Building Nitridic Networks with Phosphorus and Germanium-from GeIIP2N4 to GeIVPN3.

Nitridophosphates and nitridogermanates attract high interest in current research due to their structural versatility. Herein, the elastic properties of GeP2N4 were investigated by single-crystal X-ray diffraction (XRD) upon compression to 44.4(1) GPa in a diamond anvil cell. Its isothermal bulk modulus was determined to be 82(6) GPa. At 44.4(1) GPa, laser heating resulted in the formation of multiple crystalline phases, one of which was identified as unprecedented germanium nitridophosphate GePN3. Its structure was elucidated from single-crystal XRD data (C2/c (no. 15), a = 8.666(5), b = 8.076(4), c = 4.691(2) Å, ß = 101.00(7)°) and is built up from layers of GeN6 octahedra and PN4 tetrahedra. The GeN6 octahedra form double zigzag chains, while the PN4 tetrahedra are found in single zigzag chains. GePN3 can be recovered to ambient conditions with a unit cell volume increase of about 12%. It combines PV and GeIV in a condensed nitridic network for the first time.

High-pressure Synthesis of Cobalt Polynitrides: Unveiling Intriguing Crystal Structures and Nitridation Behavior.

In this study, we conduct extensive high-pressure experiments to investigate phase stability in the cobalt-nitrogen system. Through a combination of synthesis in a laser-heated diamond anvil cell, first-principles calculations, Raman spectroscopy, and single-crystal X-ray diffraction, we establish the stability fields of known high-pressure phases, hexagonal NiAs-type CoN, and marcasite-type CoN2 within the pressure range of 50-90 GPa. We synthesize and characterize previously unknown nitrides, Co3N2, Pnma-CoN and two polynitrides, CoN3 and CoN5, within the pressure range of 90-120 GPa. Both polynitrides exhibit novel types of polymeric nitrogen chains and networks. CoN3 feature branched-type nitrogen trimers (N3) and CoN5 show π-bonded nitrogen chain. As the nitrogen content in the cobalt nitride increases, the CoN6 polyhedral frameworks transit from face-sharing (in CoN) to edge-sharing (in CoN2 and CoN3), and finally to isolated (in CoN5). Our study provides insights into the intricate interplay between structure evolution, bonding arrangements, and high-pressure synthesis in polynitrides, expanding the knowledge for the development of advanced energy materials.

Stabilization of Pr4+ in Silicates─High-Pressure Synthesis of PrSi3O8 and Pr2Si7O18.

The reaction between PrO2 and SiO2 was investigated at various pressure points up to 29 GPa in a diamond anvil cell using laser heating and in situ single-crystal structure analysis. The pressure points at 5 and 10 GPa produced Pr2III(Si2O7), whereas Pr4IIISi3O12 and Pr2IV(O2)O3 were obtained at 15 GPa. Pr4IIISi3O12 can be interpreted as a high-pressure modification of the still unknown orthosilicate Pr4III(SiO4)3. PrIVSi3O8 and Pr2IVSi7O18 that contain praseodymium in its rare + IV oxidation state were identified at 29 GPa. After the pressure was released from the reaction chamber, the Pr(IV) silicates could be recovered, indicating that they are metastable at ambient pressure. Density functional theory calculations of the electronic structure corroborate the oxidation state of praseodymium in both PrIVSi3O8 and Pr2IVSi7O18. Both silicates are the first structurally characterized representatives of Pr4+-containing salts with oxoanions. All three silicates contain condensed networks of [SiO6] octahedra which is unprecedented in the rich chemistry of lanthanoid silicates.

Stabilization of Guanidinate Anions [CN3 ]5- in Calcite-Type SbCN3.

The stabilization of nitrogen-rich phases presents a significant chemical challenge due to the inherent stability of the dinitrogen molecule. This stabilization can be achieved by utilizing strong covalent bonds in complex anions with carbon, such as cyanide CN- and NCN2- carbodiimide, while more nitrogen-rich carbonitrides are hitherto unknown. Following a rational chemical design approach, we synthesized antimony guanidinate SbCN3 at pressures of 32-38 GPa using various synthetic routes in laser-heated diamond anvil cells. SbCN3 , which is isostructural to calcite CaCO3 , can be recovered under ambient conditions. Its structure contains the previously elusive guanidinate anion [CN3 ]5- , marking a fundamental milestone in carbonitride chemistry. The crystal structure of SbCN3 was solved and refined from synchrotron single-crystal X-ray diffraction data and was fully corroborated by theoretical calculations, which also predict that SbCN3 has a direct band gap with the value of 2.20 eV. This study opens a straightforward route to the entire new family of inorganic nitridocarbonates.

