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ATOMIC GLASS FORMATION AND LOCAL REARRANGEMENT :
        AMORPHIZATION IN PLANAR CLUSTERS
 
Eunji Sim, Alexander Z. Patashinski and Mark A. Ratner
Department of Chemistry and Materials Research Center
Northwestern University
Evanston, IL 60208-3113
U.S.A.
 

        Structural and dynamical spects of amorphization in a cluster model of a disordered phase are presented. Molecular dynamics simulations were done within a mobile cluster static boundary (MCSB, Fig. 1) model, in which static, disordered particles at the boundary of the simulation cell cause particles within the cell to adjust themselves structurally[1]. With sufficient boundary disorder(d0), both the radial distribution function and the orientational order parameter[2] show onset of amorphization behavior (Fig. 2). For Lennard-Jones particles, which are not good glass formers, the amorphization occurs only within a certain healing distance of the disordered boundary. With increasing boundary  disorder, the orientational correlation function and the radial distribution function indeed show orientational phase scrambling, disappearance of long-range order, and the onset of glass-like, amorphized structure. The correlation function shows stretched-exponential relaxation behavior[3], which is typical for glass systems, is observed(Fig. 3). The fundamental processes of relaxation in glass forming systems consists of the system point crossing over the barriers that intervene between basins of attraction on the multi-dimensional energy landscape[4]. Such rearrangements are local in nature: they do not correspond to the entire material undergoing some transition, but rather to a local modification of the structure, occurring in a reduced-dimensional space. Pictorial understanding of the local rearrangements, in the case of atomic glass, has obtained. We see a chain of local structural rearrangements in a small cluster of eight particles (Fig. 4) representing characteristic elementary kinetic events in this very simple model of glass behavior.  The possibility to interpret these events in terms of defect motion is discussed[5].

[1] E. Sim, A. Z. Patashinski and M. A. Ratner, J. Chem. Phys. (in press).
[2] A. Z. Patashinski and M. A. Ratner, J. Chem. Phys. 103, 10779 (1995).
[3] F. H. Stillinger, Phys. Rev. E52, 46685 (1995); M. F. Shlesinger and J. T. Bendler, Phase Transitions in Soft Condensed Matter, T. Riste and D. Sherrington, eds. (Plenum, 1989).
[4] M. Goldstein, J. Chem. Phys. 51, 3728 (1969); F. H. Stillinger and T. A. Weber, Phys. Rev. A25, 978 (1982).
[5] E. Sim, A. Z. Patashinski and M. A. Ratner, J. Chem. Phys. (submitted).
 

 
                       (a) d0 = 0.0                                                 (b) d0 = 0.5

Figure 1    MCSB model of the Lennard-Jones system with 46 mobile particles and 108 static boundary particles, open circles are mobile particles and solid brown circles are static boundary particles. (a) Initial structure without the boundary disorder. (b) One equilibrium configuration snapshot at temperature T = 0.1 with boundary disorder amplitude d0 = 0.5.
 

Figure 2    Bond order parameter probability distribution function with d0 = 0.5 where x and y axes represent real and imaginary part of the bond order parameter, respectively. The distribution becomes quite homogeneous, such that the phase is almost a random variable, corresponding to complete amorphization of the hexadic structure.
 

Figure 3    Temperature dependence of the bond order parameter correlation function with d0 = 0.5. Notice non-monotonic structure: at the lowest temperature, velocity overshoot is seen; this is simply a remnant of the phonon-like excitation at short times. With further increase in T, the correlation function persists to longer times, but loses any velocity memory. With yet further increase in temperature, the system effectively melts for T > 0.7, so that correlations vanish very rapidly in the system at higher T.
 


                (a) t = 680                                       (b) t = 695                                      (c) t = 710

Figure 4    Snapshot of configuration at chosen times with T = 0.5. Eight chosen particles are shown in blue solid circles. During the course of number of short lifetime jumps, the inner cluster gets to many other minima in energy landscape. Notice that Figure 4(c) has more ordered structure than the initial structure.



email to: eunji@chem.nwu.edu
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