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Magnetite, Fe 3 O 4 , is the oldest known magnetic mineral and archetypal mixed-valence oxide. Despite its recognized role in deep Earth processes, the behavior of magnetite at extreme high-pressure high-temperature (HPHT) conditions remains insufficiently studied. Here, we report on single-crystal synchrotron X-ray diffraction experiments up to ∼80 GPa and 5000 K in diamond anvil cells, which reveal two previously unknown Fe 3 O 4 polymorphs, γ-Fe 3 O 4 with the orthorhombic Yb 3 S 4 -type structure and δ-Fe 3 O 4 with the modified Th 3 P 4 -type structure. The latter has never been predicted for iron compounds. The decomposition of Fe 3 O 4 at HPHT conditions was found to result in the formation of exotic phases, Fe 5 O 7 and Fe 25 O 32 , with complex structures. Crystal-chemical analysis of iron oxides suggests the high-spin to low-spin crossover in octahedrally coordinated Fe 3+ in the pressure interval between 43 and 51 GPa. Our experiments demonstrate that HPHT conditions promote the formation of ferric-rich Fe-O compounds, thus arguing for the possible involvement of magnetite in the deep oxygen cycle.

High-pressure nitrogen chemistry has expanded at a formidable rate over the past decade, unveiling the chemical richness of nitrogen. Here, the Zn-N system is investigated in laser-heated diamond anvil cells by synchrotron powder and single-crystal X-ray diffraction, revealing three hitherto unobserved nitrogen compounds: β-Zn 3 N 2 , α-ZnN 4 , and β-ZnN 4 , formed at 35.0, 63.5, and 81.7 GPa, respectively. Whereas β-Zn 3 N 2 contains the N 3- nitride, both ZnN 4 solids are found to be composed of polyacetylene-like [ N 4 ] ∞ 2- chains. Upon the decompression of β-ZnN 4 below 72.7 GPa, a first-order displacive phase transition is observed from β-ZnN 4 to α-ZnN 4 . The α-ZnN 4 phase is detected down to 11.0 GPa, at lower pressures decomposing into the known α-Zn 3 N 2 (space group Ia 3̅) and N 2 . The equations of states of β-ZnN 4 and α-ZnN 4 are also determined, and their bulk moduli are found to be K 0 = 126(9) GPa and K 0 = 76(12) GPa, respectively. Density functional theory calculations were also performed and provide further insight into the Zn-N system. Moreover, comparing the Mg-N and Zn-N systems underlines the importance of minute chemical differences between metal cations in the resulting synthesized phases.

At high pressures, autoionization - along with polymerization and metallization - is one of the responses of simple molecular systems to a rise in electron density. Nitro-sonium nitrate (NO + NO 3 - ), known for this property, has attracted a large interest in recent decades and was reported to be synthesized at high pressure and high temperature from a variety of nitro-gen-oxygen precursors, such as N 2 O 4 , N 2 O and N 2 -O 2 mixtures. However, its structure has not been determined unambiguously. Here, we present the first structure solution and refinement for nitro-sonium nitrate on the basis of single-crystal X-ray diffraction at 7.0 and 37.0 GPa. The structure model ( P 2 1 / m space group) contains the triple-bonded NO + cation and the NO 3 - sp 2 -trigonal planar anion. Remarkably, crystal-chemical considerations and accompanying density-functional-theory calculations show that the oxygen atom of the NO + unit is positively charged - a rare occurrence when in the presence of a less-electronegative element., (© Laniel et al. 2021.)

Feldspars are rock-forming minerals that make up most of the Earth's crust. Along the mantle geotherm, feldspars are stable at pressures up to 3 GPa and may persist metastably at higher pressures under cold conditions. Previous structural studies of feldspars are limited to ~10 GPa, and have shown that the dominant mechanism of pressure-induced deformation is the tilting of AlO 4 and SiO 4 tetrahedra in a tetrahedral framework. Herein, based on results of in situ single-crystal X-ray diffraction studies up to 27 GPa, we report the discovery of new high-pressure polymorphs of the feldspars anorthite (CaSi 2 Al 2 O 8 ), albite (NaAlSi 3 O 8 ) , and microcline (KAlSi 3 O 8 ). The phase transitions are induced by severe tetrahedral distortions, resulting in an increase in the Al and/or Si coordination number. High-pressure phases derived from feldspars could persist at depths corresponding to the Earth upper mantle and could possibly influence the dynamics and fate of cold subducting slabs.

