Molecular Dynamics Study of Polarizable Ion Models for Molten AgBr

Abstract

Three different polarizable ion models for molten AgBr have been studied by molecular dynamics simulations. The three models are based on a rigid ion model (RIM) with a pair potential of the type proposed by Vashishta and Rahman for α-AgI, to which the induced dipole polarization of the ions is added. In the first (PIM1) the dipole moments are only induced by the local electric field, while in the other two (PIM1s and PIM2s) a short-range overlap induced polarization opposes the electrically induced dipole moments. In the PIM1 and the PIM1s only the anions are assumed polarizable, while in the PIM2s both species are polarizable. Long molecular dynamics simulations show that the PIM2s is an unphysical model since, for some improbable but possible critical configurations, the ions become infinitely polarized. The results of using the PIM1, the PIM1s, as well as those of the simple RIM, have been compared for the static structure and ionic transport properties. The PIM1 reproduces the broad main peak of the total structure factor present in the neutron diffraction data, although the smoothed three-peak feature of this broad peak is slightly overestimated. The structural results for the PIM1s are intermediate between those for the RIM and the PIM1, but fail to reproduce the experimental features within the broad principal peak. Concerning the ionic transport properties, the value of the conductivity obtained using PIM1 is in good agreement with experimental values, while the self-diffusion coefficients and the conductivity for the PIM1s are lower than the corresponding values using the PIM1 and the RIM..

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].…”

Dielectric Screening Properties and Many Body Effects in Molten Salts

“…Consequently, the condition  + ~ 0 would be strictly applicable to the pair potentials +-(r) and ++ (r). From the considerations based on these facts, the polarizable ion model has been applied to AgBr and AgCl (Bitrián and Trullàs, 2006 ;Wilson et al, 1996).…”

“…The theory that Bitrián and Trullàs (2006) have developed is effective to revise the rigid ion model to some extent. However, as we have pointed out in the previous section, such dispersion called the polarization catastrophe is ascribed to the fact that the iteration has been done disregarding that the inter ionic distance r ij is required to be much longer than the distance between the positive and negative dipole charges.…”

“…(63) is different from those Wilson et al (1996), or Bitrián and Trullàs (2006) have defined, the function which satisfies the following condition will be appropriate; when r approaches to 0, then f ab (r) approaches to 1; when r increases (about twice of the inter ionic distance), f ab (r) approaches to 0. In any case, by setting f ab (r) as Eq.…”