Synthesis of FeN₄ at 180 GPa and its crystal structure from a submicron-sized grain

Iron tetranitride, FeN4, 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\overline{1}. In the asymmetric unit, the Fe atom occupies an inversion centre (Wyckoff position 1d), while two N atoms occupy general positions (2i). The structure is made up from edge-sharing [FeN6] octahedra forming chains along [100] and being interconnected through N—N bridges. N atoms form catena-poly[tetraz-1-ene-1,4-diyl] anions [–N=N—N—N–]∞2− running along [001]. In comparison with the previously reported structure of FeN4 at 135 GPa [Bykov et al. (2018). Nat. Commun. 9, 2756], the crystal structure of FeN4 at 180 GPa is similar but the structural model is significantly improved in terms of the precision of the bond lengths and angles

Crystal structures of cristobalite-type and coesite-type PON redetermined on the basis of single-crystal X-ray diffraction data

Hitherto, phosphorus oxonitride (PON) could not be obtained in the form of single crystals and only powder diffraction experiments were feasible for structure studies. In the present work we have synthesized two polymorphs of phosphorus oxonitride, cristobalite-type (cri-PON) and coesite-type (coe-PON), in the form of single crystals and reinvestigated their crystal structures by means of in house and synchrotron single-crystal X-ray diffraction. The crystal structures of cri-PON and coe-PON are built from PO2N2 tetrahedral units, each with a statistical distribution of oxygen and nitrogen atoms. The crystal structure of the coe-PON phase has the space group C2/c with seven atomic sites in the asymmetric unit [two P and three (N,O) sites on general positions, one (N,O) site on an inversion centre and one (N,O) site on a twofold rotation axis], while the cri-PON phase possesses tetragonal I-42d symmetry with two independent atoms in the asymmetric unit [the P atom on a fourfold inversion axis and the (N,O) site on a twofold rotation axis]. In comparison with previous structure determinations from powder data, all atoms were refined with anisotropic displacement parameters, leading to higher precision in terms of bond lengths and angles

Lonsdaleite is faulted and twinned cubic diamond and does not exist as a discrete material

Lonsdaleite, also called hexagonal diamond, has been widely used as a marker of asteroidal impacts. It is thought to play a central role during the graphite-to-diamond transformation, and calculations suggest that it possesses mechanical properties superior to diamond. However, despite extensive efforts, lonsdaleite has never been produced or described as a separate, pure material. Here we show that defects in cubic diamond provide an explanation for the characteristic d-spacings and reflections reported for lonsdaleite. Ultrahigh-resolution electron microscope images demonstrate that samples displaying features attributed to lonsdaleite consist of cubic diamond dominated by extensive {113} twins and {111} stacking faults. These defects give rise to nanometre-scale structural complexity. Our findings question the existence of lonsdaleite and point to the need for re-evaluating the interpretations of many lonsdaleite-related fundamental and applied studies

High-pressure polymeric nitrogen allotrope with the black phosphorus structure

Studies of polynitrogen phases are of great interest for fundamental science and for the design of novel high energy density materials. Laser heating of pure nitrogen at 140 GPa in a diamond anvil cell led to the synthesis of a polymeric nitrogen allotrope with the black phosphorus structure, bp-N. The structure was identified in situ using synchrotron single-crystal X-ray diffraction and further studied by Raman spectroscopy and density functional theory calculations. The discovery of bp-N brings nitrogen in line with heavier pnictogen elements, resolves incongruities regarding polymeric nitrogen phases and provides insights into polynitrogen arrangements at extreme densities

Synthesis, crystal structure and structure-property relations of strontium orthocarbonate, Sr₂CO₄

Carbonates containing CO4 groups as building blocks have recently been discovered. A new orthocarbonate, Sr2CO4 is synthesized at 92 GPa and at a temperature of 2500 K. Its crystal structure was determined by in situ synchrotron single-crystal X-ray diffraction, selecting a grain from a polycrystalline sample. Strontium orthocarbonate crystallizes in the orthorhombic crystal system (space group Pnma) with CO4, SrO9 and SrO11 polyhedra as the main building blocks. It is isostructural to Ca2CO4. DFT calculations reproduce the experimental findings very well and have, therefore, been used to predict the equation of state, Raman and IR spectra, and to assist in the discussion of bonding in this compound.Funding Agencies|Alexander von Humboldt-StiftungAlexander von Humboldt Foundation; Bundesministerium fur Bildung und ForschungFederal Ministry of Education &amp; Research (BMBF) [05K19WC1]; Deutsche ForschungsgemeinschaftGerman Research Foundation (DFG) [DU 954-11/1, DU 393-9/2, DU 393-13/1, FOR2125, WI1232]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]</p