Carbon nitrides featuring 3D frameworks of CN4 tetrahedra were identified as one of the great aspirations of materials science, expected to have a hardness greater than or comparable to diamond. After more than three decades of efforts to synthesize them, no unambiguous evidence of their existence has been delivered. Here, we report the high-pressure high-temperature synthesis of three carbon-nitrogen compounds, tI14-C3 N4 , hP126-C3 N4 , and tI24-CN2 , in laser-heated diamond anvil cells. Their structures were solved and refined using synchrotron single-crystal X-ray diffraction. Physical properties investigations showed that these strongly covalently bonded materials, ultra-incompressible and superhard, also possess high energy density, piezoelectric, and photoluminescence properties. The novel carbon nitrides are unique among high-pressure materials, as being produced above 100 GPa they are recoverable in air at ambient conditions. This article is protected by copyright. All rights reserved.

Synthesis of Ultra-Incompressible and Recoverable Carbon Nitrides Featuring CN4 Tetrahedra.

Metal pentazolate compounds that contain 5-nitrogen singly charged anion rings are an interesting class of materials from a thermodynamics perspective. These compounds are believed to be equilibrium phases only at extremely high pressures, and thus, they constitute exotic states of matter at ambient pressure and temperature. Moreover, the energy released by these pentazolate compounds upon relaxation to the equilibrium state at ambient conditions is in the range 1 to 10 MJ per kg, rivaling that of combustion reactions. The synthesis of pentazolates is not an easy task, and there are only two known methods, each of which has significant challenges. The development of new scalable synthesis routes could enable the production of these exotic materials in sufficient quantity to explore their properties more widely beyond the context of explosives and propellants. In this Perspective, it is argued that nonequilibrium plasma is a promising reaction medium for the synthesis of metal pentazolate compounds, for example from the corresponding metal azide and N2 gas activated by the discharge at low background temperature. 

Polynitrogen High Energy Density Materials Synthesized by Nonequilibrium Plasma

More than thirty years ago, carbon nitrides featuring 3D frameworks of tetrahedral CN 4 units were identified as one of the great aspirations of materials science, expected to have a hardness greater than or comparable to diamond 1 . Since then, no unambiguous experimental evidence of their existence has been delivered. Here, we report the high-pressure high-temperature synthesis of the long-sought-after covalent carbon nitrides, tI 14-C 3 N 4 , hP 126-C 3 N 4 , and tI 24-CN 2 , in laser-heated diamond anvil cells. Their structures were solved and refined using synchrotron single-crystal X-ray diffraction. In these solids, carbon atoms— all sp 3 -hybridized—and nitrogen atoms are fully saturated, forming four and three covalent bonds, respectively, leading to three-dimensional arrangements of corner-sharing CN 4 tetrahedra. These carbon nitrides are ultra-incompressible, with hP 126-C 3 N 4 and tI 24-CN 2 even rivalling diamond’s incompressibility, and superhard. These novel compounds are recoverable to ambient conditions in crystalline form and chemically stable in air. Being wide-band gap semiconductors with intriguing features in their electronic structure, they are expected to exhibit multiple exceptional functionalities besides their mechanical properties, opening new perspectives for materials science. dimers, and repeating in the [001] direction, forming square-shaped channels along this direction. The four C-N bonds of the CN 4 tetrahedra are identical within their uncertainty, with two of 1.379(7) Å and two others of 1.389(7) Å; bond lengths very similar to those of oP 8-CN, tI 14-C 3 N 4 and hP 126-C 3 N 4 . The N-C-N bond angles of the CN 4 tetrahedra vary between 104.0(3)° and 116.7(6)°, with the average value being 109.5(3)°—in line with sp 3 hybridization within regular CN 4 tetrahedra. The N-N bond length is 1.272(11) Å; slightly shorter than what is expected from a single bond in 3D nitrogen polymeric solids, found to have a length of 1.3410(15) Å in cg-N (122 GPa) 42 and of 1.338(6) Å and 1.435(7) Å in bp-N (140 GPa) 39 . Still at 134 GPa, tI 24-CN 2 has an average angle formed by the C/N-N-C bonds of 117.3(4)°, suggesting predominantly sp 2 -hybridization of nitrogen.

Synthesis of Ultra-Incompressible Carbon Nitrides Featuring Three-Dimensional Frameworks of CN4 Tetrahedra Recoverable at Ambient Conditions

Postulated in 1992 and synthesized in 2004 above 2000 K and 110 GPa, the singly-bonded nitrogen cubic gauche crystal (cg-PN) is still considered to be the ultimate high energy density material (HEDM). The search however has continued for a method to synthesize cg-PN at more ambient conditions or find HEDMs which can be synthesized at lower pressure and temperature. Here, using ab initio evolutionary crystal prediction techniques, a simpler nitrogen-based molecular crystal consisting of N[Formula: see text] and N[Formula: see text] molecules is revealed to be a more favorable polynitrogen at lower pressures. The energetic gain of 534 meV/atom over cg-PN and 138 meV/atom over the N[Formula: see text] molecular crystal at zero pressure makes the N[Formula: see text]-N[Formula: see text] system more appealing. Dynamical and mechanical stabilities are investigated at 5 and 0 GPa, and vibrational frequencies are assessed for its Raman and IR spectra. The prospects of an experimental synthesis of the N[Formula: see text]-N[Formula: see text] polymeric system compared to cg-PN is higher because the C[Formula: see text] symmetry of N[Formula: see text] within this crystal would be easier to target from the readily available N[Formula: see text] azides and the observed N[Formula: see text] and N[Formula: see text] radicals. 

/pdf/a-polymeric-nitrogen-n-6-n-2-system-with-enhanced-stability-1sbpx53e.pdf

A polymeric nitrogen N$$_6$$–N$$_2$$ system with enhanced stability at low pressure

/pdf/a-polymeric-nitrogen-n-formula-see-text-n-formula-see-text-1pjla67d.pdf

A polymeric nitrogen N[Formula: see text]-N[Formula: see text] system with enhanced stability at low pressure.

Synthesis and Stabilization of Cubic Gauche Polynitrogen under Radio-Frequency Plasma

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Synthesis and Stabilization of Cubic Gauche Polynitrogen under Radio-Frequency Plasma