Ionic Overlayers on Corrugated Surfaces III: Dynamics in the Fluid Phase
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IONIC OVERLAYERS ON CORRUGATED SURFACES III: DYNAMICS IN THE FLUID PHASE Hyangsuk Seong* and S.D. Mahanti* of Physics and Astronomy and the Center for Fundamental Materials Research, Michigan State University, East Lansing, MI 48824
"Department
ABSTRACT Using molecular dynamics simulation, we have studied the dynamics in the fluid phase of ionic overlayers on corrugated substrates. This is a good model to probe the dynamics of intercalants inside the galleries of intercalated systems. We have calculated the diffusion constant from the incoherent dynamic structure factor. de Genne oscillations in the q-dependence of the Rayleigh peak width and dispersion of Brillouin peaks have been obtained from the coherent dynamic structure factor. These results are compared with neutron scattering measurements in stage-2 RbC 24 and earlier simulation results. INTRODUCTION Molecular dynamics (MD) simulations have been performed to study the dynamics in the fluid phase of ionic intercalants inside the galleries of intercalated systems. Details of MD simulations and the potentials are described in the first of the series of three papers in this proceedings [1]. For these dynamic studies a larger system containing 864 particles was chosen. To understand the dynamics we can study several quantities such as the time dependent pair correlation function, velocity-auto correlation function, intermediate time scattering function, and dynamic structure factor. Here, we focus on the dynamic structure factor, S(q, v), which is directly related to the neutron scattering cross section. In addition, we also look at Ss(q, v) which is related to the incoherent scattering cross section. S(q, v) is defined as follows. S(q, v)
=
J N_
F(q, t)e
2
1rl~dt
(1)
1• eiq-(r_(t-r.(°)) + N"Zeiq'(rjt)-r.•(°)) NN
F3 (q,t)+Fd(q,t).
(2)
We will consider the q and v dependence of both the incoherent S 3(q, v) (obtained from Eq. (1) with F(q,t) replaced by F.(q,t)) and the coherent S(q,v) dynamic structure factors which describe single particle and collective dynamics, respectively. Since we will focus on the fluid dynamics, we will take the angular average over q. We will compare MD results for two systems (systems I and II) and the experiment [2], to see the effect of the corrugation potential on the dynamics of the intercalants. System I has only interactions between the intercalants (i.e. corrugation-free system) whereas system II has a corrugation potential appropriate for stage-2 RbC 24 . Temperatures of the systems I and II are fixed at 254K and 299K where the liquid to solid transition temperatures for these systems are 200K and 220K[3], respectively. Thus the systems are not too far above their transition temperatures. INCOHERENT DYNAMIC STRUCTURE FACTOR For large values of q (- 4A-'), a simple fluid such as the system I shows a Gaussian spectrum for S.(q, v) (see Figure 1) which is characteristic of a free particle behavior. This result is very Mat. Res. Soc. Symp. Proc. Vol. 291. @1993 Materials Research Society
304
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