High-Pressure Polymeric Nitrogen Allotrope with the Black Phosphorus Structure

At a Glance

Metadata	Details
Publication Date	2020-05-28
Journal	Physical Review Letters
Authors	Dominique Laniel, B. Winkler, Timofey Fedotenko, Anna Pakhomova, Stella Chariton
Institutions	Linköping University, Dassault Systèmes (United Kingdom)
Citations	176
Analysis	Full AI Review Included

Executive Summary

This research paper reports the successful synthesis and unambiguous structural characterization of a new polymeric nitrogen allotrope, bp-N (black phosphorus structure nitrogen), under extreme conditions.

Structural Resolution: The study definitively identifies the polymeric nitrogen phase previously known as LP-N (Layered Polymeric Nitrogen) as the black phosphorus structure (bp-N, space group Cmce), resolving long-standing incongruities in the phase diagram.
Synthesis Conditions: bp-N was synthesized by laser heating pure molecular nitrogen in a diamond anvil cell (DAC) at 140 GPa to temperatures up to ~4000 K.
Material Classification: bp-N is confirmed to be a wide bandgap semiconductor with a calculated bandgap of 2.2 eV at 150 GPa, contrasting sharply with the narrow bandgap (0.3 eV) of black phosphorus.
Structural Anisotropy: The bp-N structure exhibits significant N-N bond length disparity (7.2% difference) between the zigzag (ZZ) and armchair (AC) chains, suggesting enhanced structural and electronic anisotropy compared to black phosphorus or black arsenic.
Pnictogen Alignment: The discovery brings nitrogen in line with heavier pnictogen elements (P, As, Sb, Bi), confirming the high-pressure trend for elements to adopt the structures of their heavier counterparts at lower pressures.
HEDM Relevance: Polymeric nitrogen phases are crucial prototypes for developing novel High Energy Density Materials (HEDMs) due to the large energy stored in their single N-N bonds.

Technical Specifications

Parameter	Value	Unit	Context
Synthesis Pressure	140	GPa	Optimized pressure for bp-N formation.
Synthesis Temperature (Max)	~4000	K	Maximum temperature reached during laser heating.
Crystal Structure	Orthorhombic	N/A	Isostructural to black phosphorus.
Space Group	Cmce	N/A	Experimentally determined structure.
Unit Cell Volume (V)	39.88(6)	Angstrom³	Experimental value at 140 GPa.
Calculated Bandgap	2.2	eV	bp-N at 150 GPa (Wide bandgap semiconductor).
Shortest N-N Bond (ZZ chain)	1.338(6)	Angstrom	Intralayer bond length at 140 GPa.
Longest N-N Bond (AC chain)	1.435(7)	Angstrom	Intralayer bond length at 140 GPa.
Bond Length Difference (ZZ vs AC)	7.2	%	Indicates high structural anisotropy.
Shortest Interlayer N-N Distance	2.329(6)	Angstrom	At 140 GPa, preventing interlayer covalent bonding.
Primary Raman Mode 1	1001	cm^-1	Intense vibrational mode at 140 GPa.
Primary Raman Mode 2	1308	cm^-1	Intense vibrational mode at 140 GPa.

Key Methodologies

The bp-N allotrope was synthesized and characterized using a combination of extreme pressure/temperature techniques and advanced structural analysis:

High-Pressure Setup: A BX90-type diamond anvil cell (DAC) with 80 µm diameter diamond culets was employed to achieve extreme pressures.
Sample Preparation: A 200 µm thick rhenium gasket was indented to 12 µm, creating a 40 µm diameter sample cavity. Pure nitrogen gas was loaded at ~1200 bars.
Laser Heating and Gauging: Submicron-sized gold particles (~2 µm) were loaded to serve dual roles: YAG laser absorbers for heating and internal pressure gauges.
Synthesis Protocol: The sample was compressed to 140 GPa and laser-heated to temperatures up to ~4000 K to rupture the N≡N triple bonds and form the polymeric network.
Structural Characterization (in situ): Synchrotron single-crystal X-ray diffraction (SC-XRD) was performed on the resulting polycrystalline sample, following established procedures for unambiguous structural determination.
Vibrational Analysis: Raman spectroscopy was used to measure vibrational modes, confirming the bp-N structure by matching experimental spectra to Density Functional Theory (DFT) calculations for bp-N (and ruling out the previously proposed LP-N structure).
Theoretical Validation: DFT calculations were used to confirm dynamic stability, predict relaxed structural parameters, calculate Raman spectra, and determine the electronic band structure.

Commercial Applications

The discovery and characterization of bp-N have implications for several advanced materials engineering fields, particularly those requiring high energy density or novel 2D semiconductor properties:

High Energy Density Materials (HEDMs): Polymeric nitrogen is a leading candidate for next-generation, environmentally clean propellants and explosives. The energy stored in the polymeric single N-N bonds is significantly higher than conventional chemical explosives.
Wide Bandgap Electronics: As a wide bandgap semiconductor (2.2 eV), bp-N could potentially be used in high-power, high-frequency electronic devices, or UV photodetectors, offering advantages in environments where silicon or narrow-bandgap materials fail.
2D Materials and Flexible Electronics: Given its layered structure and analogy to black phosphorus, bp-N is a candidate for mechanical exfoliation into few-layer or monolayer 2D sheets. These sheets could be utilized in flexible electronics, transistors, and sensors, leveraging the material’s inherent structural anisotropy.
Spintronics: The enhanced structural and electronic anisotropy resulting from the large bond length differences (7.2%) between the ZZ and AC chains suggests potential for directionally dependent electronic transport, which is critical for spintronic applications.
Extreme Condition Synthesis: The successful methodology (DAC + laser heating + SC-XRD) provides a robust platform for synthesizing and characterizing other novel, metastable high-pressure phases of light elements, opening pathways for new material discovery.

View Original Abstract

Studies of polynitrogen phases are of great interest for fundamental science and for the design of novel high energy density materials. Laser heating of pure nitrogen at 140 GPa in a diamond anvil cell led to the synthesis of a polymeric nitrogen allotrope with the black phosphorus structure, bp-N. The structure was identified in situ using synchrotron single-crystal x-ray diffraction and further studied by Raman spectroscopy and density functional theory calculations. The discovery of bp-N brings nitrogen in line with heavier pnictogen elements, resolves incongruities regarding polymeric nitrogen phases and provides insights into polynitrogen arrangements at extreme densities.

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevlett.124.216001

References

2006 - Polymeric Nitrogen: The Ultimate, Green High Performing Energetic Material