Two New Molecular Nitrogen Phases near Megabar Pressures

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, $tζ$-N$_2$, is a polytype of monoclinic $C2/c$ $ζ$-N$_2$, characterized by a tripled $c$ axis and 96 atoms per unit cell. The second, $ξ$-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 $ζ$-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 $tζ$-N$_2$ phase likely corresponds to the previously reported $κ$-N$_2$ phase.

• The paper identifies two novel molecular nitrogen phases (E-N₂ and tt-N₂) with unprecedentedly large unit cells at pressures up to 98 GPa.
• It employs single-crystal X-ray diffraction, Raman spectroscopy, and DFT calculations to resolve complex host-guest structures and orientational modulations.
• The study underscores the impact of pressure–temperature history on kinetic trapping and phase stability, expanding the high-pressure nitrogen phase diagram.

Structural Elucidation of New Molecular Nitrogen Phases at Megabar Pressures

Introduction

Molecular nitrogen exhibits a remarkable diversity of crystalline phases under extreme conditions, particularly near the molecular-to-polymeric transition regime. This complexity is a result of competing kinetic and thermodynamic factors that favor the formation of multiple metastable molecular polymorphs. The increasing capability to resolve large and complex unit cell structures with single-crystal X-ray diffraction (SCXRD) at multi-megabar pressures has advanced the characterization of these elusive nitrogen phases far beyond what was achievable using powder XRD or Raman spectroscopy alone.

This work reports the identification, structural characterization, and first-principles analysis of two previously unobserved crystalline phases of molecular nitrogen, named tt-N$_2$ and E-N$_2$, synthesized at pressures of 78–98 GPa and high temperatures in diamond anvil cells. Both phases entail unprecedentedly large unit cells among all known elemental molecular crystals and introduce new topological motifs in the molecular nitrogen family.

Experimental Approach and Synthesis

Both tt-N$_2$ and E-N$_2$ were synthesized by heating nitrogen samples—loaded with either Ag or Cu foils as heat absorbers—in diamond anvil cells to temperatures of 1800–3000 K at controlled pressures up to 98 GPa. The use of metallic absorbers enabled efficient localized laser heating, which facilitated the access to high-temperature, high-pressure regions of the phase diagram and suppressed kinetic trapping in metastable or glassy states.

The SCXRD measurements were performed at advanced synchrotron facilities using micron-scale beams to probe single-crystalline domains. Raman spectroscopy complemented the structural data by providing vibrational fingerprints linked to intramolecular (vibron) and lattice (translational and librational) dynamics. All structures were further supported and their vibrational and thermodynamic properties rationalized with DFT-based first-principles calculations.

Crystallographic Characterization

E-N$_2$ Phase (P6cc Symmetry):

• The E-N$_2$ phase crystallizes in the hexagonal space group P6cc with lattice parameters $a = b = 14.105$ \AA, $c = 4.786$ \AA, and contains 56 N$_2$ molecules per unit cell—the highest count of any reported elemental molecular crystal.
• Its structure adopts a highly unusual topology for a diatomic element, displaying channels or cages that confine inner chains of N$_2$ molecules, effectively forming a host-guest arrangement. One subset of molecules is distributed over three symmetry-related positions, reflecting pronounced orientational disorder.

tt-N$_2$0 Phase (C2/c Symmetry):

• tt-N$_2$1 is a monoclinic polymorph (C2/c) with a unit cell tripled along the $_2$2-axis relative to the parent $_2$3-N$_2$4 phase. The cell contains 96 atoms (48 N$_2$5 molecules), an exact threefold increase versus $_2$6-N$_2$7.
• The structural modulation underlying this tripling is a periodic variation in the orientations of specific molecular sites along the $_2$8 direction, a feature absent in the parent phase. The orientation modulations were directly resolved in the SCXRD structure.

Spectroscopic and Computational Analysis

Raman spectroscopy confirmed the uniqueness of the vibrational spectra for both phases. E-N$_2$9 exhibits broader and weaker lattice modes, consistent with the vast number of predicted Raman-active modes for such a complex unit cell. The tt-N$_2$0 polymorph showed only subtle but distinct changes compared to $_2$1-N$_2$2, with extra sharp Raman peaks and an additional low-frequency shear mode, consistent with the increased number of vibrational degrees of freedom per unit cell.

DFT calculations provided enthalpy-pressure relationships and phonon dispersions for the new phases. E-N$_2$3 is metastable with respect to the denser $_2$4-N$_2$5 polymorph but energetically competitive with other known complex molecular phases like ι-N$_2$6 and $_2$7-N$_2$8 in the 30–80 GPa regime. Notably, tt-N$_2$9 becomes slightly more stable than $_2$0-N$_2$1 above 35 GPa, which explains its formation upon prolonged annealing at 98 GPa. Both phases showed no imaginary phonon modes, confirming their dynamic stability at the relevant pressures.

Implications and Theoretical Context

The discovery of E-N$_2$2 and tt-N$_2$3 significantly enriches the known high-pressure phase diagram of nitrogen. These phases expand the compositional and structural diversity accessible to a relatively simple diatomic element and demonstrate that the complexity of the nitrogen phase diagram is still not fully mapped, especially near the molecular-to-polymeric transition.

Noteworthy implications include:

• Unit Cell Complexity: Both structures set records for the numbers of molecules per primitive cell, challenging the predictive power of current first-principles structural search methodologies, which struggle with topologies of this size and disorder.
• Host-Guest Motifs: The E-N$_2$4 phase marks the first observation of a host-guest motif for elemental nitrogen, which could have implications for understanding dense molecular packing, guest-induced stabilization, and potentially broaden perspectives on the stabilization of atypical structures in other elemental systems.
• Kinetic Accessibility and Metastability: The results reiterate the critical role of pressure-temperature history and kinetic barriers in shaping which phases can form or persist, suggesting that experimental protocols exploring variable temperature profiles at constant pressure may yield further unexpected nitrogen phases.
• Reinterpretation of Past Results: Given the subtle diffraction and spectroscopic differences between $_2$5-N$_2$6 and tt-N$_2$7, prior XRD and Raman assignments—such as the so-called K-N$_2$8 phase above 120 GPa—may need reinterpretation in light of these new structures.

Outlook

From a fundamental perspective, the expanding repertoire of molecular nitrogen polymorphs in the megabar regime presents new opportunities for benchmarking quantum chemical calculations under extreme conditions. Moreover, these findings may inform potential routes to stabilizing high-energy-density phases relevant for materials science and planetary modeling, given the role of polymeric and molecular nitrogen in planetary interiors.

Further explorations targeting finer pressure-temperature grids, kinetic pathways, and in situ characterization during phase transformation could yield additional insights and perhaps reveal further molecular or even intermediate partially polymeric phase topologies.

Conclusion

The identification of E-N$_2$9 and tt-N$_2$0 demonstrates that even for well-studied diatomic systems like nitrogen, the high-pressure phase space is far from saturated. These results underline the utility of combining advanced laser-heated high-pressure synthesis with single-crystal diffraction and ab initio calculations to uncover and rationalize the complexities of elemental solids at extreme conditions. Theoretical modeling, ab initio crystal prediction, and experimental advances in probe sensitivity and sample handling will be essential for fully mapping the nitrogen phase diagram and for understanding the deep connections between structure, dynamics, and stability in this prototypical system.