Pressure-Induced Symmetry-Lowering Transition in Dense Nitrogen to Layered Polymeric Nitrogen (LP-N) with Colossal Raman Intensity

We present the discovery of a novel nitrogen phase synthesized using laser-heated diamond anvil cells at pressures between 120–180 GPa well above the stability field of cubic gauche $(cg)\text{−}\mathrm{N}$. This new phase is characterized by its singly bonded, layered polymeric (LP) structure similar to the predicted Pba2 and two colossal Raman bands (at $\sim 1000$ and $1300{\mathrm{cm}}^{-1}$ at 150 GPa), arising from two groups of highly polarized nitrogen atoms in the bulk and surface of the layer, respectively. The present result also provides a new constraint for the nitrogen phase diagram, highlighting an unusual symmetry-lowering 3D $cg\text{−}\mathrm{N}$ to 2D LP-N transition and thereby the enhanced electrostatic contribution to the stabilization of this densely packed LP-N ($\rho =4.85\mathrm{g}/{\mathrm{cm}}^3$ at 120 GPa).

Figure 1

(Left) Raman spectra of laser-heated nitrogen at 150 GPa and ambient temperature, obtained in three distinctive regions in the inset: (A) black amorphous, (B) new LP-N, and (C) $cg\text{−}\mathrm{N}$. Note that LP-N shows two characteristic peaks with exceptionally strong intensities. (Right) The pressure-dependent shifts of two characteristic ${\nu }_s(\mathrm{N}\text{−}\mathrm{N})$ of LP-N shown in comparison with that of $cg\text{−}\mathrm{N}$. The red circles are reproduced from Refs. [7, 8].

Figure 2

Angle-resolved x-ray diffraction patterns obtained from laser-heated nitrogen in the pressure range between 137 and 52 GPa as pressure decreases, plotted after subtracting the background (see Fig. S3 in the Supplemental Material [21]).

Figure 3

(Top) The angle-resolved x-ray diffraction pattern of laser-heated nitrogen at 112 GPa, shown together with the three-phase refined and difference patterns. The refined results of individual phases are also plotted below for the major LP-N phase in Pba2 in black and two minor phases, $cg\text{−}\mathrm{N}$ ($I{2}_{1}3$) in red, and its metastable form in $C2/c$ in orange. The $hkl$’s of each phase are labeled, and their positions marked as the vertical bars. (Bottom) Crystal structures of the proposed phases showing the layered Pba2 structure made of seven-membered N-N (${\mathrm{N}}_7$) rings in comparison with the $I{2}_{1}3$ and $C2/c$ structures made of even-numbered ${\mathrm{N}}_{10}$ and ${\mathrm{N}}_{12}$ rings in 3D networks, respectively.

Figure 4

The pressure-volume compression curves and third-order Birch-Murnaghan EOS fits of LP-N (circles and solid line) and $cg\text{−}\mathrm{N}$ (squares and dotted line), plotted in comparison with the previous data (triangles in Ref. [7] and diamonds in Ref. [8]) and the calculated Pba2 structure (cross in Ref. [14]). Inset: The proposed phase diagram to emphasize the presence of new extended LP-N above the stability field of $cg\text{−}\mathrm{N}$. The dotted lines are from the previous calculations [25, 26] and the dashed lines signify the kinetic lines.