Melting Curve and Liquid Structure of Nitrogen Probed by X-ray Diffraction to 120 GPa

Synchrotron x-ray diffraction measurements of nitrogen are performed up to 120 GPa to determine the melting curve and the structural changes of the solid and liquid phases along it. The melting temperature exhibits a monotonic increase up to the triple point where the epsilon molecular solid, the cubic gauche covalent solid, and the fluid meet at 116 GPa, 2080 K. Above, the stability of the cubic gauche phase induces a sharp increase of the melting curve. The structural data on liquid nitrogen show that the latter remains molecular over the whole probed domain, which contradicts the prediction of a liquid-liquid transition at 88 GPa, 2000 K. These findings thus largely revisit the phase diagram of hot dense nitrogen and challenge the current understanding of this model system.

Figure 1

Drawing of a cross-sectional view of the sample cavity located between the two diamond anvils of the DAC. The part of the sample probed by the x-ray beam is at the intersection of the two slits engraved on the C:B disks. The two disks form a capsule heated from both sides by yttrium lithium fluoride (YLF) laser beams. The laser focus was adjusted in order to fully illuminate the C:B plates and reduce the radial temperature gradient.

Figure 2

Determination of the nitrogen melting curve from five experiments. (a) X-ray diffraction patterns collected at several temperatures around melting at 106 GPa. Solid nitrogen ($ϵ-{\mathrm{N}}_2$), diamond, and LiF diffraction peaks are labeled. The broad diffuse ring centered at $2\theta =10.5°$ at 2005 K and above comes from melted nitrogen. (b) Background-subtracted x-ray diffraction 2D images collected at several temperatures around melting at 115 GPa and the corresponding integrated patterns. (c) Pressure-temperature conditions at which x-ray diffraction patterns have been collected. Each color represents a different sample. The solid (empty) circles represent the solid (liquid) phase. The lozenges indicate the $P\text{−}T$ conditions at which the cg-N melting is crossed during the pressure increase. The black line is a fit of the Kechin equation to the melting points (see text).

Figure 3

Revisited phase diagram of nitrogen. The present determination of the melting curve is shown as a black line. Previous determinations from experiments [9, 10, 11] and from simulations [8] are shown as various color lines for comparison. The calculations of the low density liquid to high density liquid transition line (LLT) [7] are also presented as dashed lines. The $\delta \text{−}\epsilon$ transition line is omitted for clarity.

Figure 4

Structural evolution of fluid nitrogen. (a) Experimental structure factors of fluid nitrogen collected at various $P\text{−}T$ conditions along the melting line. The structure factor obtained at 4.5 GPa in a distinct experiment under resistive heating is shown for comparison. (b) Evolution of the position ($Q_m$) of the main diffraction peak with pressure. Inset: $(Q_m/Q_{m_0}{)}^3$ plotted against $\rho /{\rho }_0$, where $\rho$ is the fluid density computed by a classical molecular dynamics simulation (see text). The dashed lines are a guide for the eyes.