Effect of Hydroxyl-Containing Fragments on the Structure and Properties of Membrane-Forming Polyamide-Imides.

The structural features and thermophysical and transport properties of dense nonporous membranes of the casting type from (co)polyamide-imides synthesized by the polycondensation of the diacid chloride of 2-(4-carboxyphenyl)-1,3-dioxoisoindoline-5-carboxylic acid and diamines 5,5'-methylene-bis (2-aminophenol) (DADHyDPhM) and 4,4'-methylenebis(benzeneamine) (DADPhM), taken in molar ratios of 7:3, 1:1, and 3:7, have been studied. The effect of hydroxyl-containing modifying fragments of dihydroxy diphenylmethane introduced in various amounts into the main polymer chain on the pervaporation properties of the formed films is discussed. It has been shown that the presence of the residual solvent N-methyl-2-pyrrolidone in the films not only has a plasticizing effect on the characteristics of film membranes but also promotes the preferential transmembrane transport of polar liquids, primarily methanol (permeation rate over 2 kg for a copolymer with a ratio of DADHyDPhM:DADPhM = 7:3). The removal of the residual solvent from the polymer film, both thermally (heating to 200 °C) and by displacement with another solvent as a result of sequential pervaporation, led to a significant decrease in the rate of transfer of polar liquids and a decrease in the selectivity of the membrane. However, the dehydrocyclization reaction resulted in more brittle films with low permeability to penetrants of different polarities. The results of our comprehensive study made it possible to assume the decisive influence of structural changes in membranes occurring in connection with the competitive formation of intra- and intermolecular hydrogen bonds.

Unraveling the Bonding Complexity of Polyhalogen Anions: High-Pressure Synthesis of Unpredicted Sodium Chlorides Na2Cl3 and Na4Cl5 and Bromide Na4Br5.

The field of polyhalogen chemistry, specifically polyhalogen anions (polyhalides), is rapidly evolving. Here, we present the synthesis of three sodium halides with unpredicted chemical compositions and structures (tP10-Na2Cl3, hP18-Na4Cl5, and hP18-Na4Br5), a series of isostructural cubic cP8-AX3 halides (NaCl3, KCl3, NaBr3, and KBr3), and a trigonal potassium chloride (hP24-KCl3). The high-pressure syntheses were realized at 41-80 GPa in diamond anvil cells laser-heated at about 2000 K. Single-crystal synchrotron X-ray diffraction (XRD) provided the first accurate structural data for the symmetric trichloride Cl3- anion in hP24-KCl3 and revealed the existence of two different types of infinite linear polyhalogen chains, [Cl]∞n- and [Br]∞n-, in the structures of cP8-AX3 compounds and in hP18-Na4Cl5 and hP18-Na4Br5. In Na4Cl5 and Na4Br5, we found unusually short, likely pressure-stabilized, contacts between sodium cations. Ab initio calculations support the analysis of structures, bonding, and properties of the studied halogenides.

High-pressure synthesis of dysprosium carbides.

Chemical reactions between dysprosium and carbon were studied in laser-heated diamond anvil cells at pressures of 19, 55, and 58 GPa and temperatures of ∼2500 K. In situ single-crystal synchrotron X-ray diffraction analysis of the reaction products revealed the formation of novel dysprosium carbides, Dy4C3 and Dy3C2, and dysprosium sesquicarbide Dy2C3 previously known only at ambient conditions. The structure of Dy4C3 was found to be closely related to that of dysprosium sesquicarbide Dy2C3 with the Pu2C3-type structure. Ab initio calculations reproduce well crystal structures of all synthesized phases and predict their compressional behavior in agreement with our experimental data. Our work gives evidence that high-pressure synthesis conditions enrich the chemistry of rare earth metal carbides.

Electronic properties of single-crystalline Fe4O5.

