Calculation of the melting curve, shock Hugoniot, and ramp adiabat of nickel up to the super-Earth pressure–temperature range

Nickel is one of the most plentiful elements in the metallic core of rocky planets, but its geophysical properties remain ambiguous at elevated pressures and temperatures. In this study, we develop the statistical moment method to explore geophysically important routes on the phase diagram of nickel up to 4000 GPa and 20 000 K. First, the solid–liquid boundary is constructed via the modified work-heat equivalent principle. Our theory shows a dramatic growth in the melting point of nickel during compression, consistent with the latest static experiments and atomistic simulations. It also reveals the high risk of carbon contamination in diamond anvil cells and the vital role of nickel as a fcc stabilizer in iron–nickel binaries. Next, we revisit the principal Hugoniot in pressure–density and pressure–temperature spaces. The onset of melting is detected at 258 GPa, 13.50 g/cm3, and 6050K, which agree well with recent thermodynamic calculations. It is feasible to access warmer and denser crystalline regions by going along second-shock Hugoniot branches. Finally, we provide detailed information about the solid adiabats of nickel under extreme conditions. Our numerical data would be valuable for developing dynamic compression techniques to capture the geophysical characteristics of nickel in the high-energy-density regime.