|dc.description.abstract||When a liquid is cooled quickly so that it avoids crystallization, the motion of the atoms or molecules slow until they eventually become kinetically trapped in an amorphous solid state at the glass transition temperature. There is no thermodynamic signature associated with the glass transition and the glass structure remains similar to that of the supercooled liquid. This raises questions about the fundamental nature of the glass phenomena. One approach to understanding the glass transition suggests that the formation of locally favoured structures, consisting of low energy clusters of atoms that are unable to tile space, compete with the formation of crystal nuclei and prevent the formation of the crystal.
The work described in this thesis uses computer simulation to explore the role of locally favoured structures in the glass forming ability of binary Lennard-Jones clusters formed from large A type atoms and small B type atoms with Kobb-Andersen interaction parameters. The phase diagram for the clusters with a size $N = 600$ is mapped out as a function of composition to identify the glass forming region over the range of B atom mole fractions $x_B=0-0.5$. Caloric curves, measuring the potential energy per atom as a function of temperature, exhibit large discontinuities suggesting freezing for compositions $x_B<0.1$ and $x_B>0.35$. At intermediate compositions, the potential energy varies continuously, but rapidly changes slope at a low temperature, which is indicative of a glass transition. The glass transition temperature of these clusters increases with increasing $x_B$. The amorphous nature of the clusters in the $x_B=0.1-0.3$ composition range is confirmed on the basis of measurements of the radial distribution function. The radial distribution function for the $x_B=0.4$ shows that the cluster freezes to a CsCl type crystal. Furthermore, measurements of the density profile and composition distribution as a function of radius from the centre of mass of the clusters show that the B atoms exhibit a significant enrichment at the core for all compositions which may influence the overall phase behaviour of the system relative to the bulk.
The local structure around an atom was studied using the local, average and disorder orientational bond order parameters which are based on the Steinhardt bond order parameters. The order parameters decrease as a function of decreasing temperature in the liquid phase, then increase at the freezing temperature for those compositions that crystallize. In the glass phase, the order parameters plateau at the glass transition temperature. The exception is the $x_B=0.1$ composition. These clusters exhibit a small amount of ordering at the glass transition. Additional analysis shows that this is caused by the partial ordering of the A atoms at the cluster surface, while the core remains amorphous. The disorder parameter effectively captures the same phenomena but in reverse and also shows that, at low temperatures, the $x_B=0.3$ clusters are the most disordered.
Finally, a Voronoi analysis, quantified in terms of the Voronoi index, is used to describe the geometric nature of the cage formed by neighbouring atoms around the B type atoms in the core of the glass forming clusters. The number of different types of polyhedra increases with increasing $x_B$, but the ten most abundant polyhedra remain the same and consist of 9-11 sided structures. It is notable that icosahedral structures are not observed. Instead, the $(0,2,8,0)$ polyhedron, which is consistent with a bicapped antiprism, is the most abundant structure for all compositions and exhibits the largest increase as a function of temperature, essentially doubling in concentration as the liquid approaches the glass transition temperature. However, the overall fraction of B atoms involved in $(0,2,8,0)$ polyhedra in the glass decreases from 0.35 to 0.15 as $x_B$ increases from 0.1 to 0.3, The probability distribution for all polyhedra also becomes more uniform. This is consistent with the clusters becoming more disordered as measured by the orientational bond order parameters.
To conclude, this thesis maps out the glass forming compositions for binary Lennard-Jones clusters and shows that composition has a significant effect on the types and distribution of local structure observed in the glass forming systems.||