Group V elements in crystal structure isostructural to black phosphorus with unique puckered two-dimensional layers exhibit exciting physical and chemical phenomena. However, as the first element of group V, nitrogen has never been found in the black phosphorus structure. Here, we report the synthesis of the black phosphorus–structured nitrogen at 146 GPa and 2200 K. Metastable black phosphorus–structured nitrogen was retained after quenching it to room temperature under compression and characterized in situ during decompression to 48 GPa, using synchrotron x-ray diffraction and Raman spectroscopy. We show that the original molecular nitrogen is transformed into extended single-bonded structure through gauche and trans conformations. Raman spectroscopy shows that black phosphorus–structured nitrogen is strongly anisotropic and exhibits high Raman intensities in two Ag normal modes. Synthesis of black phosphorus–structured nitrogen provides a firm base for exploring new type of high-energy-density nitrogen and a new direction of two-dimensional nitrogen.

High performance composite of nanosized Li 4 Ti 5 O 12 (LTO)and graphene nanosheets wasfabricated using a novel atomic layer deposition (ALD) seeded process incorporated with hydrothermal lithiation for the first time.TiO 2 nanoislands as seeds were anchored on graphene by ALD process, triggering the unique structure formation of subsequent LTO. The synergistic effects of nanosized LTO and graphene endow the composite with a short lithium ion diffusion path and efficiently conductive network for electron and ion transport, boosting the excellent reversible capacity, rate capability, and cyclic stability as anode materials for lithium ion capacitors (LICs). The reversible capacity of 120.8 mAh g-1 at an extremely high current rate of 100 C was achievedsuccessfully, and the electrode can be charged/discharged to about 70% of the theoretical capacity of LTO in 25 s. Meanwhile, the composite exhibited excellent cyclic stability of 90% capacity retention at 20 C with nearly 100% Coulombic efficiency after 2500 cycles. The sintering treatment after hydrothermal reaction has significant effects on the crystallinity, defect density, microstructureandelectrochemical property of the composite, which is also supported by theoreticalcalculations. The results provide a versatile roadmap for synthesis of high performance LTO based composite and new insights into LICs.

Nitride materials are of considerable interest due to their fundamental importance and practical applications. However, synthesis of transition metal nitrides often requires extreme conditions, e.g., high temperature and/or high pressure, slowing down the experimental discovery. Using global structure search methods in combination with first-principles calculations, we systematically explore the stoichiometric phase space of iron–nitrogen compounds on the nitrogen-rich side at ambient and high pressures up to 100 GPa. Diverse stoichiometries in the Fe–N system are found to emerge in the phase diagram at high pressures. Significantly, FeN4 is found to be stable already at ambient pressure. It undergoes a polymerization near 20 GPa which results in a high energy density. Accompanying the polymerization, FeN4 transforms from a direct band gap semiconductor to ferromagnetic metal. We also predict several phase transitions in FeN and FeN2 at high pressure, and the results explain the previous experimental observations by comparing the X-ray diffraction patterns. Stepwise formation of polynitrogen species is observed following the increment of nitrogen content in the stoichiometry, from isolated N atoms in FeN, to the N2 unit in FeN2 and Fe3N8, to the N6 unit in Fe3N8 and FeN3, and to the N∞ chain in FeN4, FeN6, and FeN8. Ultra-incompressibility is found in marcasite-FeN2, FeN3, and FeN4 along particular crystalline directions, while high energy density, 1.37–2.02 kJ g–1, is expected for FeN4, FeN6, and FeN8. Our results shed light on understanding the chemistry of transition metal polynitrides under pressure and encourage experimental synthesis of newly predicted iron nitrides in the near future.

The discovery of electrides, in particular, inorganic electrides where electrons substitute anions, has inspired striking interests in the systems that exhibit unusual electronic and catalytic properties. So far, however, the experimental studies of such systems are largely restricted to ambient conditions, unable to understand their interactions between electron localizations and geometrical modifications under external stimuli, e.g., pressure. Here, pressure‐induced structural and electronic evolutions of Ca2N by in situ synchrotron X‐ray diffraction and electrical resistance measurements, and density functional theory calculations with particle swarm optimization algorithms are reported. Experiments and computation are combined to reveal that under compression, Ca2N undergoes structural transforms from R 3true¯ m symmetry to I 4true¯2d phase via an intermediate Fd 3true¯ m phase, and then to Cc phase, accompanied by the reductions of electronic dimensionality from 2D, 1D to 0D. Electrical resistance measurements support a metal‐to‐semiconductor transition in Ca2N because of the reorganizations of confined electrons under pressure, also validated by the calculation. The results demonstrate unexplored experimental evidence for a pressure‐induced metal‐to‐semiconductor switching in Ca2N and offer a possible strategy for producing new electrides under moderate pressure.

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