On the Existence of Layered Polymeric Nitrogen
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摘要: 在高温高压的极端条件下，分子晶体氮会打破传统三键机制向单键聚合态转变。氮在高压下的独特解离机制使聚合氮的研究意义超越了含能材料范畴，在基础物理学领域亦有深刻的科学意义。继立方偏转聚合氮cg-N（空间群I213）之后，第2个在实验上被发现的层状结构聚合氮LP-N（空间群Pba2）一直存在争议。主要的问题在于，LP-N的结构除了没有被高压X射线衍射实验验证之外，还与随后被发现的黑磷结构聚合氮BP-N（空间群Cmca）具有相近的合成温压条件和合成路径以及几乎相同的拉曼光谱特征。LP-N的合成很可能具有独特的相变动力学势垒。为此，选择独辟蹊径，从低温固态分子氮λ-N2出发，利用双面激光加热金刚石压砧技术，结合高压同步辐射X射线衍射和高压拉曼光谱分析方法，在约140 GPa、2600 K的条件下观察到了具有Pba2结构的聚合氮LP-N。结合第一性原理计算，分析了它的原子体积随压力的变化关系（p-V曲线），并探讨了LP-N高温高压合成动力学因素。研究结果不仅使我们更全面地认识LP-N，还进一步揭示了聚合氮的高压路径依赖特性。Abstract: Under the extreme conditions of high temperature and high pressure, molecular crystal nitrogen breaks the traditional three-bond mechanism and transforms into a single-bond polymerization state. The unique dissociation mechanism of nitrogen under high pressure makes the research significance of polymeric nitrogen beyond the scope of energetic materials, and also has profound scientific significance in the field of fundamental physics. Following the cubic gauche polymeric nitrogen cg-N (space group I213), the second experimentally discovered layered structure polymeric nitrogen LP-N (space group Pba2) has been controversial. The main problem is that, in addition to not being verified by other high-pressure X-ray diffraction experiments, LP-N structure has the similar synthesis temperature and pressure conditions and synthesis pathways, as well as almost the same Raman spectral characteristics as the subsequently discovered black phosphorus structure polymeric nitrogen BP-N (space group Cmca). The high-temperature and high-pressure synthesis of LP-N is likely to have a unique phase transition kinetic barrier. In this work, we started from the low-temperature solid-state molecular nitrogen λ-N2, used double-sided laser-heated diamond anvil cell (LHDAC) technology, combined with high-pressure X-ray diffraction based on the synchrotron radiation and high-pressure Raman scattering spectroscopy, supplemented by first-principles calculations, and observed the Pba2 structure of polymeric nitrogen LP-N at the conditions of about 141 GPa and about 2600 K. Combined with first-principles calculations, we compared and analyzed its pressure-dependent evolution of the volume per atom (p-V curve) and discussed the kinetic factors of LP-N synthesis at high temperature and high pressure. In addition to a more comprehensive understanding of LP-N, this paper further reveals the high-pressure path-dependent characteristics of polymeric nitrogen.
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图 1 (a) 氮的p-T相图与聚合氮的合成条件（相图的部分数据来自文献[13–14, 16–17, 34, 37–39]），(b)～(c) 双面LHDAC实验示意图，(d)～(e) 激光加热过程中的光学照片，(f) 黑体辐射法测定温度的拟合曲线
Figure 1. (a) p-T phase diagram of nitrogen and the synthesis conditions of polymeric nitrogen (Part of the phase diagram is obtained from Refs. [13–14, 16–17, 34, 37–39].); (b)−(c) schematic diagram of double-sided LHDAC experiments; (d)−(e) optical photos during laser heating; (f) fitting curve of temperature determined by black body radiation method
图 2 (a) 激光加热前样品在55、115和140 GPa压力下的拉曼光谱，(b) 激光加热后样品在141 GPa压力下的拉曼光谱（插图为对应压力下样品腔的光学图像）
Figure 2. (a) Raman spectra of sample at 55, 115 and 140 GPa before laser heating, (b) Raman spectra of sample at 141 GPa after laser heating (Inset: optical images of sample chamber at corresponding pressure respectively.)
图 3 (a) 激光加热后样品在卸压过程中的高压XRD谱，(b) 样品卸压至123 GPa时的XRD精修谱，(c) BP-N和LP-N在123 GPa压力下的计算XRD谱与对应的晶体结构（插图为二维XRD图）
Figure 3. (a) High-pressure XRD spectra of the sample after laser heating on decompression, (b) XRD refined spectrum of the sample decompressed to 123 GPa, (c) calculated XRD spectra of BP-N and LP-N at 123 GPa and the corresponding crystal structure (Inset: two dimension XRD diagram)
图 4 聚合氮cg-N、LP-N、HLP-N和BP-N的原子体积随压力的变化关系（插图为聚合氮cg-N、BP-N和LP-N的相对计算焓值（ΔH），以LP-N（HPba2）的焓值[30]为基准）
Figure 4. (a) Volume per atom dependence of pressure on polymeric nitrogen cg-N, LP-N, HLP-N and BP-N (Inset: the relative calculated enthalpy value (ΔH) of polymeric nitrogen cg-N, BP-N and LP-N based on LP-N (HPba2) [30])
表 1 由XRD谱精修得到的晶格参数
Table 1. Lattice parameters obtained from Rietveld refinement
Material Space group a/Å b/Å c/Å α/(°) β/(°) γ/(°) V/Å3 LP-N Pba2 4.2403(2) 4.3840(7) 4.4600(7) 90 90 90 82.93(5) cg-N I213 3.1501(3) 3.1501(3) 3.1501(3) 90 90 90 31.25(9) Re P63/mmc 2.8391(3) 2.8391(3) 3.8520(4) 90 90 120 26.89(2) 
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