Thermodynamic potentials and non-monotonic melting curve of sodium at high pressure

Abstract

The free energy of sodium and its melt are reconstructed in the semi-empirical form up to high pressures. The room temperature–high-pressure isotherm was used for the reconstruction of the free energy of solid bcc-Na. Hugoniot was used for the reconstruction of the free energy of liquid sodium. The intersecting line of chemical potential surfaces of bcc-Na and liquid sodium is calculated in the pressure–temperature–chemical potential space. This line represents a melting curve of sodium at pressures from−1.3 to 60 GPa and temperatures 140–1000 K. The calculated melting curve is a non-monotonic convex curve. The curve includes the maximum near 30 GPa and 900 K and practically coincides with the literature experimental melting curve of dense bcc-Na.

Keywords:

• melting curve
• phase transition
• free energy
• thermodynamics
• phase diagram

Acknowledgements

This study was partially supported by a Government Contract, No. 2.552.11.7090. The author thanks the anonymous referees for helpful criticisms of the paper.