Preparation, crystal structure, and properties of barium pernitride, BaN(2)

Synthesis and high-pressure studies of strontium diazenide by synchrotron X-ray diffraction and DFT calculations

In this work, strontium diazenide (SrN2) was synthesized using strontium azide as a starting material in a Walker-type module under high-pressure and high-temperature conditions. The synthesized SrN2 was further studied under high pressure up to 43.2 GPa using in situ synchrotron X-ray diffraction to supplement the high-pressure exploration of alkaline earth diazenides. The SrN2 underwent a possible phase transition from a tetragonal structure into an orthorhombic structure at 12.0 GPa. SrN2 shows anisotropic compressibility due to the orientation of the diazenide anions. The bulk modulus of SrN2 is 132.4 (10.2) GPa, which is larger than that of Sr(N3)2. The larger bulk modulus of SrN2 is attributed to the stronger covalent strength between Sr and N atoms in SrN2, which is confirmed by our theoretical calculations.

Direct Reaction between Copper and Nitrogen at High Pressures and Temperatures

Transition-metal nitrides have applications in a range of technological fields. Recent experiments have shown that new nitrogen-bearing compounds can be accessed through a combination of high temperatures and pressures, revealing a richer chemistry than was previously assumed. Here, we show that at pressures above 50 GPa and temperatures greater than 1500 K  elemental copper reacts with nitrogen, forming copper diazenide (CuN₂). Through a combination of synchrotron X-ray diffraction and first-principles calculations we have explored the stability and electronic structure of CuN₂. We find that the novel compound remains stable down to 25 GPa before decomposing to its constituent elements. Electronic structure calculations show that CuN₂ is metallic and exhibits partially filled N₂ antibonding orbitals, leading to an ambiguous electronic structure between Cu⁺/Cu²⁺. This leads to weak Cu-N bonds and the lowest bulk modulus observed for any transition-metal nitride.

Electrides with Dinitrogen Ligands

Electrides are a class of materials which contain excess electrons occupying the cavities in the crystal and playing the role of anions. To achieve electron-rich conditions, it usually requires a positive total formal charge in electride materials. However, the assignment of charges relies on a detailed analysis on chemical bonding. Herein, we present a survey on potential electrides which may be overlooked if no bonding analysis is performed. By applying various structure sampling techniques in conjunction with first-principles calculation, we predicted two compounds Ba₂N₂:e^- and Li₂Ca₃N₆:2e^-, both of which are featured by the presence of dinitrogen ligands [N₂], to be potential electrides. While Li₂Ca₃N₆:2e^- with [N₂]^2- ions has been synthesized in the past, its electride nature was discovered for the first time based on our high-throughput screening. On the other hand, Ba₂N₂:e^- with [N₂]^3- ions is a new compound entirely from first-principles structure prediction. The different valence states of dinitrogen ligands identified in these two compounds suggest a novel route to tune the concentration and anisotropic properties of anionic interstitial electrons. Our discovery does not only establish a new class of inorganic electrides but also demonstrates the predictive power of modern crystal structure sampling techniques toward rational material design.

Penta-Pt2N4: an ideal two-dimensional material for nanoelectronics

Since the discovery of graphene, two-dimensional (2D) materials have paved new ways to design high-performance nanoelectronic devices. To facilitate applications of such devices, there are three key requirements that a material needs to fulfill: sizeable band gap, high carrier mobility, and robust environmental stability. However, among the most popular 2D materials studied in recent years, graphene is gapless, hexagonal boron nitride has a very large band gap, transition metal dichalcogenides have low carrier mobility, and black phosphorene is ambience-sensitive. Thus far, these three characteristics could seldom be satisfied by only a single material. Therefore, it is a great challenge to find an ideal 2D material that can overcome these limitations. In this study, we theoretically predicted a novel planar 2D material penta-Pt2N4, which was designed using the Cairo pentagonal tiling as well as the rare nitrogen double bonds. Most significantly, 2D penta-Pt2N4 exhibits excellent intrinsic properties, including large direct band gap (up to 1.51 eV), high carrier mobility (up to 105 cm2·V-1·s-1), very high Young's modulus (up to 0.70 TPa), and robust dynamic, thermal, and ambient stabilities. Moreover, penta-Pt2N4 is the global minimum structure among 2D materials with PtN2 stoichiometry. We also propose a CVD/MBE scheme to enable its experimental synthesis. We envision that 2D penta-Pt2N4 may find wide applications in the field of nanoelectronics.

