Structural and Electronic Competing Mechanisms in the Formation of Amorphous Carbon Nitride by Compressing s-Triazine

Abstract

The pressure-induced transformation of molecular crystals can give rise to new materials characterized by intriguing hardness or energetic properties. Mechanisms regulating these reactions at the molecular level result from a complex interplay among crystal structure, lattice dynamics, and electronic properties. Here, we show that the formation of a three-dimensional amorphous carbon nitride by compressing phase II s-triazine is controlled by the competition between two different mechanisms, one entirely structural and the other electronic, representing the first example where such occurrence is demonstrated. Temperature drives the reactivity below 8 GPa by ruling the lattice dynamics, whereas above 8 GPa the electronic modifications, uniquely governed by pressure, trigger the chemical transformation. The amorphous material synthesized has a bonding structure characterized by a bulk typical of a strongly conjugated three-dimensional carbon nitride with hydrogen atoms migrated to saturate C and N terminations.

One-dimensional diamondoid polyaniline-like nanothreads from compressed crystal aniline

“… 6 , 20 – 22 This occurrence is likely related to the longer C–C contacts between adjacent molecules in aniline, a consequence of the strong and directional H-bonds along the c -axis. 15 In order to estimate the P – T reactivity threshold of aniline at several P and T conditions, we have employed a model accounting for thermal displacements already used for s -triazine 5 and benzene 10 crystals adopting the critical distance, 2.5–2.6 Å between the closest intermolecular C–C contacts found in these cases. As reported in ref.…”

“…Solid-state chemistry induced at high-pressure and high-temperature has been successfully used in the search for new and fascinating materials such as confined polymers and extended amorphous networks. 5 – 9 A possible advantage of these reactions is represented by the topochemical constraints posed by the crystal that can give rise to products closely recalling the symmetry of the molecular crystal from which it is formed. 6 , 10 , 11 In some cases the pressure and temperature conditions required for the synthesis are such that they can be easily scaled up, thus representing a ‘green’ method appealing to industrial chemical synthesis, since the use of additional and polluting compounds such as initiators, catalysts and solvents is avoided.…”

Evolution of Interatomic and Intermolecular Interactions and Polymorphism of Melamine at High Pressure

“…The compressional behavior of melamine closely resembles that of other hydrogen-bonded molecular solids, where individual molecular subunits and internal covalent bonds are not significantly altered with pressure [33,35,54,63,64]. Instead, hydrogen bonds can readily accommodate the increase in pressure, becoming more stable with decreasing donor-acceptor distances and linearization of donor-hydrogen-acceptor angles up to a critical pressure [48,65]. Melamine is notable for its extensive network of hydrogen bonds when compared to other substituted aromatics [54,63], which allows it to remain structurally stable over the same pressure range where many other molecular crystals irreversibly amorphize or decompose [44].…”

“…Additionally, this increase in hydrogen bonding stability (and by extension, covalent character) seems to overrule or direct other steric and electronic interactions that would otherwise exhibit more control over compressional behavior and reactivity. This is especially apparent when comparing against non-hydrogen bonded molecular crystals; for instance, although s-triazine shares the same fundamental aromatic ring system as melamine, its behavior is controlled by π interactions, resulting in susceptibility to electronic modification with pressure and increased reactivity [53,65,66], ultimately becoming irreversibly amorphous at 15.2 GPa. Similarly, benzene amorphizes into extended polyaromatic compounds at 23 GPa.…”

From Molecules to Carbon Materials—High Pressure Induced Polymerization and Bonding Mechanisms of Unsaturated Compounds

“…Because of the anisotropic compressibility of the unit cell determined by the effect of H-bonds, at lower pressure, molecules react along the bc plane which is favor the formation of a layered structure, while reacting along the a-axis at higher pressure will result in the nanothread. In addition, the reactive distance of 2.49 Å of aniline is consistent with benzene [105] and triazine [111], suggesting its popularity. Those studies show that the stress anisotropy can be induced to gain crystalline product, and high temperature is also an important way to facilitate the crystallization.…”