⇦Chinese Physics Letters, 2020, Vol. 37, No. 6, Article code 068101 Evidence for a New Extended Solid of Nitrogen * Li Lei (雷力)1,2**, Qi-Qi Tang (唐琦琪)1, Feng Zhang (张峰)1, Shan Liu (刘珊)1, Bin-Bin Wu (吴彬彬)1, Chun-Yin Zhou (周春银)3 Affiliations 1Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China 2Key Laboratory of High Energy Density Physics and Technology (Ministry of Education), Sichuan University, Chengdu 610064, China 3Shanghai Synchrotron Radiation Facility, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201204, China Received 14 February 2020, online 26 May 2020 *Supported by the National Natural Science Foundation of China (Grant No. 11774247) and the Chinese Academy of Sciences (Grant No. 2019-SSRF-PT-009588).
**Corresponding author. Email: lei@scu.edu.cn, Citation Text: Lei L, Tang Q Q, Zhang F, Liu S and Wu B B et al 2020 Chin. Phys. Lett. 37 068101    Abstract A new extended solid nitrogen, referred to as post-layered-polymeric nitrogen (PLP-N, or Panda-N), was observed by further heating the layered-polymeric nitrogen (LP-N) to above 2300 K at 161 GPa. The new phase is found to be very optically transparent and exhibits ultra-large $d$-spacings ranging from 2.8 to 4.9 Å at 172 GPa, suggesting a lower-symmetry large-unit-cell 2D chain-like or 0D cluster-type structure with wide bandgap. However, the observed x-ray diffraction pattern and Raman scattering data cannot match any predicted structures in the published literature. This finding further complicates the phase diagram of nitrogen and also highlights the path dependence of the high-pressure dissociative transition in nitrogen. In addition, the phase transition from cubic gauche nitrogen (cg-N) to LP-N is observed at 157 GPa and 2000 K. DOI:10.1088/0256-307X/37/6/068101 PACS:81.30.-t, 78.30.-j, 61.50.Ks © 2020 Chinese Physics Society Article Text Understanding the pressure-induced dissociative transition from dense molecular solids to nonmolecular extended solids in low-Z elements, such as hydrogen[1–3] and nitrogen,[4–6] is an important objective in high-pressure physics. The high-pressure dissociation and metallization transitions in nitrogen behave similarly in many respects to hydrogen.[7] However, nitrogen differs from hydrogen in its chemical bonding property forming rich solid polymorphous.[8] The intermolecular dissociation transition in nitrogen involves the rearrangement of triple bonds, resulting in various molecular solids, such as $\beta$,[9,10] $\delta$,[11–13] $\delta_{\rm loc}$,[13,14] $\varepsilon $,[14,15] $\zeta $,[16,17] $\zeta '$ [18,19] $\kappa $,[17,19] $\lambda$,[20,21] $\eta $,[18,19] $\theta $[22] and $\iota $.[22,23] The intramolecular dissociation transition in nitrogen is correlated with the rupture of strong covalent triple bonds, resulting in different densely packed, wide bandgap, single-bonded polymeric phases, such as cubic gauche nitrogen (cg-N),[6,19,24,25] layered-polymeric nitrogen (LP-N)[26,27] and hexagonal layered polymeric nitrogen (HLP-N).[27] The resulting polymeric phases typically have high energy density (33 kJ/cm$^{3}) $[28] and are promising as a high-energy-density material (HEDM). Direct compression molecular nitrogen to megabar often yields extended solids associated with the pressure-induced symmetry breaking.[19] Although considerable theoretical efforts have been made on the prediction of polymeric structures with rings,[29] layers,[30] clusters,[31] chains[32] and cages,[33] the structures of many high-pressure nitrogen phases still require unambiguous determination. More than 20 polymeric phases have been predicted from theoretical calculations,[29–34] but only three types of polymeric phases have been experimentally observed so far.