Laser-shock compression of magnesium oxide in the warm-dense-matter regime

Abstract

Magnesium oxide has been experimentally and computationally investigated in the warm-dense solid and liquid ranges from 200 GPa to 1 TPa along the principal Hugoniot. The linear approximation between shock velocity and particle velocity is validated up to a shock velocity of 15 km/s from the experimental data, this suggesting that the MgO B1 structure is stable up to the corresponding shock pressure of ∼350 GPa. Moreover, our Hugoniot data, combined with ab initio simulations, show two crossovers between MgO Hugoniot and the extrapolation of the linear approximation line, occurring at a shock pressures of approximately 350 and 650 GPa, with shock temperatures of 8000 and 14,000 K, respectively. These crossover regions are consistent with the solid-solid (B1-B2) and the solid-liquid (B2-melt) phase boundaries predicted by the ab initio calculations.

Thermodynamically complete equation of state of MgO from true radiative shock temperature measurements on samples preheated to 1850 K

“…The experimental data reported by Miyanishi et al [17] have uncertainties too large to constrain the principal Hugoniot of MgO or the locations of its phase boundaries. The latter were determined solely by the results of their ab initio calculations.…”

“…The predicted melting temperature for MgO at the Earth's core-mantle boundary pressure, ≈135 GPa, ranges from 6000 to 9000 K [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] (Fig. 1).…”

“…None of the recently reported shock experiments to approximately 1 TPa pressure [17,18,[21][22][23][24] was designed to probe even indirectly the actual melting of B1 phase MgO. These studies probed decaying [17,22,23], steady [18], or almost steady [21] states along the principal Hugoniot or quasi-isentrope [24] and so they only characterize, at best, the B2 melting curve and the B1-B2 solid-solid phase transition.…”

“…These studies probed decaying [17,22,23], steady [18], or almost steady [21] states along the principal Hugoniot or quasi-isentrope [24] and so they only characterize, at best, the B2 melting curve and the B1-B2 solid-solid phase transition. Moreover, all the laser shock experiments so far published have very large uncertainties on MgO pressure and temperature due to the severe challenges associated with such measurements.…”

“…McWilliams et al [22] had poor constraints on the mechanical state of their MgO samples at shock speeds below ≈17.3 km/s (i.e., pressures below ≈525 GPa) due to loss of VISAR reflectivity, high uncertainties on all pressures due to sparse calibration assuming a linear Hugoniot despite the [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] and conventional shock temperature data [25,26]. Typical uncertainties of the predicted melt lines are 100-200 K. Thicker lines indicate several melting curves that are close enough to be grouped and shown together.…”