Nuclear quantum effects in hydrated nanocrystals

• Main Author: Schaack, Sofiane
• Other Authors: Institut des Nanosciences de Paris (INSP), Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS), Sorbonne Université, Philippe Depondt, Fabio Finocchi
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: HAL CCSD 2019
• Subjects: Nuclear quantum effects; Path integrals; Hydrated systems; Effets quantiques nucléaires; Simulations ab-initio; Intégrales de chemin; Brucite; Clathrate de méthane; Soude; [PHYS.QPHY]Physics [physics]/Quantum Physics [quant-ph]; Methane hydrate
• Online Access: https://theses.hal.science/tel-03008642; https://theses.hal.science/tel-03008642/document; https://theses.hal.science/tel-03008642/file/SCHAACK_Sofiane_2019.pdf

The quantum nature of nuclei yields unexpected and often paradoxical behaviors. Due to the lightness of its nucleus, the hydrogen is a most likely candidate for such effects. During this thesis, we focus on complexe hydrated systems, namely, the brucite minerals (Mg(OH)2), the methane hydrate (CH4-H2O) and the sodium hydroxide (NaOH), which display complex mechanisms driven by the proton quantum properties. Brucite exhibits the coexistence of thermally activated hopping and quantum tunneling with opposite behaviors as pressure is increased. The unforeseen consequence is a pressure sweet spot for proton diffusion. Simultaneously, pressure gives rise to a «quantum» quasi two-dimensional hydrogen plane, non-trivially connected with proton diffusion. Upon compression, methane hydrate displays an important increase of the inter-molecular interactions between water and enclosed methane molecules. In contrast with ice, the hydrogen bond transition does not shift by H/D isotopic substitution. This is explained by an important delocalization of the proton which also triggers a transition toward a new MH-IV methane hydrate phase, stable up to 150 GPa which represents the highest pressure reached to date by any hydrate. Sodium hydroxide has a phase transition below room temperature at ambient pressure only in its deuterated version. This radical isotope effect can be explained by the quantum delocalization of the proton as compared with deuteron shifting the temperature-induced phase transition of NaOD towards a pressure-induced one in NaOH. La nature quantique des noyaux produit des comportements inattendus et souvent paradoxaux. Du fait de sa légèreté, l'hydrogène est le candidat le plus susceptible de présenter de tels comportements. Nous avons étudié trois systèmes hydratés dont les mécanismes sont déterminés par les propriétés quantiques des protons (NQEs) : la Brucite (Mg(OH)2), l'hydrate de méthane (CH4-H2O) et l'hydroxyde de sodium (NaOH). Au sein des Brucites coexistent deux effets en compétition : un mécanisme ...