<p dir="ltr">Electronic structure calculations, specifically plane wave Density functional theory (DFT) in conjunction with data science resources on the nanoHUB platform, are a powerful tool for the study of materials at extreme conditions. nanoHUB’s resources for FAIR (Findable, Accessible, Interoperable and Reproducible) data and workflows accelerate research and sharing of results. With these combined resources we studied the high-pressure properties and stability of various ceramic materials, such as the polytypes of silicon carbides (SiC).</p><p dir="ltr">Silicon carbides are of interest for their exceptional mechanical strength, thermal stability and chemical resistance making them attractive for applications at extreme conditions. To understand the temperature- and pressure-induced phase transitions we studied the electronic structure, phonon dispersion and elastic constants of major synthesizable SiC polytypes as a function of pressure. In addition, we report on the elastic constants as a function of pressure for the main polymorphs. This DFT workflows are published online, not only enhances the reproducibility of findings, but can also accelerates the discovery and development of new material properties.</p>

The majority of structural studies on DNA have been carried out using fibre diffraction, while studies of its dynamics and thermal behaviour have been mainly performed in solution. When the DNA double helix is heated, it exhibits local separation of the two strands that grow in size with temperature and lead to their complete separation. This work has investigated various aspects of this phenomenon. The experiments reported in this thesis were carried out on films of oriented fibres of DNA prepared with the Wet Spinning Apparatus. Thus, sample preparation and characterisation are essential parts of the research. The structures of two forms of DNA, A and B, have been explored as a function of relative humidity at fixed ionic conditions. A method to eliminate traces of ever-present B-form contamination in A-form samples was established. The high orientation of the DNA molecules within the samples allowed us to investigate dynamical fluctuations and the melting transition of DNA using neutron scattering, which can provide the spatial information crucial to understand a phase transition, probing the static correlation length along the molecule as a function of temperature. The transition has been investigated for A and B-forms in order to understand its dependence on molecular configuration.Furthermore, after the first melting, denatured DNA films show typical glass behaviour. Their thermal relaxation has been explored using calorimetry.Neutron and X-ray inelastic scattering (INS and IXS) were used in the past to measure longitudinal phonons in fibre DNA, and the results shown disagreement. Recent INS measurements supported with phonon simulations have been crucial to understand the different dispersion curves reported to date. Experiments using INS and IXS have been carried out to continue with this investigation. Attempts to observe the transverse fluctuations associated to the thermal denaturing of DNA, never experimentally investigated before, have been made.

La majorité des études structurales sur l’ADN avaient été réalisées par diffraction sur des fibres tandis que ses propriétés dynamiques thermiques avaient été étudiées en solution. Lorsque la double hélice d’ADN est chauffée elle présente des séparations locales des deux brins, dont la taille augmente avec la température jusqu’à la séparation complète des brins. Ce travail étudie différents aspects de ce phénomène. Les expériences présentées dans cette thèse ont été réalisées sur des films formés de fibres orientées d’ADN préparés par la méthode du ”filage humide“. La préparation et la caractérisation des échantillons en deux formes A et B de l’ADN ont constitué une partie importante de la recherche. Une méthode pour éliminer la contamination résiduelle de la forme B dans les échantillons de forme A a été mise au point. La bonne orientation des molécules d’ADN dans les échantillons nous a permis d’étudier les fluctuations dynamiques et la transition de dénaturation thermique de l’ADN par diffraction de neutrons, sensibles à la longueur de corrélation statique le long de la molécule en fonction de la température. La transition a été étudiée pour les formes A et B pour déterminer comment elle dépend de la conformation. De plus, après la première dénaturation thermique, les films d’ADN présentent un comportement typique d’un verre. Leur relaxation thermique a été étudiée par calorimétrie. La diffusion inélastique de neutrons et de rayons X (INS et IXS) avaient été utilisées antérieurement pour mesurer les phonons longitudinaux dans des fibres d’ADN, avec des désaccords entre les résultats. Des mesures INS récentes, complétées par des simulations, avaient été cruciales pour comprendre les différentes courbes de dispersion observées. Nous avons mené des expériences INS et IXS pour poursuivre cette analyse. Des tentatives pour observer les mouvements transversaux associés à la dénaturation thermique de l’ADN, jamais observés expérimentalement, ont également été faites. / The majority of structural studies on DNA have been carried out using fibre diffraction, while studies of its dynamics and thermal behaviour have been mainly performed in solution. When the DNA double helix is heated, it exhibits local separation of the two strands that grow in size with temperature and lead to their complete separation. This work has investigated various aspects of this phenomenon. The experiments reported in this thesis were carried out on films of oriented fibres of DNA prepared with the Wet Spinning Apparatus. Thus, sample preparation and characterisation are essential parts of the research. The structures of two forms of DNA, A and B, have been explored as a function of relative humidity at fixed ionic conditions. A method to eliminate traces of ever-present B-form contamination in A-form samples was established. The high orientation of the DNA molecules within the samples allowed us to investigate dynamical fluctuations and the melting transition of DNA using neutron scattering, which can provide the spatial information crucial to understand a phase transition, probing the static correlation length along the molecule as a function of temperature. The transition has been investigated for A and B-forms in order to understand its dependence on molecular configuration.Furthermore, after the first melting, denatured DNA films show typical glass behaviour. Their thermal relaxation has been explored using calorimetry.Neutron and X-ray inelastic scattering (INS and IXS) were used in the past to measure longitudinal phonons in fibre DNA, and the results shown disagreement. Recent INS measurements supported with phonon simulations have been crucial to understand the different dispersion curves reported to date. Experiments using INS and IXS have been carried out to continue with this investigation. Attempts to observe the transverse fluctuations associated to the thermal denaturing of DNA, never experimentally investigated before, have been made.