First principles calculations were performed to investigate the structural, elastic, and electronic properties of IrN2 for various space groups: cubic Fm-3m and Pa-3, hexagonal P3(2)21, tetragonal P4(2)/mnm, orthorhombic Pmmn, Pnnm, and Pnn2, and monoclinic P2(1)/c. Our calculation indicates that the P2(1)/c phase with arsenopyrite-type structure is energetically more stable than the other phases. It is semiconducting (the remaining phases are metallic) and contains diatomic N-N with the bond distance of 1.414 A. These characters are consistent with the experimental facts that IrN2 is in lower symmetry and nonmetallic. Our conclusion is also in agreement with the recent theoretical studies that the most stable phase of IrN2 is monoclinic P2(1)/c. The calculated bulk modulus of 373 GPa is also the highest among the considered space groups. It matches the recent theoretical values of 357 GPa within 4.3% and of 402 GPa within 7.8%, but smaller than the experimental value of 428 GPa by 14.7%. Chemical bonding and potential displacive phase transitions are discussed for IrN2. For IrN3, cubic skutterudite structure (Im-3) was assumed.

Crystal structures and elastic properties of superhard IrN2 and IrN3 from first principles

Energy storage devices are the key components for successful and sustainable energy systems. Some of the best types of energy storage devices right now include lithium-ion batteries and supercapacitors. Research in this area has greatly improved electrode materials, enhanced electrolytes, and conceived clever designs for device assemblies with the ever-increasing energy and power density for electronics. Electrode materials are the fundamental key components for energy storage devices that largely determine the electrochemical performance of energy storage devices. Various materials such as carbon materials, metal oxides and conducting polymers have been widely used as electrode materials for energy storage devices, and great achievements have been made. Recently, metal nitrides have attracted increasing interest as remarkable electrode materials for lithium-ion batteries and supercapacitors due to their outstanding electrochemical properties, high chemical stability, standard technological approach and extensive fundamental importance. This review analyzes the development and progress of metal nitrides as suitable electrode materials for lithium-ion batteries and supercapacitors. The challenges and prospects of metal nitrides as energy storage electrode materials are also discussed.

Recent advances in metal nitrides as high-performance electrode materials for energy storage devices

Diamond-anvil-cell and shock-wave technologies now permit the study of matter under multimegabar pressure (that is, of several hundred GPa). The properties of matter in this pressure regime differ drastically from those known at 1 atm (about 105 Pa). Just how different chemistry is at high pressure and what role chemical intuition for bonding and structure can have in understanding matter at high pressure will be explored in this account. We will discuss in detail an overlapping hierarchy of responses to increased density: a) squeezing out van der Waals space (for molecular crystals); b) increasing coordination; c) decreasing the length of covalent bonds and the size of anions; and d) in an extreme regime, moving electrons off atoms and generating new modes of correlation. Examples of the startling chemistry and physics that emerge under such extreme conditions will alternate in this account with qualitative chemical ideas about the bonding involved.

The chemical imagination at work in very tight places.

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.

High-pressure chemistry of nitride-based materials

Based on the detailed analysis of chemical bonds, we present a Vickers hardness expression for the covalency-dominant crystals such as transition-metal carbides and nitrides. Hardness is dependent not only on bond length, bond density, and ionicity of bond [F. M. Gao et al., Phys. Rev. Lett. 91, 015502 (2003)] but also on the metallicity of bond and orbital form in the crystal structure of a compound, and all of these parameters can be determined by first-principles calculations. The calculated hardness using our expression has a good agreement with the experimental values for known monocarbides, mononitrides of transition metals, and cubic Zr3N4 with Th3P4 structure. In addition, we have predicted the Vickers hardness of the recently predicted tetragonal BC3 and tetragonal B2CN, and the recently synthesized pyrite PtN2 and marcasite OsN2. Our method offers one useful technique to search for superhard materials in transition-metal carbides and nitrides.

Timon Schröter

Papers

Prediction of Novel Phases of Tantalum(V) Nitride and Tungsten(VI) Nitride That Can Be Synthesized under High Pressure and High Temperature

Synthesen bei hohem Druck und hoher Temperatur führen zu neuen Phasen von Tantal(V)‐nitrid und Wolfram(VI)‐nitrid