High-Pressure Melting Curve of Nitrogen and the Liquid-Liquid Phase Transition

The melting curve of nitrogen was measured up to 71 GPa, a fourfold increase in pressure over previous measurements. The measurements were made using the laser-heated diamond anvil cell and melting was detected in situ by the laser speckle method. The melting temperature rises linearly up to a maximum at 50 GPa and 1920 K, and with increasing pressure suddenly decreases linearly to 1400 K at 71 GPa. This sharp drop in the melting slope (dT/dP) above 50 GPa indicates the appearance of a liquid denser than the solid and of a liquid-liquid phase transition. The sharpness of the changes suggests that the transition is first order and is a liquid-liquid polymer transition. This conclusion is consistent with earlier theoretical studies and experimental evidence that pressure transforms molecular nitrogen into a chainlike polymeric form.