Prediction of o-N16: A layered polymeric nitrogen phase

XY Li and QP Yang and D Jin and SC Ding and A Bergara and YS Yao and L Zhu and JJ Zhao and GC Yang, PHYSICAL REVIEW B, 111, 134111 (2025).

DOI: 10.1103/PhysRevB.111.134111

Nitrogen allotropes, especially under extreme conditions, play a crucial role in understanding atomic configurations and driving innovations in high-energy materials. The phase composition of layered polymeric nitrogen (LP-N) remains unresolved, with Pba2, C2/c, and Pccn structures proposed as candidates. However, their relative stabilities and ambiguous x-ray-diffraction (XRD) patterns-consistent with experimental data but not definitely-along with multiple possible explanations, have been topics of ongoing debate, particularly under varying pressures. In this study, we present an unexpected orthorhombic phase, o-N16, predicted using machine-learning driven swarm-intelligence approach. Gibbs free-energy calculations, kinetic barrier analysis during decompression, and simulated XRD patterns reveal o-N16 and Pba2 as dominant LP-N phases at elevated pressures, with a partial transition from o-N16 to C2/c upon decompression. Notably, the relative stabilities of nitrogen allotropes cg-N, o-N16, C2/c, and Pccn are dictated by configurational and conformational isomerism of armchair chains and interchain bonding. These findings shed light on the evolution of atomic configurations under high-pressure conditions. Beyond its favorable energy density, o-N16 exceeds all polymeric nitrogen structures in terms of detonation velocity and detonation pressure.

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