Crystallization of a binary Lennard-Jones mixture

Abstract

Transition interface path sampling combined with straightforward molecular dynamics simulation was applied to study the mechanism and kinetics of the crystallization of an undercooled 3:1 binary Lennard-Jones mixture with diameter ratio 0.85 and equal interaction strengths. We find that this mixture freezes via the formation of crystalline clusters consisting of a fcc-rich core and a bcc-rich surface layer, with an excess of large particles and particle species distributed randomly. A detailed comparison reveals that the transition mechanism is similar to that of the pure fluid but occurs with much smaller nucleation rates even at comparable degrees of undercooling. Also, the growth of the crystalline cluster in the mixture proceeds at a pace about 1 order of magnitude slower than in the pure system. Possibly, this slow dynamics of the mixture is related to the occurrence and subsequent relaxation of icosahedral structures in the growing crystal as well as in the liquid surrounding it.

Observation of crystallization slowdown in supercooled parahydrogen and orthodeuterium quantum liquid mixtures

“…A number of simulation studies [15,17,18,28] have suggested that in one possible scenario crystallization might be hindered by the emergence in the bulk supercooled liquid of locally preferred structures that eventually are incompatible with long-range crystalline order [34]. One important example is the icosahedron with its fivefold symmetry [35], which has been recently found to be a fundamental geometrical motif in the structure of bulk metallic glasses [36].…”

“…One important example is the icosahedron with its fivefold symmetry [35], which has been recently found to be a fundamental geometrical motif in the structure of bulk metallic glasses [36]. To explore this point we have performed a microscopic structural analysis based on the local bond order invariants method [4,18,28] of our simulated pH 2 -oD 2 mixtures. We have focused here on the invariant w 6 , which is most sensitive to icosahedral-like order, but the present analysis does not rule out the presence in the metastable liquid of local order with different symmetries.…”

“…Indeed, classical binary systems of particles that interact via a simple LennardJones (LJ) pair potential have been widely employed as the simplest theoretical models to investigate crystallization and glassy behavior in supercooled liquids [15][16][17][18][19]. Due to the spherical symmetry of the ground-state wave function of the pH 2 and oD 2 molecules, which are characterized by an even rotational quantum number J [20], a pH 2 -oD 2 mixture provides a neat molecular binary system in which the pair interactions can be described by the same isotropic LJ potential [20].…”

Caveats of mean first-passage time methods applied to the crystallization transition: Effects of non-Markovianity

“…The definition of the final state is relatively uncritical, since the probability to relax into the final state becomes constant after the system overcomes the free energy barrier. The value of the reaction rate we obtained in the TIS simulations 35 is JV = (1.6 ± 0.4) × 10 −4 , which differs significantly from the rates obtained with the MFPT and MLT methods. Still, the paths which are sampled in the TIS are essentially the same trajectories as those considered for the MFPT calculations.…”

“…We used Steinhardt bond order parameters 31 with the standard scheme proposed by ten Wolde, Ruiz-Montero, and Frenkel 27 to identify crystalline clusters, and monitored the progress of the reaction by considering the size of the largest crystalline cluster, n s . Details of this analysis and of the TIS simulations 2,33,34 are extensively described in our previous work 35 . Mean first-passage times were calculated from a collection of 200 MD trajectories started in the initial undercooled state.…”