[6,19,24–27] There are still controversies over the issue of the structure for the polymeric phases LP-N,[27,29] and the true crystal structure of LP-N requires unambiguous determination. Strong diffraction spots rather than powder rings often present on the high-pressure x-ray diffraction pattern due to preferred spontaneous crystallization at high temperature, and the absent Raman scattering often occurs in high-pressure Raman spectroscopy because of the extreme stress environment in the small sample cavity of diamond anvil cell (DAC). Therefore, it has been challenging to achieve convincing results for the structure of the high-pressure nitrogen phase. The complexity of phase relation in nitrogen is partially attributed to the path-dependent transformation under high pressure. The observation of the thermodynamically favorable high-pressure phase depends on the experimental pressure-temperature ($P$–$T$) path taken, so walking different $P$–$T$ paths may allow us to arrive at a different metastable phase, which would provide a better understanding of pressure-induced dissociative transition in nitrogen. Here we load the solid nitrogen in two different $P$–$T$ pathways (a red path and a blue path in Fig. 1(a)), and present the experimental observation of a novel high-pressure extended nitrogen, post-layered-polymeric nitrogen (PLP-N, or Panda-N), beyond the $P$–$T$ stability field of LP-N. High pressure was generated using a DAC with 80–100 µm culets. Rhenium was used as the gasket material precompressed to 20 µm thickness with a sample chamber drilled using laser cutting to produce a 25 µm diameter hole. The pressure was monitored by the high-frequency edge of the diamond phonon with a pressure error of $\pm$1 GPa.[35] High-pressure Raman experiments were carried out on a custom-built confocal Raman spectrometry system in the back-scattering geometry based on an Andor Shamrock triple grating monochromator with an attached Andor Newton Electron Multiplying Charge Coupled Device (EMCCD), excitation by a solid-state laser at 532 nm. The high-pressure angle dispersive x-ray diffraction (ADXRD) data were collected at the BL15U1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF, China). A monochromatic x-ray beam with an energy of 20 keV and a focused beam size of $2.9\times 3.2$ µm$^2$ was used for the ADXRD measurements. Laser heating experiments were conducted without the use of irradiation absorbing agents in a custom-build double-sided laser-heating DAC system with two 1064 nm CW fiber lasers with $\sim $5 µm laser-heating spot size. Laser heating temperature was either measured by the spectroradiometric method or estimated with an uncertainty of 10%. Because the laser-sample coupling is unstable, temperature fluctuations make precise measurement difficult in experiments. About 3–5 GPa pressures were dropped at the transition point after laser heating due to the structural relaxation and the relieving of stress. The pressure release was stopped due to a diamond anvil failure at $\sim $70 GPa, and no apparent evidence was found in the spectral and diffraction data, suggesting chemical reaction of nitrogen with diamond or rhenium gasket.
cpl-37-6-068101-fig1.png
Fig. 1. (a) $P$–$T$ phase diagram of nitrogen and two experimental $P$–$T$ routes. Gray solid and dashed lines are the phase boundaries, the red and blue arrow lines indicate the two $P$–$T$ paths, the blue zone shows the forming region of PLP-N. Microscope images of PLP-N at 172 GPa (b) and cg-N at 130 GPa (c) with the same transmitting light for the sample size $\sim$10 µm.
