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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.

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The oxygen evolution reaction (OER) plays a key role in many electrochemical energy conversion systems, but it is a kinetically sluggish reaction and requires a large overpotential to deliver appreciable current, especially for the non‐noble metal electrocatalysts. In this study, the authors report a surface phase engineering strategy to improve the OER performance of transition metal nitrides (TMNs). The iron‐nickel nitrides/alloy nanospheres (FeNi3‐N) wrapped in carbon are synthesized, and the optimized FeNi3‐N catalyst displays dual‐phase nitrides on the surface induced by atom migration phenomenon, resulting from the different migration rates of metal atoms during the nitridation process. It shows excellent OER performance in alkaline media with an overpotential of 222 mV at 10 mA cm−2, a small Tafel slope of 41.53 mV dec−1, and long‐term durability under high current density (>0.5 A cm−2) for at least 36 h. Density functional theory (DFT) calculations further reveal that the dual‐phase nitrides are favorable to decrease the energy barrier, modulate the d‐band center to balance the absorption and desorption of the intermediates, and thus promote the OER electrochemical performance. This strategy may shed light on designing OER and other catalysts based on surface phase engineering.

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Transition-metal (TM) phosphides attract increasing attention with applications for energy conversion and storage, due to their outstanding physical, chemical, and electronic properties. The 3d transition metal tetraphosphides (TMP, TM = V, Cr, Mn, and Fe) possess multiple allotropies and rich electronic properties. Here, we perform the investigations of the structural, electronic, and elastic properties for 3d-TMP (TM = V, Cr, Mn, and Fe) using density functional theory (DFT) calculations. These compounds are featured with alternating buckled phosphorus sheets with ten-numbered phosphorus rings and varied transition-metal layers. Hybrid DFT calculations reveal that TMP compounds exhibit a wide range of electrical properties, ranging from metallic behavior for VP to semiconducting behavior for CrP with the narrow direct band gap of 0.63 eV to enlarged semiconducting MnP and FeP with band gap of 1.6-2.1 eV. The bonding analysis indicates that P-P and TM-P covalent interactions dominate in the phosphorus sheets and TMP octahedrons, which are responsible for the structural and electronic diversity.

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