We synthesized single and polycrystals of iron oxide with an unconventional Fe4O5 stoichiometry under high-pressure high-temperature (HP-HT) conditions. The crystals of Fe4O5 had a CaFe3O5-type structure composed of linear chains of iron with octahedral and trigonal-prismatic oxygen coordinations. We investigated the electronic properties of this mixed-valence oxide using several experimental techniques, including measurements of electrical resistivity, the Hall effect, magnetoresistance, and thermoelectric power (Seebeck coefficient), X-ray absorption near edge spectroscopy (XANES), reflectance and absorption spectroscopy, and single-crystal X-ray diffraction. Under ambient conditions, the single crystals of Fe4O5 demonstrated a semimetal electrical conductivity with nearly equal partial contributions of electrons and holes (σn ≈ σp), in line with the nominal average oxidation state of iron as Fe2.5+. This finding suggests that both the octahedral and trigonal-prismatic iron cations contribute to the electrical conductivity of Fe4O5via an Fe2+/Fe3+ polaron hopping mechanism. A moderate deterioration of crystal quality shifted the dominant electrical conductivity to n-type and considerably worsened the conductivity. Thus, alike magnetite, Fe4O5 with equal numbers of Fe2+ and Fe3+ ions can serve as a prospective model for other mixed-valence transition-metal oxides. In particular, it could help in the understanding of the electronic properties of other recently discovered mixed-valence iron oxides with unconventional stoichiometries, many of which are not recoverable to ambient conditions; it can also help in designing novel more complex mixed-valence iron oxides.

Novel Class of Rhenium Borides Based on Hexagonal Boron Networks Interconnected by Short B2 Dumbbells.

Transition metal borides are known due to their attractive mechanical, electronic, refractive, and other properties. A new class of rhenium borides was identified by synchrotron single-crystal X-ray diffraction experiments in laser-heated diamond anvil cells between 26 and 75 GPa. Recoverable to ambient conditions, compounds rhenium triboride (ReB3) and rhenium tetraboride (ReB4) consist of close-packed single layers of rhenium atoms alternating with boron networks built from puckered hexagonal layers, which link short bonded (∼1.7 Å) axially oriented B2 dumbbells. The short and incompressible Re-B and B-B bonds oriented along the hexagonal c-axis contribute to low axial compressibility comparable with the linear compressibility of diamond. Sub-millimeter samples of ReB3 and ReB4 were synthesized in a large-volume press at pressures as low as 33 GPa and used for material characterization. Crystals of both compounds are metallic and hard (Vickers hardness, H V = 34(3) GPa). Geometrical, crystal-chemical, and theoretical analysis considerations suggest that potential ReB x compounds with x > 4 can be based on the same principle of structural organization as in ReB3 and ReB4 and possess similar mechanical and electronic properties.

Verwey-Type Charge Ordering and Site-Selective Mott Transition in Fe4O5 under Pressure.

The metal-insulator transition driven by electronic correlations is one of the most fundamental concepts in condensed matter. In mixed-valence compounds, this transition is often accompanied by charge ordering (CO), resulting in the emergence of complex phases and unusual behaviors. The famous example is the archetypal mixed-valence mineral magnetite, Fe3O4, exhibiting a complex charge-ordering below the Verwey transition, whose nature has been a subject of long-time debates. In our study, using high-resolution X-ray diffraction supplemented by resistance measurements and DFT+DMFT calculations, the electronic, magnetic, and structural properties of recently synthesized mixed-valence Fe4O5 are investigated under pressure to ∼100 GPa. Our calculations, consistent with experiment, reveal that at ambient conditions Fe4O5 is a narrow-gap insulator characterized by the original Verwey-type CO. Under pressure Fe4O5 undergoes a series of electronic and magnetic-state transitions with an unusual compressional behavior above ∼50 GPa. A site-dependent collapse of local magnetic moments is followed by the site-selective insulator-to-metal transition at ∼84 GPa, occurring at the octahedral Fe sites. This phase transition is accompanied by a 2+ to 3+ valence change of the prismatic Fe ions and collapse of CO. We provide a microscopic explanation of the complex charge ordering in Fe4O5 which "unifies" it with the behavior of two archetypal examples of charge- or bond-ordered materials, magnetite and rare-earth nickelates (RNiO3). We find that at low temperatures the Verwey-type CO competes with the "trimeron"/"dimeron" charge ordered states, allowing for pressure/temperature tuning of charge ordering. Summing up the available data, we present the pressure-temperature phase diagram of Fe4O5.

Pressure-induced high-spin/low-spin disproportionated state in the Mott insulator FeBO3.

