High-Pressure Polymeric Nitrogen Allotrope with the Black Phosphorus Structure

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.

Figure 1

Crystal structure of bp-N. (a) The zigzag and armchair arrangements forming the bp-N layers. (b) The crystal structure of bp-N, where the zigzag and armchair chains are visible. (c) Two superimposed layers of the bp-N structure, highlighting the puckered honeycomb arrangement of fused ${\mathrm{N}}_6$ hexagons.

Figure 2

Raman spectroscopy data. (a) Comparison of the experimental (black) and calculated Raman spectrum of bp-N (blue) at 140 GPa with the Raman spectrum calculated in this work for LP-N with the structure reported in Ref. [6] (red). The experimental spectrum of LP-N reported in Refs. [6, 7] resembles that of bp-N (black) but does not match the red spectrum, implying the incorrectness of the structural model of LP-N suggested in Ref. [6]. Inset shows an enlargement of the 750 to $1500{\mathrm{cm}}^{-1}$ frequency range. For clarity, arrows indicate the correspondence between the calculated and experimental low intensity modes ($B_{1g}$ and two $B_{3g}$). (b) Pressure dependence of the bp-N Raman modes obtained in this study (black squares) in comparison with those previously attributed to the LP-N structure [6, 7]. The identity of the experimental Raman fingerprints of bp-N and LP-N allows us to conclude that these phases have the same bp-N type structure.

Figure 3

Experimental and DFT calculated volume per nitrogen atom in function of pressure for cg-N, bp-N, and HLP-N [6, 7]. The blue triangles represent experimental data obtained here on bp-N, while the full and dashed lines are fits to the experimental and calculated data, respectively. Notably, the volume per nitrogen atom decreases when transforming from cg-N to bp-N, as well as from bp-N to HLP-N.

Figure 4

Schematic diagram illustrating the phases of the pnictogen elements experimentally observed at high pressure. When possible, each phase is identified with its commonly employed name, space group, and structure type. The terms p-sc, sc, IM, bcc, and sh refer to pseudosimple cubic, simple cubic, incommensurate, body-centered cubic, and simple hexagonal, respectively. Phases indicated with the same color possess the same structure type. The figure was modified after Refs. [73] and [74].