Étude de la structure et des propriétés des polymorphes de SiO2 et B2O3 par méthodes ab initio

Abstract : During this PhD I use density functional theory and quantum Monte Carlo to evaluate the importance of van der Waals effects on the structures, the energies, and the properties of SiO2 and B2O3 polymorphs. I show that exchange-correlation functionals including dispersion effects lead to an error cancellation between an overestimation of the Si-O distances and an underestimation of the Si-O-Si angles in low densities SiO2 polymorphs. By using quantum Monte Carlo calculations, I have predicted with high accuracy the relative energy of a new B2O3 polymorph, which allowed me to evaluate the performances of different exchange-correlation functionals on this material. I then use the best functional possible to compute the mechanical and electronic properties of 25 predicted B2O3 polymorphs. Some of the predicted polymorphs exhibit intriguing mechanical properties, such as negative linear compressibility, auxeticity and anisotropy. These calculations allow me to make a hypothesis explaining the crystallization anomaly in B2O3. They underline a seemingly universal link between low energy polymorphism and ease of vitrification.
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Henri Hay. Étude de la structure et des propriétés des polymorphes de SiO2 et B2O3 par méthodes ab initio. Matériaux. Université Pierre et Marie Curie - Paris VI, 2016. Français. ⟨NNT : 2016PA066318⟩. ⟨tel-01470131⟩

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