Fe-N system at high pressure reveals a compound featuring polymeric nitrogen chains

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.

Crystal structures of transition metal pernitrides predicted from first principles

We have extensively explored the stable crystal structures of early-transition metal pernitrides (TMN₂, TM = Ti, V, Cr, Mn, Zr, Nb, Mo, Hf, and Ta) at ambient and high pressures using effective CALYPSO global structure search algorithm in combination with first-principles calculations. We identified for the first time the ground-state structures of MnN₂, TaN₂, NbN₂, VN₂, ZrN₂, and HfN₂ pernitrides, and proposed their synthesis pressures. All predicted crystal structures contain encapsulated N₂ dumbbells in which the two N atoms are singly bonded to a [N₂]⁴⁻ pernitride unit utilizing the electrons transferred from the transition metals. The strong nature of the single dinitrogen bond and transition metal–nitrogen charge transfer induce extraordinary mechanic properties in the predicted transition metal pernitrides including large bulk modulus and high Vickers hardness. Among the predictions the hardness of MnN₂ is 36.6 GPa, suggesting that it is potentially a hard material. The results obtained in the present study are important to the understanding of structure–property relationships in transition metal pernitrides and will hopefully encourage future synthesis of these technologically important materials.

We identified for the first time the ground-state structures of MnN₂, TaN₂, NbN₂, VN₂, ZrN₂ and HfN₂ pernitrides and proposed their synthesis pressures.

High-Pressure NiAs-Type Modification of FeN

The combination of laser-heated diamond anvil cells and synchrotron Mössbauer source spectroscopy were used to investigate high-temperature high-pressure chemical reactions of iron and iron nitride Fe2 N with nitrogen. At pressures between 10 and 45 GPa, significant magnetic hyperfine splitting indicated compound formation after annealing at 1300 K. Subsequent in situ X-ray diffraction reveals a new modification of FeN with NiAs-type crystal structure, as also rationalized by first-principles total-energy and chemical-bonding studies.

Pyrite form of group-14 element pernitrides synthesized at high pressure and high temperature

The incompressible pyrite form of group 14 elemental pernitrides synthesized at high pressures and high temperatures.

Exotic stable cesium polynitrides at high pressure

New polynitrides containing metastable forms of nitrogen are actively investigated as potential high-energy-density materials. Using a structure search method based on the CALYPSO methodology, we investigated the stable stoichiometries and structures of cesium polynitrides at high pressures. Along with the CsN3, we identified five new stoichiometric compounds (Cs3N, Cs2N, CsN, CsN2, and CsN5) with interesting structures that may be experimentally synthesizable at modest pressures (i.e., less than 50 GPa). Nitrogen species in the predicted structures have various structural forms ranging from single atom (N) to highly endothermic molecules (N2, N3, N4, N5, N6) and chains (N∞). Polymeric chains of nitrogen were found in the high-pressure C2/c phase of CsN2. This structure contains a substantially high content of single N-N bonds that exceeds the previously known nitrogen chains in pure forms, and also exhibit metastability at ambient conditions. We also identified a very interesting CsN crystal that contains novel N4(4-) anion. To our best knowledge, this is the first time a charged N4 species being reported. Results of the present study suggest that it is possible to obtain energetic polynitrogens in main-group nitrides under high pressure.

Synthesis, structure, and properties of SrC(NH)3 , a nitrogen-based carbonate analogue with the trinacria motif

Strontium guanidinate, SrC(NH)3 , the first compound with a doubly deprotonated guanidine unit, was synthesized from strontium and guanidine in liquid ammonia and characterized by X-ray and neutron diffraction, IR spectroscopy, and density-functional theory including harmonic phonon calculations. The compound crystallizes in the hexagonal space group P63 /m, constitutes the nitrogen analogue of strontium carbonate, SrCO3 , and its structure follows a layered motif between Sr(2+) ions and complex anions of the type C(NH)3 (2-) ; the anions adopt the peculiar trinacria shape. A comparison of theoretical phonons with experimental IR bands as well as quantum-chemical bonding analyses yield a first insight into bonding and packing of the formerly unknown anion in the crystal.

