Ionic Overlayers on Corrugated Surfaces I: Structure

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IONIC OVERLAYERS ON CORRUGATED SURFACES I: STRUCTURE Tahir Cagin*, Surajit Sen**, Hyangsuk Seong**, and S.D. Mahanti** "*Molecular Simulations Inc., Suite 540, 199 S Robles Av., Pasadena, CA 91101 "*Department of Physics and Astronomy and the Center for Fundamental Materials Research, Michigan State University, East Lansing, MI 48824 ABSTRACT Structural and dynamic properties of a large class of intercalation compounds can be modelled by a collection of ions interacting via a screened Coulomb potential and subjected to the corrugation potential produced by the host atoms. In this paper we discuss the results of Molecular Dynamics simulation studies of the ground state structure of RbC 24 and related systems focussing primarily on the micro-structure i.e. on the nature of the domains and domain walls. INTRODUCTION Over the past decade there have been extensive experimental and theoretical studies on the structural and dynamic properties of binary and ternary graphite intercalation compounds (GIC's) [11-[5]. One of the important features of these systems is the competition between the substrate potential and the intercalant interaction. Like rare gas atoms physisorbed on a graphite substrate [6], the GIC's show both commensurate and incommensurate structures. Although structural properties of stage-i binary alkali GIC's (MCs, M = K, Rb, Cs) and their ternary counterparts (MM{_.Cs) are reasonably well understood [1, 5], the detail structure of the stage2 compounds is still not completely clear. Of particular interest here is the nature of the ground state in the incommensurate phase, the size of the intercalant domains and the domain walls. Also of interest is to find out how the ground state structure changes by changing the intercalant density. We have used extensive Molecular Dynamics (MD) simulation to investigate some of these issues and here we summarize our major findings. The corrugation potential used in this work was obtained by Moss and his coworkers [7] from a careful analysis of the liquid structure data. The two-particle potential is of Yukawa form [8] and differs slightly from the two-particle potential used in the simulation study of Moss et al ([7], see also reference [1]). In a series of three papers in this proceedings we adress three basic questions pertaining to the physical properties of stage-n (n > 2) GIC's; (1) intercalant structure in the solid phase (paper I), (2) nature of the solid-fluid melting (paper II), and (3) dynamics in the fluid phase (paper III). In particular we focus on the role of the substrate corrugation by tuning its strength from zero to that seen in real systems. For the sake of specificity we take stage-2 RbC 2 4 and a related compound RbC 24.57 as our test systems but most of our results are quite general. THE MODEL The chemical stoichiometry of stage-n (n > 2) compounds is in general MC 12n, and the layer stoichiometry is nominally MC1 2 giving the planar density p = 1/6 (in units of 1/area of the graphite planar unit cell). Unfortunately it is not possible to continuou