Poly-nitrogen compounds have been considered as potential high energy density materials for a long time due to the large number of energetic N–N or N=N bonds. In most cases high nitrogen content and stability at ambient conditions are mutually exclusive, thereby making the synthesis of such materials challenging. One way to stabilize such compounds is the application of high pressure. Here, through a direct reaction between Fe and N₂ in a laser-heated diamond anvil cell, we synthesize three ironnitrogen compounds Fe₃N₂, FeN₂ and FeN₄. Their crystal structures are revealed by single-crystal synchrotron X-ray diffraction. Fe₃N₂, synthesized at 50 GPa, is isostructural to chromium carbide Cr₃C₂. FeN₂ has a marcasite structure type and features covalently bonded dinitrogen units in its crystal structure. FeN₄, synthesized at 106 GPa, features polymeric nitrogen chains of [N₄²⁻]_n units. Based on results of structural studies and theoretical analysis, [N₄²⁻]_n units in this compound reveal catena-poly[tetraz-1-ene-1,4-diyl] anions.

Although chemically very simple, Fe₂O₃ is known to undergo a series of enigmatic structural, electronic and magnetic transformations at high pressures and high temperatures. So far, these transformations have neither been correctly described nor understood because of the lack of structural data. Here we report a systematic investigation of the behaviour of Fe₂O₃ at pressures over 100 GPa and temperatures above 2,500 K employing single crystal X-ray diffraction and synchrotron Mössbauer source spectroscopy. Crystal chemical analysis of structures presented here and known Fe(II, III) oxides shows their fundamental relationships and that they can be described by the homologous series nFeO·mFe₂O₃. Decomposition of Fe₂O₃ and Fe₃O₄ observed at pressures above 60 GPa and temperatures of 2,000 K leads to crystallization of unusual Fe₅O₇ and Fe₂₅O₃₂ phases with release of oxygen. Our findings suggest that mixed-valence iron oxides may play a significant role in oxygen cycling between earth reservoirs.

Deep divide in fate of iron: A large component of Earth's atmosphere comes from the interior, where the gas species are dictated by the redox state of the mantle. After formation of Earth's iron core, the mantle became several orders of magnitude more oxidized. Armstrong et al. conducted a set of experiments looking at the redox state of silicate melt representative of Earth's early magma oceans. They found that at some depth, iron oxide disproportionates into iron(III) oxide and metallic iron. The reduced iron sinks to the core, leaving an oxidized rocky mantle that emits carbon dioxide and water instead of more reduced species. Science , this issue p. 903

Reduction and oxidation (redox) reactions play an important role in geological systems, such as Earth9s mantle. In her Perspective, McCammon reviews recent advances toward understanding the oxidation state of the upper and the lower mantle. Apparently contradictory observations can be understood when one takes into account the ability of the most abundant minerals to take up iron in different oxidation states. The results shed light on how Earth9s mantle became oxidized and may provide clues to the early history of our planet, including the rise of atmospheric oxygen.