Two New Molecular Nitrogen Phases near Megabar Pressures

TL;DR

This study reports two newly discovered molecular nitrogen phases near megabar pressures, characterized by advanced diffraction and spectroscopy techniques, expanding understanding of nitrogen's structural diversity under extreme conditions.

Contribution

The paper introduces two novel nitrogen phases, $t\zeta$-N$_2$ and $\xi$-N$_2$, with detailed structural determination at high pressures and temperatures.

Findings

Discovered two new nitrogen phases near 80-100 GPa.

Determined crystal structures using X-ray diffraction and Raman spectroscopy.

Supported findings with first-principles calculations.

Abstract

Molecular nitrogen exhibits remarkable structural diversity near the polymeric transition, where multiple phases are metastable. Here, we report two new molecular phases. The first, $t\zeta$-N$_2$, is a polytype of monoclinic $C2/c$ $\zeta$-N$_2$, characterized by a tripled $c$ axis and 96 atoms per unit cell. The second, $\xi$-N$_2$, is a previously unreported hexagonal phase ($P6cc$) containing 112 atoms per unit cell. Both phases were synthesized in a diamond anvil cell by laser heating $\zeta$-N$_2$ to 1800--2500~K at pressures of 78--98~GPa. Their crystal structures were determined using single-crystal X-ray diffraction, corroborated by Raman spectroscopy, and supported by first-principles calculations. The $t\zeta$-N$_2$ phase likely corresponds to the previously reported $\kappa$-N$_2$ phase.