Size-dependent the amorphous-amorphous phase transitions in tetrahedrally and octahedrally coordinated nanoparticles |
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The project is aimed at the investigation of
pressure-induced polyamorphism, which is an unusual and intriguing
phenomenon and many works have been devoted to understanding the
underlying mechanisms. It consists in a first-order transition between a
low-density amorphous state (LDA) and a high-density amorphous state (HDA)
through pressure application with significant changes in structure and
physical properties of the amorphous solids. As in their crystalline
counterparts, some structures provide interesting properties for potential
applications. In the case of amorphous states, it is important to be able
to vary some physical properties (for instance, electrical conductivity as
in the case of the LDA-HDA transformation in amorphous silicon) associated
with intrinsic properties linked to their amorphous nature (mechanical
properties or thermal conductivity). With increasing pressure, the
coordination of the structural units are usually sharply or smoothly
rising. We will use materials containing the polyhedra with different
coordination to get the LDA and HDA phases with different topologies and
compare their properties. As nanocrystals containing octahedrally and
tetrahedrally coordinated polyhedra, we will investigate the titanium
dioxide TiO2, obtained by sol-gel technology, its polymorphic
modifications: anatase, rutile, brookite, and Si, Ge, respectively.
Therefore, in the frame of our project during the reporting period was
obtained the nanocrystals TiO2, Ge and Si-containing octahedrally and
tetrahedrally coordinated polyhedra. The review of the literature was made
to determine the currently existing problems that to be solved in carrying
out this project. Also a selection of the literature was made to develop
an appropriate theoretical model for the simulation of an
amorphous-amorphous transitions in tetrahedrally and octahedrally
coordinated nanoparticles. In addition, the interaction between LDA and
HDA phases is quite complex for bulk materials, has added complexity
associated with nano-sized particles under investigation. Surface effect,
when the particle size reaches nanometer, strongly influences the
stability of phases. It leads to the observation of new stabilized
structures or metastable states. However, obtaining a starting amorphous
state to generate amorphous-amorphous transformations may be difficult,
especially from materials such as TiO2 that have remarkable technological
applications but are poor glass formers. Reducing the size of the
particles TiO2 till 10 nm led to the stabilization of the anatase
structure with respect to the rutile and brookite at ambient conditions.
Amorphous TiO2 nanoparticles for the experiments can be prepared by a
mechanical pressure increases on anatase, and can be prepared without
pressure, by a chemical sol-gel technology. Recently, it was found that,
depending on whether chemically or mechanically were prepared the initial
amorphous nanoparticles the polyamorphic transformations at high pressures
will be differ. This observation indicates that pressure is a suited tool
to discriminate between nanomaterials apparently similar at ambient
conditions.
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