Molecular Dynamics Study of Polarization Effects on AgI

Abstract

Three different models of AgI are studied by molecular dynamics simulations. The first one is the rigid ion model (RIM) with the effective pair potential of the Vashishta and Rahman form and the parametrization proposed by Shimojo and Kobayashi. The other two are polarizable ion models in which the induced polarization effects have been added to the RIM effective pair potential. In one of them (PIM1), only the anions are assumed to be polarizable by the local electric field. In the other one (PIM2s), the silver polarization is also included, and a short-range overlap-induced polarization opposes the electrically induced dipole moments. This short-range polarization is proved to be necessary to avoid overpolarization when both species are assumed to be polarizable. The three models reproduce the superionic character of alpha-AgI at 573 K and the liquid behavior of molten AgI at 923 K. The averaged spatial distribution of the cations in the alpha-phase obtained for PIM1 appears to be in better agreement with experimental data analysis. The PIM1 also reproduces the structure factor prepeak at about 1 A(-1) observed from neutron diffraction data of molten AgI. The three models retain in the liquid phase the superionic character of alpha-AgI, as the mobility of the cations is significantly larger than that for the anions. The ionic conductivity for the polarizable ion models is in better agreement with experimental data for alpha-AgI and molten AgI.

Ionic Conductivities of Molten CuI and AgI-CuI Mixtures

“…It is difficult to explain why the E a for molten CuI is larger than that for AgI by considering the difference in the ionic size and mass between Ag and Cu, because an Ag ion is larger and heavier than a Cu ion, which seems to be a disadvantage for the ionic diffusion. Furthermore, the valence charges of Ag and Cu ions are almost the same [12,13]. However, the concentration dependence of E a for (AgI) 1-x (CuI) x mixtures observed in this study would not be the error, because the similar behavior has also been reported for the solid solution of AgI-CuI system [5].…”

“…Furthermore, weak covalencies of cation-cation pairs have also been reported [14,15], and finally the resulting first correlation lengths of cationcation and cation-I correlations are different for molten AgI and CuI [16]. Moreover, it has been reported that the electric field generated from the dipole-moment of anions affects the liquid structure and cationic diffusion in molten Ag and Cu halides [12,13,[17][18][19]. These complex properties of Ag and Cu halides compared to typical molten salts might be associated with the activation energy in AgI-CuI system.…”

Including many-body effects in models for ionic liquids

“…36-38 They were also used by Trullas and co-worker to study a series of silver and copper halides, [39][40][41][42][43] and the PIM model now is implemented in the CP2K code. 44 Going back to the case of oxides, when attention is restricted to a single phase or where similar materials are being compared it is often sufficient to neglect the full complexity of the AIM model, and we have also successfully used some PIM-type potentials in the case of amorphous GeO 2 45,46 and of doped zirconia crystals.…”

Structure and Ionic Diffusion in Molten NaI, RbI, and NaI-RbI mixture

“…Although we used the rigid ion model in this study, if we consider a polarizable ion model, the cation diffusion would become large. However, polarization effects on NaI, RbI, and their mixture would be weak compared to silver (or cupper) halides [13][14][15][16][17][18][19][20] and zinc halides [21,22]. Ciccotti et al have reported that D Na and D I for pure NaI at 1081K are 9.4 × 10 -5 and 6.8 × 10 -5 cm 2 /s, respectively [3].…”