No abstract

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 NiN 2 which possesses atomic-thick layers comprised of Ni 2 N 3 pentagons forming Cairo-type tessellation. The layers of NiN 2 are weakly bonded with the calculated exfoliation energy of 0.72 J/m 2 , 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 NiN 2 is a direct-band-gap semiconductor, while the bulk material is metallic. This indicates the promise of NiN 2 to be a precursor of a pentagonal 2D material with a tunable direct band gap.

With the exception of Li, alkali metals do not react with elemental nitrogen neither at ambient conditions nor at elevated temperatures, requiring the search for alternative synthetic routes to their nitrogen-containing compounds. Here using a controlled decomposition of sodium azide NaN3 at high pressure conditions we synthesize two novel compounds Na3(N2)4 and NaN2 both containing dinitrogen anions. NaN2 synthesized at 4 GPa might be the common intermediate in high-pressure solid-state metathesis reactions where NaN3 is used as a source of nitrogen, while Na3(N2)4 opens a new class of compounds, where [N2] units accommodate a non-integer formal charge of -0.75. This finding can dramatically extend the expected compositions in other group 1-2 metal-nitrogen systems. Electronic structure calculations show the metallic character for both compounds.

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,t heir synthesis and stabilization is af undamental challenge.P olymeric nitrogen units whichm ay be stabilized in compounds with metals at high pressure are now restricted to non-branched chains with an average NÀNb ond order of 1.25, limiting their HEDM performances.Herein, we demonstrate the synthesis of an ovel polynitrogen compound TaN 5 via ad irect reaction between tantalum and nitrogen in adiamond anvil cell at circa 100 GPa. TaN 5 is the first example of am aterial containing branched all-single-bonded nitrogen chains [N 5 5À ] 1 .A part from that we discover two novel TaN compounds:T aN 4 with finite N 4 4À chains and the incommensurately modulated compound TaN 2Àx ,which is recoverable at ambient conditions.

Knowledge of the thermal conductivity of Ar under conditions of high pressures and temperatures (P-T) is important for model calculations of heat transfer in the laser heated diamond anvil cell (DAC) as it is commonly used as a pressure transmitting medium and for thermal insulation. We used a modified transient heating technique utilizing microsecond laser pulses in a symmetric DAC to determine the P-T dependent thermal conductivity of solid Ar up to 50 GPa and 2500 K. The temperature dependent thermal conductivity of Ar was obtained by fitting the results of finite element calculations to the experimentally determined time dependent temperature of a thin Ir foil surrounded by Ar. Our data for the thermal conductivity of Ar are larger than that theoretically calculated using the Green-Kubo formalism, but they agree well with those based on kinetic theory. These results are important for ongoing studies of the thermal transport properties of minerals at pressures and temperatures native to the mantle and core.