Shock Hugoniot of osmium up to 800 GPa from first principles calculations

First principles total energy calculations on hcp, ω (a three atom simple hexagonal), β (bcc) and fcc phases of osmium have been performed as a function of hydrostatic compression employing the FP-LAPW method. The comparison of total energies of these phases up to a maximum compression V/V(0) = 0.58 (pressure∼700 GPa) shows that the hcp structure remains stable up to this compression. The 300 K isotherm is determined after adding finite temperature thermal contributions to the total energy calculated as a function of volume at 0 K. From the theoretically determined isotherm, we have derived the shock Hugoniot of this metal and determined the shock parameters C(0) and s to be 4.48 km s(-1) and 1.32, respectively. Employing the theoretically calculated Gruneisen parameter in the differential form of the Lindemann melting rule, we have determined the variation of melting point of the osmium with pressure. The theoretically derived melting curve and the temperature rise along the Hugoniot predict the shock melting of osmium at ∼447 GPa with a corresponding temperature of ∼9203 K.