Shock and melting behaviour of beryllium and magnesium oxide at megabar pressures*

We used ab initio molecular dynamic simulations to study the phase diagrams of beryllium and magnesium oxide at megabar pressures. We employ the thermodynamic integration (TDI) method to obtain the free energies of the liquid and the solid phases and to derive phase boundaries. We find that both materials exhibit very similar properties. There is a solid-to-solid phase transition that is strongly affected by anharmonic effects that shift the triple point to high pressure, which makes it more difficult to observe the higher-pressure solid phase with single-shock experiments. Conversely quasi-harmonic calculations underestimate the stability of lower-pressure phase.

 

Furthermore we calculated the principal and secondary shock Hugoniot curves of both materials and determine the offset between solid and liquid branches in pressure-temperature space. We find that this offset is much larger for secondary Hugoniot curves that start from a higher initial pressure. We also compare our secondary Hugoniot curves with recent shock experiments of MgO. We find good agreement in liquid region only.

 

We also make predictions for ramp compression experiments assuming they are isentropic. We determine the pressure-temperature interval where they follow the melting line while remaining in solid-liquid mixed state. Finally, we compare the melting line that we computed with TDI method with predictions based on Lindemann's law.