Understanding high pressure effects on simple molecular system is of great interest for condensed matter science and geophysics. Accessing the static pressure and temperature regions found in planetary interiors is made possible by the development of the Diamond Anvil Cell technique. We developed a double sided resistive heating method for the membrane DAC operating in low pressure (<0.5 mTorr) pressure environment requiring only 175 W input power to reach sample temperatures up to 1300 K. We applied this technique successfully to study molecular solids at high temperatures, such as H2, N2 and CO2. We made an attempt to determine the melting line of hydrogen and present data up to 26 GPa in agreement with literature. Raman spectroscopy of Nitrogen indicates a high stability of the ε molecular phase, while θ phase is only accessible via certain P, T paths. Studies of solid CO2 at high pressure and temperature lead to the discovery of a six-fold coordinated stishovite-like phase VI, obtained by isothermal compression of associated CO2-II above 50 GPa at 530-650 K, or by isobaric heating of CO2-III above 55 GPa. From our X-ray diffraction experiment on isothermally compressed H2O we report a coexistence of ice VII and symmetric ice X from the start of the transition pressure 40GPa to just below 100 GPa and a volume change of 4% across the transition. Vanadium, a transition metal undergoes a phase transition upon compression unlike other elements (Nb, Ta) from its group. We confirm the bcc phase transition to rhombohedral structure at 62 GPa under quasi hydrostatic compression in Ne pressure medium. Compression without pressure medium results in a much lower 30 GPa transition pressure at room temperature and 37 GPa at 425 K, pointing to a positive phase line between the bcc and rhombohedral crystalline phases.

Ce travail de thèse porte sur l’étude de la dynamique femtoseconde de photo-commutation de matériaux moléculaires bistables, à l’aide d’expériences pompe-sonde basées sur les spectroscopies optiques et rayons X. Une partie des expériences a été réalisée sur synchrotron et X-FEL (X-ray Free Electron Laser). La première partie de la thèse, qui est consacrée à l’étude de systèmes à transition de spin non-octaédriques, a révélé différents chemins de transformations sur la surface de potentiel, associés à différents mécanismes de changement d’état électronique et modulant la cohérence de la dynamique structurale pilotant le processus. La seconde partie porte sur l’étude d’analogues du bleu de Prusse (CoFe) où les expériences ultra-rapides ont permis de d’étudier les dynamiques de transformation autour des sites de fer et de cobalt. / This thesis focuses on the study of the femtosecond photoswitching dynamic in the bistable molecular materials, using the pump-probe experiments which are based on the optical and x-ray spectroscopies.  Part of these experiments was performed at synchrotron and X-FEL (X-ray Free Electron Laser). The first part of the thesis, which is devoted to the study of non-octahedral spin transition systems, revealed different pathways of transformation on the potential surface. The second part focuses on the study of the Prussian Blue Analogues (CoFe), where the ultra-fast experiments allowed to follow the dynamics around the two metal ions.