In the red path, the representative Raman spectra are shown in Fig. 2(a), upon compression to above 130 GPa, the molecular nitrogen becomes dull red and Raman silent. The high-pressure dissociation transition in nitrogen is sluggish and path-dependent. Laser heating provides enough activation energy to overcome the potential barrier of the transition. The amorphous "red"-N formed at $\sim $1000 K and 131 GPa can well absorb near-infrared laser radiation (1064 nm) and be laser heated directly without the addition of a thermal absorbing agent. After direct laser-heating the "red"-N to above 1800 K at 134 GPa, the cg-N is formed as evidenced by the observation of the fingerprint Raman $A$ mode of cg-N at about 855 cm$^{-1}$. Upon further room-temperature compression to 157 GPa, the $A$ mode of the cg-N becomes very weak. However, the cg-N transforms to the LP-N after successive heating up to $\sim $2000 K as evidenced by the presence of Raman modes of the LP-N at about 857 cm$^{-1}$, 1033 cm$^{-1}$, and 1150 cm$^{-1}$. It is shown that the forming region of LP-N is above the $P$–$T$ stability field of the cg-N, and the phase boundary between cg-N and LP-N can be determined at 157 GPa and $\sim $2000 K (Fig. 1(a)). It is also noted that the main Raman peak at 1033 cm$^{-1}$ of the LP-N exhibits colossal Raman scattering that was first reported by Tomasino et al.[26] Upon further room-temperature compression to 190 GPa, the Raman peaks of LP-N exhibits blue shift and significant broadening, indicating the wide pressure stability over the pressure range from 157 GPa to 190 GPa. The above-described case is the red $P$–$T$ path as shown in Fig. 1(a).
cpl-37-6-068101-fig2.png
Fig. 2. Representative Raman spectra of nitrogen via the red path (a) and the blue path (b). The $\uparrow$ and $\downarrow$ arrows denote the room-temperature compression and decompression, respectively.
In the blue $P$–$T$ path (Fig. 2(b)), the cg-N is initially formed under the lower pressure and higher temperature conditions (114 GPa, $\sim $2000 K) as compared with the red path. After successive laser heating to above 2200 K at 131 GPa and 151 GPa, the cg-N is still unable to transform into the LP-N due to the lack of pressure. Upon laser heating at a higher pressure (159 GPa), however, a mixed-phase of cg-N and LP-N is produced, which further confirmed the phase boundary (157 GPa, 2000 K) between the cg-N and LP-N (Fig. 1(a)). We also noted that the intensity of the Raman peak of the obtained LP-N is very low as compared with the colossal Raman scattering of the LP-N sample in the red path, suggesting the existence of a metastable state far away from thermodynamic equilibrium in the sample. We continually compressed the mixed-phase sample to 161 GPa, and further laser-heated the sample to a higher temperature ($\sim $2300 K) for the purpose of producing phase-pure LP-N. Unexpectedly, all the Raman bands for the cg-N and LP-N are found to have disappeared, but three Raman modes at 985 cm$^{-1}$, 1021 cm$^{-1}$, and 1054 cm$^{-1}$ at 161 GPa along with a strong luminescence background are observed. After the laser-heating at 161 GPa, the sample now becomes very transparent (Fig. 1(b)), even more transparent than the cg-N (Fig. 1(c)), suggesting a higher bandgap value of the new state. The newly synthesized extended solid nitrogen is referred to as the post-layered-polymeric nitrogen phase (PLP-N) because the forming condition is well above the $P$–$T$ stability field of LP-N. In general, when the band gap becomes larger, the high-pressure phase becomes more optically transparent, and vice versa.[36,37] The nitrogen transforms into an opaque state upon compression above 140 GPa at room temperature, and the band gap gradually decreases to 0.4 eV at 240 GPa.[38] For the optically transparent polymeric nitrogen phase, like cg-N, the insulating states with localized electrons are associated with the unique close-packed crystal structures. The novel extended solid phase is different from the previously reported polymeric phases, including cg-N,[6] LP-N[26] and HLP-N.[27] Recent fast optical spectroscopy experiments to just higher temperatures than the transition point suggested a metallization transition in nitrogen.