This study is devoted to the detailed in situ Raman spectroscopy investigation of propane C 3 H 8 in laser-heated diamond anvil cells in the range of pressures from 3 to 22 GPa and temperatures from 900 to 3000 K. We show that propane, while being exposed to particular thermobaric conditions, could react, leading to the formation of hydrocarbons, both saturated and unsaturated as well as soot. Our results suggest that propane could be a precursor of heavy hydrocarbons and will produce more than just sooty material when subjected to extreme conditions. These results could clarify the issue of the presence of heavy hydrocarbons in the Earth's upper mantle.

The synthesis of polynitrogen compounds is of fundamental importance due to their potential as environmentally-friendly high energy density materials. Attesting to the intrinsic difficulties related to their formation, only three polynitrogen ions, bulk stabilized as salts, are known. Here, magnesium and molecular nitrogen are compressed to about 50 GPa and laser-heated, producing two chemically simple salts of polynitrogen anions, MgN 4 and Mg 2 N 4 . Single-crystal X-ray diffraction reveals infinite anionic polythiazyl-like 1D N-N chains in the crystal structure of MgN 4 and cis-tetranitrogen N 4 4- units in the two isosymmetric polymorphs of Mg 2 N 4 . The cis-tetranitrogen units are found to be recoverable at atmospheric pressure. Our results respond to the quest for polynitrogen entities stable at ambient conditions, reveal the potential of employing high pressures in their synthesis and enrich the nitrogen chemistry through the discovery of other nitrogen species, which provides further possibilities to design improved polynitrogen arrangements.

Sulfur and nitrogen represent one of the most studied inorganic binary systems at ambient pressure on account of their large wealth of metastable exotic ring-like compounds. Under high pressure conditions, however, their behavior is unknown. Here, sulfur and nitrogen were compressed in a diamond anvil cell up to about 120 GPa and laser-heated at regular pressure intervals in an attempt to stabilize novel sulfur-nitrogen compounds. Above 64 GPa, an orthorhombic (space group Pnnm ) SN 2 compound was synthesized and characterized by single-crystal and powder X-ray diffraction as well as Raman spectroscopy. It is shown to adopt a CaCl 2 -type structure-hence it is isostructural, isomassic, and isoelectronic to CaCl 2 -type SiO 2 -comprised of SN 6 octahedra. Complementary theoretical calculations were performed to provide further insight into the physicochemical properties of SN 2 , notably its equation of state, the bonding type between its constitutive elements, and its electronic density of states. This new solid is shown to be metastable down to about 20 GPa, after which it spontaneously decomposes into S and N 2 . This investigation shows that despite the many metastable S-N compounds existing at ambient conditions, none of them are formed by pressure.

High-pressure synthesis in diamond anvil cells can yield unique compounds with advanced properties, but often they are either unrecoverable at ambient conditions or produced in quantity insufficient for properties characterization. Here we report the synthesis of metallic, ultraincompressible (K 0  = 428(10) GPa), and very hard (nanoindentation hardness 36.7(8) GPa) rhenium nitride pernitride Re 2 (N 2 )(N) 2 . Unlike known transition metals pernitrides Re 2 (N 2 )(N) 2 contains both pernitride N 2 4- and discrete N 3- anions, which explains its exceptional properties. Re 2 (N 2 )(N) 2 can be obtained via a reaction between rhenium and nitrogen in a diamond anvil cell at pressures from 40 to 90 GPa and is recoverable at ambient conditions. We develop a route to scale up its synthesis through a reaction between rhenium and ammonium azide, NH 4 N 3 , in a large-volume press at 33 GPa. Although metallic bonding is typically seen incompatible with intrinsic hardness, Re 2 (N 2 )(N) 2 turned to be at a threshold for superhard materials.

Iron tetra-nitride, FeN 4 , was synthesized from the elements in a laser-heated diamond anvil cell at 180 (5) GPa and 2700 (200) K. Its crystal structure was determined based on single-crystal X-ray diffraction data collected from a submicron-sized grain at the synchrotron beamline ID11 of ESRF. The compound crystallizes in the triclinic space group P . In the asymmetric unit, the Fe atom occupies an inversion centre (Wyckoff position 1 d ), while two N atoms occupy general positions (2 i ). The structure is made up from edge-sharing [FeN 6 ] octa-hedra forming chains along [100] and being inter-connected through N-N bridges. N atoms form catena -poly[tetraz-1-ene-1,4-di-yl] anions [-N=N-N-N-] ∞ 2- running along [001]. In comparison with the previously reported structure of FeN 4 at 135 GPa [Bykov et al. (2018). Nat. Commun. 9 , 2756], the crystal structure of FeN 4 at 180 GPa is similar but the structural model is significantly improved in terms of the precision of the bond lengths and angles.