Ab initio calculations of the B1-B2 phase transition in MgO

We present an ab initio study of MgO at high temperature and pressure, around the phase transition between the B1 and B2 phases. By means of ab initio molecular dynamic calculations, the thermal evolution of vibrational properties and thermodynamic quantities is obtained. We carefully compare our results with previous theoretical works on the phase transition curve and we analyze the differences among them. We show that anharmonic effects have been underestimated in the quasiharmonic approximation and that their inclusion in the free energy strongly straightens up the transition curve. Then, we use our B1-B2 phase boundary and our calculated Hugoniot to analyze recent decaying shock experiments on MgO. We also provide important thermodynamic quantities as the Gr\"uneisen parameter and sound velocities and we discussed their temperature dependence.