diffusion in αAg2Te for several temperatures. The density distribution of Ag ions suggests that a Ag ion locates at tetrahedral site for the most of time and then moves to its neighboring tetrahedral site through the vicinity of the octahedral site. The activation energy for an ionic diffusion also is obtained from the Arrhenius plotting of the self-diffusion coefficient of Ag ions. Its value of 0.17 eV is obtained, which is in good agreement with the experiment. © 1990.
