Atomic Motion in Amorphous Ni 81 B 19 Studied by Reverse Monte Carlo and Molecular Dynamics Simulation
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were multiplied by 1.61, 0.68, and 0.63, respectively, to obtain the well depths 0.14, 0.27, and 0.10 eV found by Li et al., 7 and ii) the attractive parts of VNiB(r) and VBB(r) were replaced by Lennard-Jones 6-12 tails. T* teq* tpr* These modifications were necessary to avoid the almost instantaneous phase separation and unphysical diffusion behavior that were found when 0.3 700 690 the original potentials were used. 8 Cut-off distances were taken as 6.7 A 0.6 500 700 for Ni-Ni and Ni-B, and 8.1 A for B-B. Contrary to many other studies on 0.9 300 700 the glass transition, 9 - 12 the pair potentials in this work are strongly non1.2 75 325 additive (well positions 2.68 A for Ni-Ni, 2.26 A for Ni-B, and 3.34 A for 1.5 50 75 B-B - 1.84 A for B-B would be "additive"), have different well depths, 1.8 50 75 and have repulsive parts of different softness (the ratios of zero-crossing 2.0 50 335 distance and well position are 0.87, 0.82, and 0.75, respectively). In addition, the density of the present system is high. In this paper, reduced quantities, denoted by an asterisk, are expressed in units based on the original Ni-Ni pair potential: 1012 K for temperature, 0.087 eV for energy, 2.68 A for length, and 0.71 ps for time. The simulation times are listed in Table I; all runs were started with the RMC configuration with zero velocities, which was immediately brought to the required temperature. Results shown here are averages over production time. Table I Equilibration and production times
CONSISTENCY OF RMC AND MD RESULTS Fig. 1 shows the effects of applying MD at room temperature (T* = 0.3) to the RMC configuration. One sees that the agreement between the MD and the RMC results remains satisfying. There are a few imperfections: the nearest-neighbor peaks in the Ni-Ni and Ni-B partial RDF's 15
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r (A) Vvor / Vsph Figure 1. Partial RDF's, neighbor-number distributions, and normalized Voronoi volume distributions for the Molecular Dynamics system at T*=0.3 (heavy curves) and for the starting configuration, obtained by Reverse Monte Carlo (light curves). Neighbors are defined as atoms within a distance of 3.3, 3.0, and 4.6 A for Ni-Ni, NiB, and B-B pairs, respectively. Voronoi volumes were calculated using the radical-plane method with effective hard-sphere radii 1.26 A and 0.85 A for Ni and B; V,,,h denotes the corresponding hard-sphere volume. Note that the boron atoms fit more loosely in their Voronoi volumes than Ni.
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become sharper, the secondary maxima at =3 A and =7 A disappear, and a small mismatch in the medium-range order oscillations originates; however, the remaining characteristics are left almost entirely intact. It is particularly reassuring that the amplitude decay of the oscillations, the shapes of the split second peaks, and the absence of B-B nearest neighbors are well reproduced; normal temperature effects aside, the same level of coincidence between MD and RMC was observed at high
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