[7] The observation of wide-bandgap PLP-N, however, cannot provide direct evidence of the insulator-to-metal transformation, the crystalline polymeric nitrogen is likely to form an insulated state with wide bandgap. The Raman peaks of the transparent PLP-N become indistinguishable from the background upon further compression to 172 GPa (Fig. 2(b)), even after repetitive laser heating at the same pressure. Note that there are at least five Raman modes at 700–1100 cm$^{-1}$ observed on the decompression due to the release of stress. Phonon splitting implies the pressure-induced symmetry lowering transition from 3D network cg-N to 2D layered LP-N, and then to PLP-N with lower symmetry (2D or 0D). The three Raman modes of the PLP-N (the modes a, b, c in Fig. 2(b) and Fig. 3) are analogous to the main Raman peak of LP-N at 1025 cm$^{-1}$ at 150 GPa. The observed Raman mode d exhibits the pressure dependence different from the $A$ mode of cg-N, especially at pressures lower than 110 GPa. The possibility of the reversible transformation into the cg-N on the decompression cannot be ruled out. The assignment of Raman modes relies on the knowledge of the structure. Although the structure of PLP-N is unable to be determined, the observed Raman data provide useful information on the new extended solid. As shown in Fig. 3, the pressure-dependent phonon frequency changes in PLP-N suggest a stiff lattice and shift nearly linearly with pressure at approximately 1.1 cm$^{-1}$/GPa. This value is smaller than those of cg-N ($\sim 1.3$ cm$^{-1}$/GPa)[6,19] and LP-N (1.2–1.6 cm$^{-1}$/GPa),[26,27] suggesting that the PLP-N may have a higher density than that of the cg-N (4.5 g/cm$^{3}$ at 120 GPa) and LP-N ($\sim $4.85 g/cm$^{3}$). In addition, a broad Raman band at about 900 cm$^{-1}$ is also observed in the high-pressure Raman spectra (Fig. 2(b)), which could come from stress-induced disordered local phonon of nitrogen.
cpl-37-6-068101-fig3.png
Fig. 3. Pressure dependent Raman shifts of PLP-N (square) shown in comparison with those of cg-N (triangle) and LP-N (circle). Red and blue color symbols represent Raman data in the red path and the blue path, respectively. Solid and open symbols represent the present Raman data on compression and decompression, respectively. Dashed lines, solid lines and dotted lines are the previously published data for cg-N,[19] LP-N[26] and HLP-N.[27] Thick lines and larger symbols indicate the main peaks of Raman spectra.
cpl-37-6-068101-fig4.png
Fig. 4. (a) The integrated x-ray diffraction patterns of PLP-N taken at room temperature at 123 GPa and 172 GPa from the center of the nitrogen sample. Black thick lines indicate the positions for the observed diffraction $d$-spacing. (b) A typical 2D x-ray diffraction image of the PLP-N at 123 GPa.
In order to probe the structure of the PLP-N, in situ high-pressure x-ray diffraction experiments have been conducted. As shown in Fig. 4, the new phase exhibits a distinctive x-ray diffraction pattern, at least eight reflections were collected from the center of the sample cavity ($\sim $10 µm). The diffraction-peak positions of the PLP-N are listed in Table 1. If we focus the x-ray spot onto the edge of the sample cavity, additional strong reflections from the rhenium gasket are observed at the smaller $d$-spacings. The reflections from the cg-N and LP-N cannot be observed. We would like to note that the PLP-N exhibits ultra-large $d$-spacings ranging from 2.8 to 4.9 Å at 172 GPa, indicating the larger primitive cell dimensions that are comparable to other previously known polymeric phases, in which the overwhelming majority of $d$-spacings are smaller than 3.0 Å. The structure of PLP-N is most likely the 2D chain-like or the 0D cluster-type structure that may have larger $d$-spacing values. Pressure-induced symmetry breaking suggests the dissociation transition from 3D cubic network (cg-N) to 2D layered structure (LP-N), and then to a lower-symmetry structure with 2D or 0D for the PLP-N.
Table 1. The diffraction-peak positions of PLP-N at 123 GPa and 172 GPa ($\lambda= 0.6199$ Å).
