THESIS
2020
xvii, 72 pages : illustrations ; 30 cm
Abstract
Phase transitions in crystalline lattices arguably cover most numerous types of
structural phase transitions. They include solid-solid transitions between two ordered
lattices or between a lattice and a disordered solid, and order-disorder transitions for
the rearrangement of different types of constituent particles in a fixed crystalline lattice.
Different lattice symmetries, defects, and strains can lead to rich behaviors that remain
not well understood. Here I study these transitions using a novel model composed of
the same sized paramagnetic and non-magnetic soft disks randomly distributed on a
two-dimensional triangular lattice by simulation. Under different mixing ratios, we
found that the system exhibits order-disorder, solid-solid, and solid-hexatic transition
as a perp...[
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Phase transitions in crystalline lattices arguably cover most numerous types of
structural phase transitions. They include solid-solid transitions between two ordered
lattices or between a lattice and a disordered solid, and order-disorder transitions for
the rearrangement of different types of constituent particles in a fixed crystalline lattice.
Different lattice symmetries, defects, and strains can lead to rich behaviors that remain
not well understood. Here I study these transitions using a novel model composed of
the same sized paramagnetic and non-magnetic soft disks randomly distributed on a
two-dimensional triangular lattice by simulation. Under different mixing ratios, we
found that the system exhibits order-disorder, solid-solid, and solid-hexatic transition
as a perpendicular magnetic field quasi-statically increases. This simple model and the
related phase transitions can be experimentally realized in binary colloids.
In the first system, we choose the 1:3 mixing ratio for the paramagnetic and non-magnetic
particles. As the magnetic field increases above a threshold, particles
experience intense local rearrangements among the first and second-nearest neighbors
and form small triangular superlattice domains. After this partially ordering transition,
the superlattice domains expand and merge, resulting in a completely ordering
transition at a higher magnetic field. Such order-disorder transition about the
distribution of different types of constituent particles in a fixed lattice is central to fast-ion
conducting solids, solar cells, and electrocatalyst, but our observed order-disorder
transition is caused by a different mechanism of long-range magnetic repulsions.
The second system with a mixing ratio 1:1 exhibits an interesting kinetic pathway of a
solid-solid transition: the initial triangular lattice breaks down to a liquid-like state in
low magnetic fields and transforms into a polycrystal of the square lattice in strong
magnetic fields.
In the third system with a mixing ratio of 1:4, we find a abnormal melting phase
transition where the long-range orientational order is lost as the magnetic field increases.
The competition between different lattice constants serves as a possible explanation for
the observed lattice distortion and the formation of the liquid-like phase.
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