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.

The presence of carbonates in inclusions in diamonds coming from depths exceeding 670 km are obvious evidence that carbonates exist in the Earth’s lower mantle. However, their range of stability, crystal structures and the thermodynamic conditions of the decarbonation processes remain poorly constrained. Here we investigate the behaviour of pure iron carbonate at pressures over 100 GPa and temperatures over 2,500 K using single-crystal X-ray diffraction and Mössbauer spectroscopy in laser-heated diamond anvil cells. On heating to temperatures of the Earth’s geotherm at pressures to ∼50 GPa FeCO₃ partially dissociates to form various iron oxides. At higher pressures FeCO₃ forms two new structures—tetrairon(III) orthocarbonate Fe₄³⁺C₃O₁₂, and diiron(II) diiron(III) tetracarbonate Fe₂²⁺Fe₂³⁺C₄O₁₃, both phases containing CO₄ tetrahedra. Fe₄C₄O₁₃ is stable at conditions along the entire geotherm to depths of at least 2,500 km, thus demonstrating that self-oxidation-reduction reactions can preserve carbonates in the Earth’s lower mantle.

Carbon-bearing solids, fluids, and melts in the Earth's deep interior may play an important role in the long-term carbon cycle. Here we apply synchrotron X-ray single crystal micro-diffraction techniques to identify and characterize the high-pressure polymorphs of dolomite. Dolomite-II, observed above 17 GPa, is triclinic, and its structure is topologically related to CaCO ₃ -II. It transforms above 35 GPa to dolomite-III, also triclinic, which features carbon in [3 + 1] coordination at the highest pressures investigated (60 GPa). The structure is therefore representative of an intermediate between the low-pressure carbonates and the predicted ultra-high pressure carbonates, with carbon in tetrahedral coordination. Dolomite-III does not decompose up to the melting point (2,600 K at 43 GPa) and its thermodynamic stability demonstrates that this complex phase can transport carbon to depths of at least up to 1,700 km. Dolomite-III, therefore, is a likely occurring phase in areas containing recycled crustal slabs, which are more oxidized and Ca-enriched than the primitive lower mantle. Indeed, these phases may play an important role as carbon carriers in the whole mantle carbon cycling. As such, they are expected to participate in the fundamental petrological processes which, through carbon-bearing fluids and carbonate melts, will return carbon back to the Earth’s surface.

Significance: A new technique has been developed to measure in situ the density of amorphous material composed of light elements under extreme conditions of pressure using the X-ray absorption method. At core–mantle boundary (CMB) pressure, the densities of MgSiO ₃ glass and melts are similar to the one of the crystalline bridgmanite, within uncertainty. Due to the affinity of iron oxide for silicate liquids, melting in the MgSiO ₃ –FeSiO ₃ system will produce dense melts that could accumulate above the CMB, leading to the formation of a dense basal magma ocean in the early Earth's mantle.