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Register a new userHigh-pressure polymeric nitrogen allotrope with the black phosphorus structure
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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.
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In a semimetal, both electron and hole carriers contribute to the density of states at the Fermi level. The small band overlaps and multi-band effects give rise to many novel electronic properties, such as relativistic Dirac fermions with linear dispersion, titanic magnetoresistance and unconventional superconductivity. Black phosphorus has recently emerged as an exceptional semiconductor with high carrier mobility and a direct, tunable bandgap. Of particular importance is the search for exotic electronic states in black phosphorus, which may amplify the materials potential beyond semiconductor devices. Here we show that a moderate hydrostatic pressure effectively suppresses the band gap and induces a Lifshitz transition from semiconductor to semimetal in black phosphorus; a colossal magnetoresistance is observed in the semimetallic phase. Quantum oscillations in high magnetic field reveal the complex Fermi surface topology of the semimetallic black phosphorus. In particular, a Dirac-like fermion emerges at around 1.2 GPa, which is continuously tuned by external pressure. The observed semi-metallic behavior greatly enriches black phosphoruss material property, and sets the stage for the exploration of novel electronic states in this material. Moreover, these interesting behaviors make phosphorene a good candidate for the realization of a new two-dimensional relativistic electron system, other than graphene.
The unique optoelectronic properties of black phosphorus (BP) have triggered great interest in its applications in areas not fulfilled by other layered materials (LMs). However, its poor stability (fast degradation, i.e. <<1 h for monolayers) under ambient conditions restricts its practical application. We demonstrate here, by an experimental-theoretical approach, that the incorporation of nitrogen molecules (N2) into the BP structure results in a relevant improvement of its stability in air, up to 8 days without optical degradation signs. Our strategy involves the generation of defects (phosphorus vacancies) by electron-beam irradiation, followed by their healing with N2 molecules. As an additional route, N2 plasma treatment is presented as an alternative for large area application. Our first principles calculations elucidate the mechanisms involved in the nitrogen incorporation as well as on the stabilization of the modified BP, which corroborates with our experimental observations. This stabilization approach can be applied in the processing of BP, allowing for its use in environmentally stable van der Waals heterostructures with other LMs as well as in optoelectronic and wearable devices.
Theoretical predictions of pressure-induced phase transformations often become long-standing enigmas because of limitations of contemporary available experimental possibilities. Hitherto the existence of a non-icosahedral boron allotrope has been one of them. Here we report on the first non-icosahedral boron allotrope, which we denoted as {zeta}-B, with the orthorhombic {alpha}-Ga-type structure (space group Cmce) synthesized in a diamond anvil cell at extreme high-pressure high-temperature conditions (115 GPa and 2100 K). The structure of {zeta}-B was solved using single-crystal synchrotron X-ray diffraction and its compressional behavior was studied in the range of very high pressures (115 GPa to 135 GPa). Experimental validation of theoretical predictions reveals the degree of our up-to-date comprehension of condensed matter and promotes further development of the solid state physics and chemistry.
Two-dimensional layered semiconductor black phosphorus (BP), a promising pressure induced Dirac system as predicted by band structure calculations, has been studied by $^{31}$P-nuclear magnetic resonance. Band calculations have been also carried out to estimate the density of states $D(E)$. The temperature and pressure dependences of nuclear spin lattice relaxation rate $1/T_1$ in the semiconducting phase are well reproduced using the derived $D(E)$, and the resultant pressure dependence of semiconducting gap is in good accordance with previous reports, giving a good confirmation that the band calculation on BP is fairly reliable. The present analysis of $1/T_1$ data with the complemental theoretical calculations allows us to extract essential information, such as the pressure dependences of $D(E)$ and chemical potential, as well as to decompose observed $1/T_1$ into intrinsic and extrinsic contributions. An abrupt increase in $1/T_1$ at 1.63GPa indicates that the semiconducting gap closes, resulting in an enhancement of conductivity.
The energy landscape of helium-nitrogen mixtures is explored by ab initio evolutionary searches, which predicted several stable helium-nitrogen compounds in the pressure range from 25 to 100 GPa. In particular, the monoclinic structure of HeN$_{22}$ consists of neutral He atoms, partially ionic dimers N$_{2}$$^{delta-}$, and lantern-like cages N$_{20}$$^{delta+}$. The presence of helium not only greatly enhances structural diversity of nitrogen solids, but also tremendously lowers the formation pressure of nitrogen salt. The unique nitrogen framework of (HeN$_{20}$)$^{delta+}$N$_{2}$$^{delta-}$ may be quenchable to ambient pressure even after removing helium. The estimated energy density of N$_{20}$$^{delta+}$N$_{2}$$^{delta-}$ (10.44 kJ/g) is $sim$2.4 times larger than that of trinitrotoluene (TNT), indicating a very promising high-energy-density material.
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