Dielectric Function of Superionic Material AgI in the Molten and alpha Phase

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(1) First we have studied the dynamical structure factors of AgI in the alpha(superionic) and molten phases. The charge-density correlation function S_<CC>(kappa,omega) has only a broad peak at about 15 ps^<-1>. These things have been understood from the partial dynamical structure factors S_<II>(kappa,omega) S_<AgAg>(kappa,omega). The sharp peak of the LO mode in S_<II>(kappa,omega) in the alpha-phase becomes extremely broad in the molten phase. The behavior of S_<II>(kappa,omega) in the molten phase is similar to that of S_<AgAg>(kappa,omega). The peaks of the LO mode in S_<II>(kappa,omega) become broad and then the LO mode does not appear clearly in the molten phase.(2) Next we have calculated the static dielectric function epsilon(kappa) of AgI using S_<CC>(kappa). The epsilon(kappa) touched the abscissa axis at some values of kappa as the melting temperature was approached. This shows the tendency of ions in the molten phase to order similarly to the solid phase. In the alpha-phase, the values of epsilon(kappa) became zero at special wave numbers which lead to the Debye peaks. Very similar behavior was found in a classical one-component plasma, and a hydrogen plasma. The study of epsilon(kappa) gave us information about ordering of ions in a regular lattice system and a sign of the forerunning phenomena to an ordering from a liquid state.(3) We have presented an ionic plasma model to describe the low-energy excitation mode in cation superionic conductors. When taking account of an effective charge of cations, the model is supported by experiments.