Pressure Peak labels $d$-spacing 2$\theta$ Relative intensity
(GPa) in Fig. 4(a) (Å) (deg)
172 P$_{1}$ 2.8432 12.4934 3
P$_{2}$ 3.0081 11.8087 26
P$_{3}$ 3.1332 11.3373 18
P$_{4}$ 3.2010 11.0972 53
P$_{5}$ 3.9427 9.0102 68
P$_{6}$ 4.3567 8.1544 95
P$_{7}$ 4.4315 8.0168 100
P$_{8}$ 4.8678 7.2986 43
123 P$_{1}$ 2.8697 12.3781 2
P$_{2}$ 3.0401 11.6844 6
P$_{3}$ 3.1977 11.1087 5
P$_{4}$ 3.2370 10.9739 11
P$_{5}$ 4.0088 8.8617 100
P$_{6}$ 4.3926 8.0878 20
P$_{7}$ 4.4722 7.9439 26
P$_{8}$ 4.9094 7.2368 51
We try to fit the measured spectra to the predicted structures. However, the observed x-ray diffraction pattern and Raman scatting data cannot match any structures in the published literature. Although the exact structure of the new phase is still an open issue. This suggests the wide bandgap and large-unit cell structure properties for the extended solid nitrogen beyond the stability field of cg-N and LP-N. This finding further complicates the phase diagram of nitrogen and also highlights the path dependence of the high-pressure dissociative transition in nitrogen. In conclusion, we have reported experimental observations of a pressure-induced structural transition of nitrogen into an optically transparent extended solid, PLP-N. The novel phase is formed above the $P$–$T$ stability field of LP-N, and should have a possible large-unit-cell 2D chain-like or 0D cluster-type structure wide band gap. We thank Jian Sun for helpful discussion. References New Low-Temperature Phase of Molecular Deuterium at Ultrahigh PressureSynchrotron infrared spectroscopic evidence of the probable transition to metal hydrogenUltrahigh-pressure isostructural electronic transitions in hydrogenPressure Dissociation of Solid Nitrogen under 1 MbarPolymeric nitrogenSingle-bonded cubic form of nitrogenMetallization and molecular dissociation of dense fluid nitrogenPolymerization in highly compressed nitrogen (Review Article)Single‐Crystal X‐Ray Diffraction Study of β NitrogenStructure of N2 at 2.94 GPa and 300 KHigh pressure x‐ray diffraction studies on solid N 2 up to 43.9 GPaThe structure of N2 at 49 kbar and 299 KThe crystal structures of δ and δ[sup ∗] nitrogenStructures of Molecular Nitrogen at High Pressures.Structures and phase diagrams of N 2 and CO to 13 GPa by x‐ray diffractionOn the ϵ-ζ transition of nitrogenHigh P-T transformations of nitrogen to 170GPaHigh-pressure amorphous nitrogenRaman study of pressure-induced dissociative transitions in nitrogenNovel high-pressure nitrogen phase formed by compression at low temperatureRaman spectroscopy and phase stability of λ-N2Raman, infrared, and x-ray evidence for new phases of nitrogen at high pressures and temperaturesUnusually complex phase of dense nitrogen at extreme conditionsTransformation of molecular nitrogen to nonmolecular phases at megabar pressures by direct laser heatingSingle-crystalline polymeric nitrogenPressure-Induced Symmetry-Lowering Transition in Dense Nitrogen to Layered Polymeric Nitrogen (LP-N) with Colossal Raman IntensityHexagonal Layered Polymeric Nitrogen Phase Synthesized near 250 GPaAIP Conference ProceedingsSingle-bonded allotrope of nitrogen predicted at high pressureNovel High Pressure Structures of Polymeric NitrogenCalculations predict a stable molecular crystal of N8Metastable high-pressure single-bonded phases of nitrogen predicted via genetic algorithmCagelike Diamondoid Nitrogen at High PressuresHigh-Pressure Phases of NitrogenPressure calibration of diamond anvil Raman gauge to 310GPaTransparent dense sodiumConductive dense hydrogenSemiconducting non-molecular nitrogen up to 240 GPa and its low-pressure stability
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