Phase Transitions of Magnesium Oxide under Shock Compression

We report an experimental study on the phase diagram of MgO using laser-based shock compression under planetary interior conditions at Omega-EP in LLE. The reflecting shock front through the quartz-MgO-quartz target package was monitored using VISAR (Velocity interferometric system for any reflector) diagnostic [1] to determine the pressure of the shock front. Streaked optical pyrometry (SOP) [2,3,4] monitored spectral radiance of the propagating shock where these quartz windows served as an in-situ calibrant [4,5]. Of particular interest in previous work is the location of shock Hugoniot with respect to the B1, B2 and liquid phases [6,7,8] due to the adjacence of corresponding Hugoniot region to the triple point. We observed apparent orientation-dependent thermal anomalies of MgO [100], [111] and [110] using decaying shock compression. The results will be discussed in the context of the phase diagram and transition kinetics, and implications for different transition pathways under different initial structures.

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