Thèses

Lundi 24 Septembre 2018 à 14h00.

Pressure-induced disorder in bulk and nanometric SnO2

''Nanosized materials have been the focus of an extensive interest research because they present new physical and chemical properties in comparison to their bulk counterpart.
When dealing with nanomaterials, the size effect and the surface energy are generally invoked, even though the underlying concepts are not clear. In this thesis, the main question that we want to answer is: what are the main parameters which govern the structural stability in the SnO2 nanometric under high-pressure in comparison to its bulk equivalent? The interest of studying nanoparticles under the high-pressure has at least two-fold: (i) to gain a fundamental understanding of thermodynamics when the interfacial energy reaches the same magnitude as the internal energy, and (ii) to stabilize new structures that may have potential interest as functional materials. In this work, we used complementary Raman and XRD spectroscopies. The former is sensitive to the anionic sublattice, the second is sensitive to cationic sublattice.
In the case of the SnO2 bulk samples, we used the concept of invasive percolation to explain the “partial” disorder of the oxygen sublattice which appears in the powders when the pressure increases; and for studying SnO2 nanoparticles, we used ab initio simulations to explain the appearance of this kind of disorder, i.e. the anionic sublattice disorder in SnO2 nanoparticle samples. In this way, we developed an original framework to describe the behavior of the SnO2 bulk and nanoparticles under pressure where both sublattices (anion and cation) are considered separately during the transition.
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