The pressure-induced Mott insulator-to-metal transitions are often accompanied by a collapse of magnetic interactions associated with delocalization of 3d electrons and high-spin to low-spin (HS-LS) state transition. Here, we address a long-standing controversy regarding the high-pressure behavior of an archetypal Mott insulator FeBO3 and show the insufficiency of a standard theoretical approach assuming a conventional HS-LS transition for the description of the electronic properties of the Mott insulators at high pressures. Using high-resolution x-ray diffraction measurements supplemented by Mössbauer spectroscopy up to pressures ~ 150 GPa, we document an unusual electronic state characterized by a "mixed" HS/LS state with a stable abundance ratio realized in the [Formula: see text] crystal structure with a single Fe site within a wide pressure range of ~ 50-106 GPa. Our results imply an unconventional cooperative (and probably dynamical) nature of the ordering of the HS/LS Fe sites randomly distributed over the lattice, resulting in frustration of magnetic moments.

Stabilization of hexazine rings in potassium polynitride at high pressure.

Polynitrogen molecules are attractive for high-energy-density materials due to energy stored in nitrogen-nitrogen bonds; however, it remains challenging to find energy-efficient synthetic routes and stabilization mechanisms for these compounds. Direct synthesis from molecular dinitrogen requires overcoming large activation barriers and the reaction products are prone to inherent inhomogeneity. Here we report the synthesis of planar N62- hexazine dianions, stabilized in K2N6, from potassium azide (KN3) on laser heating in a diamond anvil cell at pressures above 45 GPa. The resulting K2N6, which exhibits a metallic lustre, remains metastable down to 20 GPa. Synchrotron X-ray diffraction and Raman spectroscopy were used to identify this material, through good agreement with the theoretically predicted structural, vibrational and electronic properties for K2N6. The N62- rings characterized here are likely to be present in other high-energy-density materials stabilized by pressure. Under 30 GPa, an unusual N20.75--containing compound with the formula K3(N2)4 was formed instead.

Structural Diversity of Magnetite and Products of Its Decomposition at Extreme Conditions.

Magnetite, Fe3O4, 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 Fe3O4 polymorphs, γ-Fe3O4 with the orthorhombic Yb3S4-type structure and δ-Fe3O4 with the modified Th3P4-type structure. The latter has never been predicted for iron compounds. The decomposition of Fe3O4 at HPHT conditions was found to result in the formation of exotic phases, Fe5O7 and Fe25O32, with complex structures. Crystal-chemical analysis of iron oxides suggests the high-spin to low-spin crossover in octahedrally coordinated Fe3+ 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.

Synthesis of rare-earth metal compounds through enhanced reactivity of alkali halides at high pressures.

Chemical stability of the alkali halides NaCl and KCl has allowed for their use as inert media in high-pressure high-temperature experiments. Here we demonstrate the unexpected reactivity of the halides with metals (Y, Dy, and Re) and iron oxide (FeO) in a laser-heated diamond anvil cell, thus providing a synthetic route for halogen-containing binary and ternary compounds. So far unknown chlorides, Y2Cl and DyCl, and chloride carbides, Y2ClC and Dy2ClC, were synthesized at ~40 GPa and 2000 K and their structures were solved and refined using in situ single-crystal synchrotron X-ray diffraction. Also, FeCl2 with the HP-PdF2-type structure, previously reported at 108 GPa, was synthesized at ~160 GPa and 2100 K. The results of our ab initio calculations fully support experimental findings and reveal the electronic structure and chemical bonding in these compounds.

Realization of an Ideal Cairo Tessellation in Nickel Diazenide NiN2: High-Pressure Route to Pentagonal 2D Materials.

Most of the studied two-dimensional (2D) materials are based on highly symmetric hexagonal structural motifs. In contrast, lower-symmetry structures may have exciting anisotropic properties leading to various applications in nanoelectronics. In this work we report the synthesis of nickel diazenide NiN2 which possesses atomic-thick layers comprised of Ni2N3 pentagons forming Cairo-type tessellation. The layers of NiN2 are weakly bonded with the calculated exfoliation energy of 0.72 J/m2, which is just slightly larger than that of graphene. The compound crystallizes in the space group of the ideal Cairo tiling (P4/mbm) and possesses significant anisotropy of elastic properties. The single-layer NiN2 is a direct-band-gap semiconductor, while the bulk material is metallic. This indicates the promise of NiN2 to be a precursor of a pentagonal 2D material with a tunable direct band gap.

Immiscibility in N2-H2O solids up to 140 GPa.