Novel lithium-nitrogen compounds at ambient and high pressures

Using ab initio evolutionary simulations, we predict the existence of five novel stable Li-N compounds at pressures from 0 to 100 GPa (Li₁₃N, Li₅N, Li₃N₂, LiN₂, and LiN₅). Structures of these compounds contain isolated N atoms, N₂ dimers, polyacetylene-like N chains and N₅ rings, respectively. The structure of Li₁₃N consists of Li atoms and Li₁₂N icosahedra (with N atom in the center of the Li₁₂ icosahedron) – such icosahedra are not described by Wade-Jemmis electron counting rules and are unique. Electronic structure of Li-N compounds is found to dramatically depend on composition and pressure, making this system ideal for studying metal-insulator transitions. For example, the sequence of lowest-enthalpy structures of LiN₃ shows peculiar electronic structure changes with increasing pressure: metal-insulator-metal-insulator. This work also resolves the previous controversies of theory and experiment on Li₂N₂.

Structural analysis of the coordination of dinitrogen to transition metal complexes

Transition-metal complexes show a wide variety of coordination modes for the nitrogen molecule. A structural database study has been undertaken for dinitrogen complexes, and geometrical parameters around the L(n)M-N2 unit are retrieved from the Cambridge Structural Database. These data were classified in families of compounds, according to metal properties, to determine the degree of lengthening for the dinitrogen bonding. The importance of the nature of the metal center, such as coordination number and electronic configuration, is reported. Our study reveals poor activation by coordination of dinitrogen in mononuclear complexes, always having end-on coordination. However, partial weakening of nitrogen-nitrogen bonding is found for end-on binuclear complexes, whereas side-on complexes can be completely activated.

Structure prediction of binary pernitride MN2 compounds (M=Ca, Sr, Ba, La, and Ti)

Metal-pernitride compounds belong to a class of chemical systems in which both the complex ions and the non-bonding electrons may play roles in the formation of their modified crystalline structures. To investigate this issue, the energy landscapes of pernitrides of metals with different maximum valence (M=Ca, Sr, Ba, La, and Ti) were globally explored on the ab initio level at standard and high pressures, thereby yielding possible (meta)stable modifications in these systems together with information on how the landscape changed as function of the valence of the metal cation. For all of the systems in which no compounds had been synthesized so far, we predicted the existence of kinetically stable modifications that should, in principle, be experimentally accessible. In particular, TiN2 should crystallize in a new structure type, TiN2-I.

Synthesis of alkaline earth diazenides M(AE)N2 (M(AE) = Ca, Sr, Ba) by controlled thermal decomposition of azides under high pressure

The alkaline earth diazenides M(AE)N(2) with M(AE) = Ca, Sr and Ba were synthesized by a novel synthetic approach, namely, a controlled decomposition of the corresponding azides in a multianvil press at high-pressure/high-temperature conditions. The crystal structure of hitherto unknown calcium diazenide (space group I4/mmm (no. 139), a = 3.5747(6) Å, c = 5.9844(9) Å, Z = 2, wR(p) = 0.078) was solved and refined on the basis of powder X-ray diffraction data as well as that of SrN(2) and BaN(2). Accordingly, CaN(2) is isotypic with SrN(2) (space group I4/mmm (no. 139), a = 3.8054(2) Å, c = 6.8961(4) Å, Z = 2, wR(p) = 0.057) and the corresponding alkaline earth acetylenides (M(AE)C(2)) crystallizing in a tetragonally distorted NaCl structure type. In accordance with literature data, BaN(2) adopts a more distorted structure in space group C2/c (no. 15) with a = 7.1608(4) Å, b = 4.3776(3) Å, c = 7.2188(4) Å, β = 104.9679(33)°, Z = 4 and wR(p) = 0.049). The N-N bond lengths of 1.202(4) Å in CaN(2) (SrN(2) 1.239(4) Å, BaN(2) 1.23(2) Å) correspond well with a double-bonded dinitrogen unit confirming a diazenide ion [N(2)](2-). Temperature-dependent in situ powder X-ray diffractometry of the three alkaline earth diazenides resulted in formation of the corresponding subnitrides M(AE(2))N (M(AE) = Ca, Sr, Ba) at higher temperatures. FTIR spectroscopy revealed a band at about 1380 cm(-1) assigned to the N-N stretching vibration of the diazenide unit. Electronic structure calculations support the metallic character of alkaline earth diazenides.

