A MOLECULAR-DYNAMICS STUDY OF THE STRUCTURAL STABILITY OF A 2-DIMENSIONAL SOFT-SPHERES BINARY ALLOY

We present molecular dynamics calculations for a two-dimensional binar y alloy composed of soft disks. We start the simulation with a system of 108 identical particles at low temperature arranged in a close-pack ed triangular lattice. Then the diameter ratio lambda is varied in ste ps while the temperature is kept constant. We define a set of order pa rameters suitable for detecting the existence of ordered binary domain s over a wide range of lambda-values. The phase diagram thus obtained for the range 0.3 less than or equal to lambda less than or equal to 1 shows two ordered phases: one FCC structure below lambda = 0.5 and a new structure, not previously reported, for 0.5 less than or equal to lambda less than or equal to 0.85. This is not expected from Hume-Roth ery rules but is in remarkable agreement with experimental results for colloidal systems. Both transitions are first-order ones; we also stu dy hysteresis and metastable states.