Theoretical extension of the gold pressure calibration standard beyond 3 Mbars

The static-lattice equation of state and structural phase stability of gold have been calculated to 10 Mbar, using two distinct density functional models; the local density approximation (LDA) and the generalized gradient approximation (GGA). The fcc structure is predicted to be the most stable phase at zero pressure, transforming to the hcp structure at 3.5 Mbar (LDA) or 4.1 Mbar (GGA), which then remains stable to 10 Mbar. These transition pressures are roughly 50% larger than previous predictions. Once thermal effects are accounted for, the LDA model produces a room temperature isotherm that is in rather good agreement with existing data and that smoothly merges with the existing gold pressure calibration standard near 2 Mbar. The LDA room temperature isotherm should provide a reliable extension of the gold pressure calibration standard up to the fcc-hcp transition pressure, well above 3 Mbar.