High-pressure chemistry of nitride-based materials

Besides temperature at one atmosphere, the applied pressure is another important parameter for influencing and controlling reaction pathways and final reaction products. This is relevant not only for the genesis of natural minerals, but also for synthetic chemical products and technological materials. The present critical review (316 references) highlights recent developments that utilise high pressures and high-temperatures for the synthesis of new materials with unique properties, such as high hardness, or interesting magnetic or optoelectronic features. Novel metal nitrides, oxonitrides as well as the new class of nitride-diazenide compounds, all formed under high-pressure conditions, are highlighted. Pure oxides and carbides are not considered here. Moreover, syntheses under high-pressure conditions require special equipment and preparation techniques, completely different from those used for conventional synthetic approaches at ambient pressure. Therefore, we also summarize the high-pressure techniques used for the synthesis of new materials on a laboratory scale. In particular, our attention is focused on reactive gas pressure devices with pressures between 1.2 and 600 MPa, multi-anvil apparatus at P < 25 GPa and the diamond anvil cell, which allows work at pressures of 100 GPa and higher. For example, some of these techniques have been successfully upgraded to an industrial scale for the synthesis of diamond and cubic boron nitride.

The indium subnitrides Ae6In4(InxLiy)N(3-z)(Ae = Sr and Ba)

The indium nitrides Sr6In4(In(0.32)Li(0.92))N(2.49) and Ba6In(4.78)N(2.72) have been synthesized from an excess of molten sodium. They crystallize in a stuffed variant of the eta-carbide structure type in the cubic space group Fdm with Z = 8. The lattice parameters are a = 14.3752(4) and 15.216(1) A, with cell volumes 2970.6(2) and 3523.3(6) A3, respectively. In both compounds there are vacancies on some of the indium and nitrogen sites and, in the case of Sr6In4(In(0.32)Li(0.92))N(2.49), mixed lithium-indium occupancy of one metal site. It is demonstrated that the partial and mixed occupancies act to carefully tune to electron count to almost fulfill the bonding requirements of the stellar quadrangular subnets of both compounds. Four probe resistivity measurements performed upon a pellet of Sr6In4(In(0.32)Li(0.92))N(2.49) show it to have a room-temperature resistivity of 6.3 mOmega.cm with a weak temperature dependence.

New group 2 chemistry: a multiple barium-nitrogen bond in the CsNBa molecule

The existence of a series of triatomic molecules with the general formula MNM', where M is an alkaline metal (K, Rb, Cs), and M' is an alkaline earth metal (Ca, Sr, Ba), has been predicted by quantum chemical methods. Among these, the CsNBa molecule shows a feature not found before, the presence of a multiple bond between barium and nitrogen. As a consequence of this novel bonding situation, the molecule is linear. The same holds for all Ba triatomics, MNBa, independent of the nature of the alkali M atom, and for all Sr compounds, MNSr. The presence of a multiple bond makes CsNBa, and other related Ba and Sr molecules, particularly stable and appealing experimentally. The systems with the alkaline earth metal M' = Ca, on the other hand, turned out to be bent. Calculations have also been performed on the negative ions BaN(-) and CaN(-), which form a well-defined entity in the MNM' systems (M' = Ba, Ca). The results show that the two ions have a different electronic structure in the ground state, which is one reason for the different properties of the MNM' systems and explains why the molecules containing the BaN(-) moiety are linear, while those containing CaN(-) are bent.