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PhD defence: investigations of high pressure phase diagrams of MgO-SiO2 systems with laser shock compression

Riccardo Bolis (LULI)

Two decades of exoplanet discoveries brought the physics of planetary interiors among the topics of broad and current interests. To advance in this field, one of the key ingredient is the knowledge of the equation of states and physical properties of planetary constituents. At the extreme conditions of planetary interiors ( 1-100 Mbar, 10^3-10^4 K), matter lies in the Warm Dense Matter (WDM) regime and theoretical descriptions are not trivial. Important progress have been done with ab-initio calculations based on differential functional theories, but such calculations need to be validated by experiments.
In this thesis, we experimentally characterized phase diagrams and physical properties of MgO, MgSiO3 and Mg2SiO4 at conditions relevant for planetary science (0.5-10 Mbar). The studied compounds are the Mg end members of (Fe, Mg)SiO3 and (Fe, Mg)2SiO4 that are among the most abundant components of Earth’s mantle and are also thought to be abundant in Super- Earth’s mantle and giant planet cores. To bring these materials to planetary interior conditions we performed laser shock compression experiments at three high power laser facilities: LULI2000 (France), GEKKOXII (Japan), MEC at LCLS(USA). At LULI2000 and GEKKOXII we investigated the liquid properties and melting of MgO, MgSiO3 and Mg2SiO4 using decaying shocks coupled to visible diagnostics. At LULI2000 we studied with XANES spectroscopy MgO in the WDM regime highlighting its metallisation mechanism and structural properties in the liquid phase. Finally, at the MEC end station of LCLS, we used X-ray diffraction to measure shock induced structural changes on MgSiO3 and Mg2SiO4 in the solid region of their phase diagrams. Altogether these works, obtained with different diagnostics, imply a revision of the phase diagrams of the studied compounds. In particular we determined a new experimental melting point for MgO (at 470 ± 40 GPa and 9860 ± 810 K), we ruled out the occurrence of an MgSiO3 liquid-liquid transition (supposed to occur at ~ 400 GPa along the Hugoniot) and we evidenced for the first time the occurrence of an amorphous phase along the Forsterite (Mg2SiO4 crystal) Hugoniot (at ~50 GPa).

Becquerel amphi. - 2pm