Nitrogen and water are very abundant in nature; however, the way they chemically react at extreme pressure-temperature conditions is unknown. Below 6 GPa, they have been reported to form clathrate compounds. Here, we present Raman spectroscopy and x-ray diffraction studies in the H2O-N2 system at high pressures up to 140 GPa. We find that clathrates, which form locally in our diamond cell experiments above 0.3 GPa, transform into a fine grained state above 6 GPa, while there is no sign of formation of mixed compounds. We point out size effects in fine grained crystallites, which result in peculiar Raman spectra in the molecular regime, but x-ray diffraction shows no additional phase or deviation from the bulk behavior of familiar solid phases. Moreover, we find no sign of ice doping by nitrogen, even in the regimes of stability of nonmolecular nitrogen.

Stabilization of pentazolate anions in the high-pressure compounds Na2N5 and NaN5 and in the sodium pentazolate framework NaN5·N2.

Synthesis and characterization of nitrogen-rich materials is important for the design of novel high energy density materials due to extremely energetic low-order nitrogen-nitrogen bonds. The balance between the energy output and stability may be achieved if polynitrogen units are stabilized by resonance as in cyclo-N5- pentazolate salts. Here we demonstrate the synthesis of three oxygen-free pentazolate salts Na2N5, NaN5 and NaN5·N2 from sodium azide NaN3 and molecular nitrogen N2 at ∼50 GPa. NaN5·N2 is a metal-pentazolate framework (MPF) obtained via a self-templated synthesis method with nitrogen molecules being incorporated into the nanochannels of the MPF. Such self-assembled MPFs may be common in a variety of ionic pentazolate compounds. The formation of Na2N5 demonstrates that the cyclo-N5 group can accommodate more than one electron and indicates the great accessible compositional diversity of pentazolate salts.

X-ray Free Electron Laser-Induced Synthesis of ε-Iron Nitride at High Pressures.

The ultrafast synthesis of ε-Fe3N1+x in a diamond-anvil cell (DAC) from Fe and N2 under pressure was observed using serial exposures of an X-ray free electron laser (XFEL). When the sample at 5 GPa was irradiated by a pulse train separated by 443 ns, the estimated sample temperature at the delay time was above 1400 K, confirmed by in situ transformation of α- to γ-iron. Ultimately, the Fe and N2 reacted uniformly throughout the beam path to form Fe3N1.33, as deduced from its established equation of state (EOS). We thus demonstrate that the activation energy provided by intense X-ray exposures in an XFEL can be coupled with the source time structure to enable exploration of the time-dependence of reactions under high-pressure conditions.

Stabilization of Polynitrogen Anions in Tantalum-Nitrogen Compounds at High Pressure.

The synthesis of polynitrogen compounds is of great importance due to their potential as high-energy-density materials (HEDM), but because of the intrinsic instability of these compounds, their synthesis and stabilization is a fundamental challenge. Polymeric nitrogen units which may be stabilized in compounds with metals at high pressure are now restricted to non-branched chains with an average N-N bond order of 1.25, limiting their HEDM performances. Herein, we demonstrate the synthesis of a novel polynitrogen compound TaN5 via a direct reaction between tantalum and nitrogen in a diamond anvil cell at circa 100 GPa. TaN5 is the first example of a material containing branched all-single-bonded nitrogen chains [N5 5- ]∞ . Apart from that we discover two novel Ta-N compounds: TaN4 with finite N4 4- chains and the incommensurately modulated compound TaN2-x , which is recoverable at ambient conditions.

The crystal structures of Fe-bearing MgCO3 sp 2- and sp 3-carbonates at 98†GPa from single-crystal X-ray diffraction using synchrotron radiation.

The crystal structure of MgCO3-II has long been discussed in the literature where DFT-based model calculations predict a pressure-induced transition of the carbon atom from the sp 2 to the sp 3 type of bonding. We have now determined the crystal structure of iron-bearing MgCO3-II based on single-crystal X-ray diffraction measurements using synchrotron radiation. We laser-heated a synthetic (Mg0.85Fe0.15)CO3 single crystal at 2500 K and 98 GPa and observed the formation of a monoclinic phase with composition (Mg2.53Fe0.47)C3O9 in the space group C2/m that contains tetra-hedrally coordinated carbon, where CO4 4- tetra-hedra are linked by corner-sharing oxygen atoms to form three-membered C3O9 6- ring anions. The crystal structure of (Mg0.85Fe0.15)CO3 (magnesium iron carbonate) at 98 GPa and 300 K is reported here as well. In comparison with previous structure-prediction calculations and powder X-ray diffraction data, our structural data provide reliable information from experiments regarding atomic positions, bond lengths, and bond angles.